Macrophage - Definition, Structure, Mechanism, Functions

Macrophage is a specialized cell of innate immune system. It is present in all vertebrates and acts as the first line of defence against the invading microorganisms.

It is derived from hematopoietic stem cells in bone marrow and also from early embryonic cell population. These cells are present in almost all tissues of the body. They maintain the normal condition of tissues and also helps in tissue integrity.

Macrophages are professional phagocytes. They engulf and remove the foreign particles from the body. These foreign particles include bacteria, viruses, dead cells, apoptotic cells and other cellular debris.

They also take part in immune surveillance. It acts as antigen presenting cell and helps in starting the specific adaptive immune response. Macrophages also produce signalling molecules called cytokines, which recruit and regulate other immune cells during immune response.

Macrophage is a large phagocytic cell of innate immune system. It engulf and destroy microbes, dead cells and cellular debris. It also helps in antigen presentation and immune response.

History and Discovery of Macrophages

In 1882, macrophages and the concept of cellular immunity was discovered by Ilya Metchnikoff. He was a French-Russian zoologist.

He introduced a rose thorn into the transparent larva of starfish. After this he observed that some mobile cells were moving towards the foreign object. These cells tried to surround and engulf the thorn.
These engulfing cells were later understood as phagocytic cells. This observation gave the basic idea that some cells of body can directly attack and remove foreign particles.
After the discovery, macrophages were first placed under the reticuloendothelial system. This system considered that phagocytes and endothelial cells had the same origin. But this idea was not correct.

During 1960s to early 1970s, Ralph van Furth and Zanvil Cohn proposed the classical mononuclear phagocyte system (MPS). According to this theory, all tissue macrophages were formed from circulating blood monocytes.
This theory stated that blood monocytes are originated from adult bone marrow. Then they enter into tissues and finally differentiate into tissue-resident macrophages.
For many years this MPS concept was accepted as the main idea for origin of macrophages. It was considered that adult bone marrow and blood monocytes are the only source of tissue macrophages.

In modern time, this idea was changed by new techniques. These include single-cell RNA sequencing, mass cytometry and genetic fate-mapping methods.
These modern studies showed that many adult tissue-resident macrophages are not mainly derived from adult circulating monocytes. They are formed during early embryonic development.
The main embryonic sources include yolk sac and fetal liver. These macrophages enter into tissues early and remain there for long time.

At present, it is accepted that many tissue-resident macrophages maintain their number by local self-renewal. They can divide and persist in tissues throughout life, without continuous replacement from blood monocytes.

Origin and Development of Macrophages

Change in old concept
- Previously it was considered that all tissue-resident macrophages are originated from circulating blood monocytes.
- These monocytes are produced in adult bone marrow and then enter into tissues.
- But modern fate-mapping studies changed this idea.
- Now it is known that many adult tissue-resident macrophages are formed during early embryonic development.
- These cells remain in tissues for long time and maintain their number by local self-renewal.

Three waves of macrophage development
- The development of macrophages occurs in three successive waves during gestation.
- Primitive hematopoiesis
  - This is the first wave of macrophage development.
  - It occurs in extraembryonic yolk sac during embryonic day 6.5 to 8.5.
  - In this wave, primitive macrophages are formed directly from progenitor cells.
  - It does not pass through any monocyte intermediate stage.
  - These early cells enter into the embryo and colonize tissues.
  - Some of them persist into adult life, mainly as microglia in the brain.
- Pro-definitive hematopoiesis
  - This is the second wave.
  - It occurs during embryonic day 8.5 to 10.5.
  - In this wave, erythromyeloid progenitors are formed in the yolk sac.
  - These progenitors migrate to the fetal liver, which becomes the main site of embryonic blood formation.
  - In fetal liver, these cells differentiate into fetal monocytes.
  - These monocytes move through circulation and seed almost all developing organs.
  - They replace many primitive macrophages and finally differentiate into long-lived adult tissue-resident macrophages.
  - Examples are Kupffer cells in liver and alveolar macrophages in lungs.
- Definitive hematopoiesis
  - This is the third wave.
  - It starts from embryonic day 10.5 and continues up to adult life.
  - In this wave, definitive hematopoietic stem cells (HSCs) are formed.
  - They expand in fetal liver and later colonize the bone marrow around birth.
  - In adulthood, the bone marrow continuously produces circulating blood monocytes.
  - These monocytes are classical and nonclassical types.

Adult tissue replenishment
- In most organs, embryonically derived macrophages maintain themselves locally by self-renewal.
- They do not continuously depend on circulating bone marrow-derived monocytes in normal condition.
- Some organs are exception to this rule.
- The dermis of skin and intestinal lamina propria continuously recruit bone marrow-derived monocytes even in healthy state.
During injury and inflammation
- During severe inflammation, injury or therapeutic depletion, tissue-resident macrophages may be lost.
- In this condition, adult bone marrow-derived monocytes are recruited into the empty tissue niches.
- These recruited monocytes can differentiate into new tissue macrophages.
- They can also acquire the shape and function of the lost embryonic macrophages.
- After this, they may form a new self-maintaining macrophage population in that tissue.

Tissue-specific maturation
- The final identity of a macrophage is mainly controlled by its organ microenvironment.
- It is not decided only by its developmental origin.
- When precursor cells from yolk sac, fetal liver or adult bone marrow enter into a tissue, they receive local tissue signals.
- These signals include physical contact, cytokines and metabolites.
- They induce special transcription factors and make the macrophage suitable for that particular organ.

Structure and Morphology of Macrophages

General size
Macrophage is a large immune cell. The diameter of human macrophage is about 21 μm. When monocytes mature into macrophages, they become larger in size.
Shape and plasticity
Macrophages do not have a single fixed shape. Their shape varies according to tissue, function and activation condition. It may be round, irregular, elongated or spread type. The change in shape is related with their function. Elongated shape of macrophage may also help in expression of M2 repair markers.

Membrane extensions
The plasma membrane of macrophage is flexible. It can form outward projections by actin movement. These projections are called pseudopodia. Pseudopodia helps to reach, surround and engulf the foreign particle.
Intracellular organelles
The cytoplasm contains many lysosomes. Lysosomes have digestive enzymes and hydrolases. During phagocytosis, the engulfed particle is kept inside a phagosome. Then phagosome joins with lysosome and forms phagolysosome. In this structure the engulfed material is digested.
Tissue-specific morphology
Macrophages show different morphology in different tissues according to the local need.
- Microglia: These are macrophages of brain. They are star-shaped cells. They have dynamic lamellipodial network which continuously checks the surrounding neural tissue.
- Osteoclasts: These are large polygonal giant cells present in bone. They are usually multinucleated and contain about 5 to 20 nuclei. It helps in bone resorption.
- Nerve-associated macrophages: These are highly elongated macrophages. They are present along nerves and may extend up to 200 μm in length.
- Epithelioid cells: These are modified macrophages seen in granulomas. They resemble epithelial cells. They have thin eosinophilic cytoplasm, small granules and less dense nucleus than lymphocytes.

Types of Macrophages in Different Tissues

Microglia
Microglia are macrophages of brain and central nervous system. They check the neural tissue. They remove debris and also helps in neuronal development and synaptic pruning.
Kupffer cells
Kupffer cells are present in liver sinusoids. They are also called stellate macrophages. They protect against gut-derived pathogens. They also remove old erythrocytes and helps in iron recycling.

Alveolar macrophages
Alveolar macrophages are present in lung alveoli. They remove inhaled dust and microbes. Interstitial macrophages are present in connective tissue of lung parenchyma.
Osteoclasts
Osteoclasts are specialized macrophages of bone. They are large giant cells and usually multinucleated. They break down bone matrix during bone remodelling.
Splenic macrophages
Spleen contains many macrophage types. Red pulp macrophages remove aged red blood cells. Marginal zone macrophages and metallophilic macrophages trap circulating particles and antigens.

Langerhans cells
Langerhans cells are present in epidermis of skin. They act in immune surveillance. Dermal macrophages are present deeper in dermis.
Intestinal macrophages
Intestinal macrophages are present in lamina propria of intestine. They maintain gut tolerance. Muscularis macrophages remain near enteric neurons and helps in normal intestinal peristalsis.
Cardiac macrophages
Cardiac macrophages are macrophages of heart. They engulf dying cardiomyocytes. They also helps in myocardial conduction and angiogenesis.

Mesangium macrophages
Mesangium macrophages are present in kidney. They are found within the basement membrane of glomerular capillaries. They are also called intraglomerular mesangial cells.
Hofbauer cells
Hofbauer cells are macrophages of placenta. They are commonly seen during early pregnancy. They help in local placental immunity.
Lymph node macrophages
Subcapsular sinus macrophages and sinus histiocytes are present in lymph nodes. They trap particulate antigens from lymph.

Peritoneal and pleural macrophages
Peritoneal macrophages are present in peritoneal cavity. Pleural macrophages are present in pleural cavity. They maintain local immune surveillance.
Type A synovial cells
Type A synovial cells are resident macrophage cells of joint spaces. They remove debris from synovial cavity and helps in joint defence.
Histiocytes
Histiocytes are resting tissue macrophages of connective tissue. During inflammation they may change into epithelioid cells or giant cells.

Mechanism of Macrophage against Pathogens

Step 1. Recognition of pathogen
Macrophage first recognize the pathogen present in tissue. The surface receptors of macrophage bind with pathogen molecules. This may occur by pattern recognition receptors (PRRs). These receptors detect PAMPs present on microbes.
Step 2. Recognition of opsonized pathogen
Some pathogens are coated with antibody or complement protein. These coated pathogens are easily recognized by macrophage. The receptors used here are opsonic receptors. This makes phagocytosis more easy.
Step 3. Receptor activation
After binding, receptors come together at the site of attachment. This is called receptor clustering. It starts local signal inside the cell. Enzymes and small GTPases are activated near the attached membrane.

Step 4. Actin change
The signal mainly act on actin cytoskeleton. Actin fibres are arranged below the plasma membrane. This gives pushing force to the membrane. Due to this the membrane starts to extend outside.
Step 5. Formation of pseudopodia
The extended membrane parts are called pseudopodia. These pseudopodia move around the pathogen. They slowly surround the pathogen from all side.
Step 6. Phagosome formation
When pseudopodia meet with each other, they fuse. The pathogen is now enclosed inside the macrophage. This enclosed vesicle is called phagosome. In early stage it is called nascent phagosome.

Step 7. Phagosome maturation
The phagosome moves inside the cytoplasm. It fuse with early endosome and late endosome. During this process V-ATPase proton pumps are added in the membrane. These pumps send protons inside the phagosome.
Step 8. Acidification
Due to proton entry, the inside of phagosome becomes acidic. This acidic condition helps in killing of pathogen. It also makes the vesicle ready for lysosome fusion.
Step 9. Phagolysosome formation
The late phagosome now fuse with lysosome. This forms phagolysosome. It is the main digestive vesicle of macrophage.

Step 10. Digestion of pathogen
Inside phagolysosome, pH becomes about 4.5. This acidic pH activates lysosomal enzymes. Acid hydrolases, proteases and lipases digest the pathogen. ROS and radicals are also produced for killing.
Step 11. Antigen processing
After digestion, pathogen proteins are broken into small peptide fragments. Some fragments are used for antigen presentation. They are loaded on MHC class II molecule.
Step 12. Antigen presentation
The peptide-MHC-II complex moves to the surface of macrophage. It is shown to CD4+ helper T cells. In this way macrophage kill pathogen and also start adaptive immune response.

Step by Step Activation Process of Macrophages

Step 1. Sensing of stimulus
Macrophage first detect the stimulus in the local tissue area. These stimulus may be microbial product, damaged cell material and cytokines. Microbial products are called PAMPs like LPS. Damaged cell products are called DAMPs.
Step 2. Binding of receptor
The stimulus bind with the receptors present on the surface of macrophage. Toll-like receptors (TLRs) are used for the recognition of pathogen molecules. Some cytokine receptors like IFNGR, IL-4Rα and IL-10R bind with the cytokines.
Step 3. Starting of internal signal
After binding, the signal is transferred inside the cell. In M1 activation, TLRs recruit adaptor proteins such as MyD88 and TRIF. Then kinase cascade takes place and NF-κB transcription factor is activated. IFN-γ also activates JAK/STAT1 pathway.

Step 4. Alternative activation
In M2 activation, IL-4 and IL-13 bind with their receptor. This activates JAK1/JAK3 and STAT6 pathway. IL-10 activates STAT3 pathway. These signals make the macrophage anti-inflammatory and repair type.
Step 5. Metabolic reprogramming
In this step, metabolism of macrophage is changed according to its function. M1 macrophage shifts towards aerobic glycolysis and pentose phosphate pathway. It helps in quick energy production and formation of ROS. M2 macrophage depends on TCA cycle, oxidative phosphorylation (OXPHOS) and fatty acid oxidation (FAO).
Step 6. Transcriptional regulation
The transcription factors are moved into the nucleus. The important factors are NF-κB, STAT1, STAT6, PPARγ and KLF4. These factors regulate the genes of activated macrophage. DNA methylation and histone changes like acetylation also take part in this process.

Step 7. Formation of functional macrophage
Finally the macrophage becomes activated into its functional form. This is called polarization. According to the stimulus, it may become M1 macrophage or M2 macrophage.
- M1 macrophage– M1 macrophage is pro-inflammatory type. It express high amount of iNOS. It produce nitric oxide (NO) and ROS. It also secrete TNF-α, IL-1β, IL-6 and IL-12. It is used to kill pathogens and increase tissue inflammation.
- M2 macrophage– M2 macrophage is anti-inflammatory and repair type. It increase arginase-1 (Arg-1). It secrete IL-10 and TGF-β. It suppress inflammation, remove cellular debris and helps in angiogenesis and tissue healing.

Differentiation of Macrophages from Hematopoietic Stem Cells

Mechanism of Phagocytosis by Macrophages

Recognition of target
Phagocytosis starts with recognition of foreign particle by macrophage. The receptors present on the surface of macrophage binds with the particle. The recognition may be directly by pattern recognition receptors (PRRs), which detect PAMPs present on pathogen. It may also be by opsonic receptors, when the particle is already coated with antibody or complement protein.
Activation of receptors
After binding with the particle, the receptors become clustered at that site. This starts the intracellular signalling below the cell membrane. The main change occur in the actin cytoskeleton. The actin fibres are rearranged for engulfing the particle.
Formation of pseudopodia
Due to actin rearrangement, the plasma membrane of macrophage moves outward. These outward extensions are called pseudopodia. The pseudopodia extend around the foreign particle and slowly cover it from all sides.

Formation of phagosome
When both sides of pseudopodia meet, they fuse with each other. The foreign particle is now enclosed inside the cell. This enclosed vesicle is called phagosome. It is also called nascent phagosome in early stage.
Maturation of phagosome
The newly formed phagosome moves inside the cytoplasm. It fuses with early endosome and then with late endosome. During this process V-ATPase proton pumps are added in the membrane. These pumps push protons inside the phagosome and the inside becomes acidic.
Formation of phagolysosome
The late phagosome then fuses with mature lysosome. This forms phagolysosome. It is the digestive compartment of macrophage where the engulfed particle is destroyed.

Digestion and killing
The internal pH of phagolysosome becomes highly acidic, nearly pH 4.5. This acidic condition activates lysosomal enzymes. The enzymes include acid hydrolases, proteases and lipases. The pathogen is killed by acidic medium, digestive enzymes and also by reactive oxygen species (ROS).
Antigen presentation
After digestion, some small peptide fragments are formed. These fragments are processed by the macrophage and shown on its surface to T cells. In this way phagocytosis also helps to start specific adaptive immune response.

Mechanism of Antigen Processing and Presentation by Macrophages

Internalization of antigen
Macrophage first take the foreign pathogen or extracellular material inside the cell. This is done by phagocytosis or endocytosis. The antigen is now present inside the internal vesicle.

Degradation of antigen
The internalized material passes through endosomal compartments. It first enters early endosome, then late endosome and finally lysosome. These compartments become more acidic. In this acidic condition, hydrolytic enzymes digest the antigen into small peptide fragments. These peptides are usually about 13 to 18 amino acids long.
Formation of MHC-II molecule
At the same time, MHC class II molecules are formed inside the rough endoplasmic reticulum (RER). The peptide binding groove of MHC-II is blocked by a protein called Invariant chain (Ii). This prevents the binding of self internal proteins with MHC-II.
Transport to MIIC compartment
The MHC-II and Invariant chain complex moves out from the RER. Then it is carried to a special late endosomal compartment. This compartment is called MHC class II compartment (MIIC).

Cleavage of invariant chain
Inside the MIIC, protease enzymes act on the Invariant chain. These enzymes include cathepsins. The invariant chain is degraded slowly and only a small fragment remains in the binding cleft. This fragment is called CLIP or Class II-associated Invariant Chain Peptide.
Loading of antigenic peptide
The CLIP fragment is removed by a vesicle membrane protein called HLA-DM. After removal of CLIP, the binding groove becomes free. The processed foreign peptides now bind with MHC-II molecule. A peptide with strong affinity is finally loaded into the groove.
Movement to plasma membrane
After peptide loading, the MHC-II-antigen complex becomes stable. This complex moves out from the endosomal compartment. Then it is transported to the outer plasma membrane of macrophage.

Presentation to T cells
On the cell surface, macrophage presents the foreign peptide with MHC-II molecule. This complex is recognized by CD4+ T cells or helper T cells. This process connects innate immune response with adaptive immune response and starts specific immune activation.

M1 and M2 Macrophages

M1 and M2 macrophages are two activated forms of macrophages. These forms are not fixed always. They are formed according to the signal present in tissue. This change is called macrophage polarization.

Macrophage Polarization_ M1 and M2 Subtypes

M1 Macrophages

M1 macrophages are classically activated macrophages. These are pro-inflammatory type. They are mainly used in killing of microbes and tumour cells.

They are activated by LPS, DAMPs, IFN-γ and TNF. These signals are mostly present in early inflammatory condition. After activation, the macrophage becomes strong antigen presenting cell.

M1 macrophages secrete IL-1, IL-6, IL-12 and TNF-α. These are inflammatory cytokines. They increase inflammation and activate other immune cells.

They convert arginine into nitric oxide (NO). They also produce reactive oxygen species (ROS). These toxic products are used for killing of bacteria, viruses and other foreign particles.

The metabolism of M1 macrophages depends mainly on aerobic glycolysis and pentose phosphate pathway. This gives fast energy for inflammatory function.

M2 Macrophages

M2 macrophages are alternatively activated macrophages. These are anti-inflammatory and repair type macrophages. They are mainly used in wound healing, removal of debris and stopping of inflammation.

They are activated by parasitic infection, fungal infection, apoptotic cells and cytokines like IL-4, IL-10, IL-13 and TGF-β. These signals make macrophage less inflammatory and more repair type.

M2 macrophages secrete IL-10 and TGF-β. These are anti-inflammatory cytokines. They reduce inflammation and helps in tissue repair.

They contain arginase-1 (Arg1). This enzyme converts arginine into ornithine and urea. These products are used in repair of tissue and vascular regeneration.

The metabolism of M2 macrophages depends mainly on oxidative phosphorylation (OXPHOS) and fatty acid oxidation (FAO). This gives long time energy for repair function.

In some disease condition, M2 macrophages may also help in fibrosis and tumour progression. So their function depends on the local tissue condition.

Subtypes of M2 Macrophages

M2 macrophages are divided into M2a, M2b, M2c and M2d. These subtypes are made according to their activators and their exact function.

Macrophage Subtypes in Atherosclerosis — Macrophage Subtypes in Atherosclerosi

Macrophage Polarization by T-helper Cells

Role of Macrophages in Innate Immunity

Immune surveillance
Macrophages are present in different tissues and continuously check the tissue area. They detect invading microbes and damaged cell products. For this recognition, they have surface receptors like pattern recognition receptors (PRRs) and Toll-like receptors (TLRs). These receptors recognize PAMPs present on microbes.

Phagocytosis of pathogens
Macrophages act as first line defence cell. They engulf bacteria, viruses and other foreign particles. The engulfed particle is enclosed inside a vesicle called phagosome. Then phagosome fuse with lysosome and forms phagolysosome.
Killing of pathogens
Inside the phagolysosome, the trapped microbes are destroyed. Digestive enzymes act on the pathogen. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are also produced. These toxic molecules help to kill the microbes.
Initiation of inflammation
After activation, macrophages secrete many chemical messengers. These include pro-inflammatory cytokines like TNF-α, IL-1β and IL-6. They also secrete chemokines. These molecules bring other immune cells towards the infected site.

Recruitment of immune cells
The cytokines and chemokines released from macrophages activate local endothelial cells. Due to this, more immune cells can enter into the tissue from blood. This helps to form a proper defensive reaction at the site of infection.
Nutritional immunity
During infection, macrophages can store iron inside the cell. They reduce the export of iron from the cell. So the extracellular pathogens do not get enough iron for their growth. This is a type of defence mechanism called iron sequestration.
Clearance of dead cells
Macrophages remove dying cells, dead neutrophils, apoptotic cells and cellular debris. This removal is called efferocytosis. It is generally non-inflammatory process. It helps to prevent tissue damage and maintain normal tissue condition.

Connection with adaptive immunity
After killing the pathogen, macrophages process its proteins into small antigen fragments. These fragments are presented on the surface with MHC class II molecules. Then they are shown to T cells. In this way macrophages connect innate immunity with adaptive immunity.

Role of Macrophages in Adaptive Immunity

Antigen processing
Macrophages act as professional antigen-presenting cells (APCs). They engulf extracellular pathogens and take them inside the acidic endosomal compartments. Inside this compartment, hydrolytic enzymes digest the foreign proteins into small peptide fragments. These fragments are usually about 13 to 18 amino acids long.
Antigen display with MHC-II
The processed peptide fragments are loaded on MHC class II molecules. This forms peptide-MHC-II complex. Then this complex moves towards the plasma membrane of macrophage. At the surface, the antigen is displayed outside for recognition by T cells.

Activation of T cells
Macrophage presents the peptide-MHC-II complex to CD4+ helper T cells. The T-cell receptor (TCR) binds with this complex. This gives the first signal for T cell activation. This signal is called Signal 1.
Costimulatory signal
Only antigen recognition is not enough for full T cell activation. Macrophage also gives second signal by surface molecules. These molecules are CD80 (B7-1) and CD86 (B7-2). They bind with CD28 receptor present on T cell. This is called Signal 2 and it prevents clonal anergy.
Guiding of T cell differentiation
The type of macrophage also decide the type of adaptive immune response. M1 macrophages produce IL-12 and IL-23. These cytokines help in formation of Th1 cells and strong cell-mediated immunity. M2 macrophages are more related with Th2 response and immune tolerance.

Feedback from T cells
After activation, T cells release cytokines. One important cytokine is interferon-gamma (IFN-γ). It binds back to receptors on macrophage. This makes macrophage more activated. Then macrophage show more antigen presentation, produce more ROS and kill intracellular pathogens more effectively.
Help in B cell antibody production
By activating CD4+ helper T cells, macrophages indirectly helps B cells. The activated helper T cells stimulate B cells to multiply and produce specific antibodies. These antibodies coat the pathogens. This coating is called opsonization. After this, macrophages can recognize and remove the pathogens more easily.

Role of Macrophages in Inflammation

Starting of inflammation
When macrophages detect pathogen or tissue damage, they become activated. In this condition they mainly form M1 macrophages. These are pro-inflammatory macrophages and start the inflammatory response.

Release of cytokines and chemokines
Activated M1 macrophages release inflammatory cytokines. The main cytokines are TNF-α, IL-1β, IL-6 and IL-12. They also release chemokines. These molecules bring other immune cells at the site of infection or injury.
Killing of pathogens
During active inflammation, M1 macrophages engulf the microbes by phagocytosis. The engulfed microbes are then destroyed inside the cell. For this, macrophages produce toxic reactive oxygen species (ROS) and nitric oxide (NO).
Removal of dead cells
Macrophages also remove apoptotic cells and dead immune cells from inflamed tissue. This process is called efferocytosis. Dead neutrophils are also cleared by this process before they rupture.
Prevention of secondary inflammation
If dead cells rupture, toxic cellular contents and DAMPs are released. These materials can start sterile inflammation in surrounding tissue. So macrophages prevent this by rapid clearance of dead cells.
Resolution of inflammation
When the harmful agent is removed, macrophages change into M2 macrophages. They stop producing strong inflammatory signals. They release anti-inflammatory cytokines like IL-10 and TGF-β.
Repair after inflammation
M2 macrophages help in wound healing after inflammation. They remove remaining debris and support tissue repair. They also helps in repair of blood vessels.
Role in chronic disease
If macrophages remain in continuous M1 state, inflammation does not stop. If dead cells are not cleared properly, tissue damage also increases. This condition is related with chronic diseases like rheumatoid arthritis, inflammatory bowel disease, obesity-associated insulin resistance and atherosclerosis.
In atherosclerosis
In atherosclerosis, uncleared dead cells and debris collect inside blood vessels. These materials form dangerous necrotic core. This makes inflammation more severe in the vessel wall.

Role of Macrophages in Tissue Repair and Wound Healing

Change from inflammation to repair
In wound healing, macrophages first show inflammatory function. At early stage they are mainly M1 macrophages. After some time they change into M2 macrophages. This change is important for repair of tissue.
Clearing of wound area
Macrophages act as cleaning cells in the wound. They engulf apoptotic cells, necrotic tissue and cellular debris. This process is called efferocytosis. It stops leakage of toxic materials from dead cells.
Control of inflammation
When dead cells are removed, the macrophage becomes repair type. It release IL-10, TGF-β and pro-resolving mediators. These molecules suppress the inflammatory reaction. Thus the wound area becomes suitable for healing.
Formation of new blood vessels
Macrophages secrete growth factors for new vessel formation. VEGF and hepatocyte growth factor are important factors. These factors helps in angiogenesis in the injured tissue.
Cell renewal
The factors released from macrophages also act on nearby cells. They stimulate the cells to divide and replace the damaged cells. This helps in regeneration of tissue.
Production of repair molecule
M2 macrophages increase arginase-1 (Arg-1). This enzyme convert arginine into ornithine and urea. Ornithine is used for collagen synthesis. Collagen rebuild the tissue framework.
Activation of stem cells
Macrophages also produce some special metabolites. These metabolites can activate local stem cells. For example, 11,12-EET from macrophage activate muscle stem cells after injury.
Granulation tissue formation
Macrophages help in formation of granulation tissue. They also guide the deposition of new extracellular matrix. This fills the wound space and helps in sealing of wound.
Wound remodelling
In later stage, macrophages control the remodelling of tissue. They remove remaining debris and support matrix arrangement. In this way they helps in proper wound healing.

Role of Macrophages in Host Defense Against Pathogens

Recognition of pathogen
Macrophages act as first line defence cell. They check the tissue for invading microbes. They recognize the pathogen by pattern recognition receptors (PRRs). These receptors detect PAMPs present on microbes.
Use of receptors
Toll-like receptors (TLRs) and C-type lectins are important receptors. Dectin-1 and Mincle are examples. These receptors bind with microbial molecules and start the defence reaction.
Recognition of coated microbes
Some microbes are coated with antibodies or complement proteins. These are recognized by opsonic receptors. Fcγ receptors recognize antibody coated microbes. Complement receptors recognize complement coated microbes.
Engulfing of microbes
After recognition, macrophage extend its membrane around the microbe. The microbe is taken inside the cell. It is enclosed in a vesicle called phagosome.
Formation of killing vesicle
The phagosome moves inside the cytoplasm. It joins with lysosome and forms phagolysosome. This is the main place where the microbe is killed.
Killing of pathogen
Inside phagolysosome, the condition becomes acidic. Digestive enzymes act on the pathogen. ROS and RNS are also formed. Nitric oxide (NO) is one important molecule. These substances destroy the microbe.
Antigen presentation
After killing, the microbial proteins are broken into small fragments. These fragments are attached with MHC class II molecule. Then macrophage show it on the surface to CD4+ T cells. This helps to start adaptive immune response.
Release of cytokines
During infection, macrophages mostly become M1 macrophages. They release TNF-α, IL-1β, IL-6 and IL-12. These cytokines increase inflammation at the infected site.
Recruitment of immune cells
Macrophages also release chemokines like CXCL8. These chemokines attract neutrophils and natural killer cells. So more immune cells come to the infected area.
Iron holding
Many microbes need iron for growth. During infection, macrophages keep iron inside the cell. They reduce ferroportin, which normally export iron. So microbes do not get enough iron and their growth becomes limited.
Antibacterial vesicles
Activated macrophages may also release extracellular vesicles. These vesicles have antibacterial activity. It helps in controlling the pathogen in another way.

Macrophages in Chronic Diseases and Cancer

Atherosclerosis
In atherosclerosis, macrophages fail to remove dying cells properly. This defective clearance is called defective efferocytosis. Due to this, apoptotic cells collect inside blood vessels. Later secondary necrosis occur and forms inflammatory necrotic core. This makes the plaque more dangerous and may cause thrombosis.
Macrophage subsets in plaques
In atherosclerotic plaque, different macrophage types may be present. Mox macrophages have poor phagocytic activity. Mhem macrophages are protective type and helps to prevent foam cell formation.
Systemic lupus erythematosus
In systemic lupus erythematosus (SLE), macrophages cannot clear dying cells properly. These uncleared cells release nuclear antigens. These antigens stimulate formation of autoantibodies. Then immune complexes are formed and deposited in organs. This causes lupus lesions.
Rheumatoid arthritis
In rheumatoid arthritis (RA), many M1 macrophages enter into synovial tissue of joints. These macrophages produce chronic inflammation. It causes synovitis, cartilage damage and bone erosion.
Inflammatory bowel disease
In inflammatory bowel disease (IBD), macrophage function becomes abnormal. It is seen in ulcerative colitis and Crohn’s disease. Defective efferocytosis and more pro-inflammatory macrophages disturb gut homeostasis. This causes long term intestinal inflammation and mucosal tissue damage.
Obesity and type 2 diabetes
In obesity, adipose tissue macrophages (ATMs) become more inflammatory. They shift towards M1 phenotype. These macrophages produce low grade chronic inflammation. This inflammation helps in development of insulin resistance and type 2 diabetes.
Alzheimer’s and Parkinson’s disease
Microglia are macrophages of central nervous system. In Alzheimer’s disease and Parkinson’s disease, microglia become dysfunctional and persistently inflammatory. They fail to clear neurotoxic debris like beta-amyloid plaques. They also release inflammatory mediators. This increases neuroinflammation and neuronal damage.
Cancer and tumour progression
In cancer, tumour cells can change macrophages into M2-like macrophages. These are called tumour-associated macrophages (TAMs). These macrophages suppress immune response and block proper activity of T cells.
Tumour growth and metastasis
TAMs release many factors that helps tumour growth. They also helps in angiogenesis, which means formation of new blood vessels. These blood vessels supply nutrients to tumour. TAMs also help in metastasis of cancer cells.
Escape from macrophage killing
Some cancer cells avoid macrophage phagocytosis. They express “don’t eat me” signals on their surface. CD47 is one important signal. It blocks macrophage engulfing and helps cancer cells to escape immune clearance.

Cytokines and Mediators Produced by Macrophages

Pro-inflammatory M1 cytokines and mediators
These are mainly formed from M1 macrophages. These are used in killing of pathogens and in production of inflammation.
- Pro-inflammatory cytokines
  TNF-α, IL-1β, IL-6, IL-12, IL-18, IL-23 and type I interferons are the main cytokines. These cytokines produce inflammatory response and activates other immune cells.
- Chemokines
  CXCL1, CXCL2, CXCL3, CXCL5, CXCL8 (IL-8), CXCL9, CXCL10 (IP-10) and CCL3 (MIP-1) are produced. These are used to attract immune cells towards the site of infection.
- Reactive species
  Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are produced by activated macrophages. Nitric oxide (NO) is one important RNS. These are toxic molecules and helps in destruction of microbes.
- Enzymes and other mediators
  Inducible nitric oxide synthase (iNOS), matrix metalloproteinase-9 (MMP-9), macrophage migration inhibitory factor (MIF) and cyclooxygenase-2 (COX2) are produced. These mediators takes part in inflammation, tissue reaction and microbial killing.
Anti-inflammatory and reparative M2 cytokines and mediators
These are mainly formed from M2 macrophages. These are used for reducing inflammation, clearing debris and repair of damaged tissue.
- Anti-inflammatory cytokines
  IL-10, transforming growth factor-beta (TGF-β) and IL-1 receptor antagonist (IL-1Ra) are produced. These cytokines decrease inflammatory reaction.
- Chemokines
  CCL17, CCL18 and CCL22 are produced. These are related with repair type immune response and regulation of immune cells.
- Growth and repair factors
  Vascular endothelial growth factor (VEGF/VEGF-A), platelet-derived growth factor-beta (PDGF-β), connective tissue growth factor (CTGF) and amphiregulin are produced. These factors helps in new blood vessel formation and healing of tissue.
- Metabolites and lipids
  Ornithine, urea, polyamines such as putrescine, spermidine and spermine, prostaglandin E2 (PGE2) and lactate are formed. These are mainly associated with repair process.
- Pro-resolving mediators
  Resolvins, protectins and maresins are produced during resolving phase of inflammation. These molecules helps in stopping the inflammatory process.
- Enzymes
  Arginase-1 (Arg-1) is an important enzyme of M2 macrophage. It converts arginine into ornithine and urea. This is associated with tissue repair function.
Mixed or other factors
Some macrophages produce mixed type of cytokines. It depends on the type of activation and local tissue condition.
- Colony-stimulating factors
  Granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) are produced. These factors helps in growth and differentiation of immune cells.
- Mixed output
  M2b macrophages can produce both inflammatory and anti-inflammatory cytokines. They may produce TNF-α, IL-1β, IL-6 and IL-10 together.

Disorders Associated with Macrophage Dysfunction

Atherosclerosis
In atherosclerosis, macrophages fail to clear dead cells properly. This defective clearing is called efferocytosis. Dead cells and necrotic cells collect inside blood vessel. It forms inflammatory necrotic core. Due to this plaque becomes weak and may rupture.
Myocardial infarction and heart failure
After myocardial infarction, many cardiomyocytes are died. If macrophages cannot remove these dying cells, inflammation continue for long time. The repair of heart tissue becomes poor. Slowly it may progress into heart failure.
Systemic lupus erythematosus
In systemic lupus erythematosus (SLE), macrophage clearance capacity is reduced. Apoptotic cells are not removed and they release nuclear antigens. These antigens make immune complexes with autoantibodies. These complexes deposit in organs and produce lupus lesions.
Rheumatoid arthritis and osteoarthritis
In rheumatoid arthritis (RA) and osteoarthritis (OA), macrophages of synovial tissue become abnormal. They fail to remove dead cells from joint area. So inflammation remains active. It causes cartilage damage, bone erosion and destruction of joint.
Obesity and diabetes
In obesity, adipose tissue macrophages become more inflammatory. Their phagocytic activity is also reduced. Apoptotic debris are collected in metabolic tissues. This produces chronic low grade inflammation and helps in insulin resistance and type 2 diabetes.
Inflammatory bowel disease
In inflammatory bowel disease (IBD), macrophage efferocytosis is impaired. This is seen in ulcerative colitis and Crohn’s disease. Dying colonic epithelial cells remain in intestine. It increases persistent intestinal inflammation and mucosal damage.
Cancer and tumour progression
In cancer, tumour cells change macrophages into tumour-associated macrophages (TAMs). These macrophages become immunosuppressive type. They help in survival of tumour cells. They also helps in angiogenesis and metastasis. They suppress anti-tumour T cells.
Alzheimer’s and Parkinson’s disease
Microglia are macrophages of brain. In Alzheimer’s disease and Parkinson’s disease, microglia become dysfunctional. They cannot clear toxic debris like beta-amyloid plaques properly. They also release inflammatory mediators. This causes neuroinflammation and neuronal damage.
Fibrotic disorders
In liver, lung, muscle and adipose fibrosis, macrophages remain in pro-fibrotic condition. They activate fibroblasts and stellate cells. These cells produce excess collagen. So tissue scarring and fibrosis takes place.
Autoimmune liver diseases
In autoimmune liver diseases (AILD), liver macrophages cannot clear dying hepatocytes properly. So self-antigens are exposed in liver tissue. This starts inflammation again and again. Later hepatic fibrosis may occur.
Skin diseases
In atopic dermatitis and psoriasis, efferocytic macrophages do not work properly. Apoptotic debris and necrotic cells collect in skin. This increases chronic allergic and auto-inflammatory skin lesions.
Gouty arthritis
In gouty arthritis, urate crystals bring neutrophils into joint. Macrophages cannot remove the dying neutrophils properly. So inflammation is not stopped. Acute inflammation remains in joint area.
Sjögren’s syndrome
In Sjögren’s syndrome, defective efferocytosis allow autoantigens to remain in glandular tissue. Inflammation continues in exocrine glands. It causes progressive damage of salivary gland and tear gland.

Functions of Macrophages

Phagocytosis and pathogen clearance
Macrophages act as first line defence cell. They recognize foreign particles, microbes and cellular debris. Then they engulf these materials and digest them inside phagolysosome.
Antigen presentation
After digestion of pathogen, macrophages process the pathogen proteins. Small antigen fragments are formed. These fragments are attached with MHC class II molecules and shown on the surface. This helps in activation of helper T cells.
Efferocytosis
Macrophages remove apoptotic and dying cells from tissue. This process is called efferocytosis. It is silent type clearing process. It prevents rupture of dead cells and stops release of toxic cell contents.
Immune surveillance
Macrophages continuously check the tissue area. They detect infection, tissue damage and abnormal particles. When danger is detected, they become activated and start immune response.
Release of cytokines and chemokines
Activated macrophages release cytokines and chemokines. TNF-α is one important cytokine. These molecules bring other immune cells at the site of infection. They also activate nearby immune cells.
Tissue repair and wound healing
During healing phase, macrophages change into repair type M2 macrophages. They release anti-inflammatory cytokines and growth factors. These factors helps in angiogenesis, granulation tissue formation and regeneration of tissue.
Iron recycling
Special macrophages of spleen and liver remove old and damaged red blood cells. They break down hemoglobin and recycle iron. This iron is again used for formation of new red blood cells.
Iron sequestration during infection
During infection, macrophages keep iron inside the cell. This makes less iron available for microbes. As microbes need iron for growth, their multiplication becomes reduced.
Maintenance of metabolic homeostasis
Macrophages also help in normal metabolic balance. They clear lipoproteins and regulate cholesterol metabolism. They also maintain tissue condition without causing unnecessary inflammation.

References

Alam, M. Z., Devalaraja, S., & Haldar, M. (2017). The Heme Connection: Linking Erythrocytes and Macrophage Biology. Frontiers in Immunology, 8, 33. https://doi.org/10.3389/fimmu.2017.00033
Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). T Cells and MHC Proteins. In Molecular Biology of the Cell (4th ed.). Garland Science.
Antigen Processing and Presentation by MHCs. (n.d.). Thermo Fisher Scientific – US.
Checking your browser – reCAPTCHA. (n.d.).
Creative Bioarray. (n.d.). Macrophage Phagocytosis Assay.
Creative Biolabs. (n.d.). Macrophage Phagocytosis.
CZ CELLxGENE CellGuide. (n.d.). Tissue-Resident Macrophage Cell Types.
Development and function of tissue resident macrophages in mice. (n.d.). PMC – NIH.
Efferocytosis signaling in the regulation of macrophage inflammatory responses. (n.d.). PMC – NIH.
Epelman, S., Lavine, K. J., & Randolph, G. J. (2014). Origin and Functions of Tissue Macrophages. Immunity, 41(1), 21-35. https://doi.org/10.1016/j.immuni.2014.06.013
A Guide to Macrophage Markers. (n.d.). Biocompare.
Haldar, M., & Murphy, K. M. (2014). Origin, Development, and Homeostasis of Tissue-resident Macrophages. Immunological Reviews, 262(1), 25-35. https://doi.org/10.1111/imr.12215
Harris, J. (n.d.). Phagocytosis. British Society for Immunology.
He, Z., Chen, N., Zhang, Y., Du, H., & Jie, L. (2026). Impaired Macrophage Efferocytosis: Shared Mechanisms and Therapeutic Implications in Immune-Mediated Inflammatory Diseases. Journal of Inflammation Research, 19, 582297. https://doi.org/10.2147/JIR.S582297
Heme-mediated SPI-C induction promotes monocyte differentiation into iron-recycling macrophages. (n.d.). PMC.
Heme-mediated SPI-C induction promotes monocyte differentiation into iron-recycling macrophages. (n.d.). PubMed.
IL-33: a key player in the development of iron-recycling red pulp macrophages. (n.d.). PMC.
Ji, Y., Li, X., Yao, X., Sun, J., Yi, J., Shen, Y., Chen, B., & Sun, H. (2025). Macrophage polarization: molecular mechanisms, disease implications, and targeted therapeutic strategies. Frontiers in Immunology, 16, 1732718. https://doi.org/10.3389/fimmu.2025.1732718
Kulle, A., Thanabalasuriar, A., Cohen, T. S., & Szydlowska, M. (2022). Resident macrophages of the lung and liver: The guardians of our tissues. Frontiers in Immunology, 13, 1029085. https://doi.org/10.3389/fimmu.2022.1029085
Lafuente, E. M., Niedergang, F., & Rosales, C. (2020). Editorial: Phagocytosis: Molecular Mechanisms and Physiological Implications. Frontiers in Immunology, 11, 586918. https://doi.org/10.3389/fimmu.2020.586918
Lavin, Y., Mortha, A., Rahman, A., & Merad, M. (n.d.). Regulation of macrophage development and function in peripheral tissues. Nature Reviews Immunology. https://doi.org/10.1038/nri3920
Lee, K. Y. (2019). M1 and M2 polarization of macrophages: a mini-review. Medical Biological Science and Engineering, 2(1), 1-5. https://doi.org/10.30579/mbse.2019.2.1.1
Lendeckel, U., Venz, S., & Wolke, C. (2022). Macrophages: shapes and functions. ChemTexts, 8(2), 12. https://doi.org/10.1007/s40828-022-00163-4
M1/M2 macrophages and their overlaps – myth or reality? (n.d.). PMC – NIH.
Mechanisms and consequences of efferocytosis in advanced atherosclerosis. (n.d.). PMC – NIH.
Murray, P. J. (2017). Macrophage Polarization. Annual Review of Physiology, 79, 541-566. https://doi.org/10.1146/annurev-physiol-022516-034339
Niche signals and transcription factors involved in tissue-resident macrophage development. (n.d.).
Ontogeny of Tissue-Resident Macrophages. (n.d.). PMC.
Origins and diversity of macrophages in health and disease. (n.d.). PMC.
Peng, W., Song, Y., Gu, S., Zhu, Y., & Li, Y. (2026). Efferocytosis: The Silent Guardian of Tissue Homeostasis and Cardiovascular Health. Journal of Cardiovascular Development and Disease, 13(1), 21. https://doi.org/10.3390/jcdd13010021
Phagocytosis: Our Current Understanding of a Universal Biological … (n.d.).
Physiology and diseases of tissue-resident macrophages. (n.d.). PMC – NIH.
R&D Systems. (n.d.). Macrophage – Common Markers.
Resident and Monocyte-Derived Macrophages in Cardiovascular Disease. (n.d.). Circulation Research – American Heart Association Journals.
Role for Spi-C in the development of red pulp macrophages and splenic iron homeostasis. (n.d.). PubMed.
Santa, K. (2023). Macrophages: Phagocytosis, Antigen Presentation, and Activation of Immunity. In S. S. Athari & E. Mehrabi Nasab (Eds.), Phagocytosis – Main Key of Immune System. IntechOpen. https://doi.org/10.5772/intechopen.110832
The ins and outs of MHC class II-mediated antigen processing and … (n.d.).
The Plasticity, Ontogeny, and Functional Dynamics of Macrophages in Tissue Homeostasis and Pathological States. (n.d.).
The role of efferocytosis-fueled macrophage metabolism in the … (n.d.).
The Role of Microglia and Macrophages in CNS Homeostasis, Autoimmunity, and Cancer. (n.d.).
The roles of tissue resident macrophages in health and cancer. (n.d.). ResearchGate.
Tissue-resident macrophages. (n.d.). PMC – NIH.
Wang, N., Liang, H., & Zen, K. (2014). Molecular Mechanisms That Influence the Macrophage M1–M2 Polarization Balance. Frontiers in Immunology, 5, 614. https://doi.org/10.3389/fimmu.2014.00614
Wei, Q., Deng, Y., Yang, Q., Zhan, A., & Wang, L. (2023). The markers to delineate different phenotypes of macrophages related to metabolic disorders. Frontiers in Immunology, 14, 1084636. https://doi.org/10.3389/fimmu.2023.1084636
Zhu, B. T. (2025). Molecular Mechanism for the Selective Presentation of Antigenic Peptides by Major Histocompatibility Complex Class I and Class II Molecules: A Hypothesis. Current Issues in Molecular Biology, 47(11), 945. https://doi.org/10.3390/cimb47110945

Macrophage – Definition, Structure, Mechanism, Functions