Non-Specific Immune Response - Definition, Types, Mechanism

Non-specific immune response is the inborn defence mechanism of body. It gives immediate and general protection against different pathogens. It does not need previous exposure and does not form immunological memory.

Non-specific immune response is the immediate and natural defence system of the body. It is also called innate immunity. It is present from birth and gives first protection against different types of pathogens.

This response does not need previous exposure to infection or vaccination. It does not form immunological memory. So, the reaction remains almost same whenever the body face the harmful microbe.

The non-specific immune response works by recognizing broad foreign patterns present on pathogens. It is not specific for one particular antigen like adaptive immunity. It gives a rapid and general type of protection.

The first line of this defence includes physical and chemical barriers. Skin, mucous membrane, saliva, and stomach acid are some important barriers. These barriers block, trap, dissolve or kill the microbes before they enter deep inside the body.

When pathogens cross these barriers, then internal defence system become active. In this condition phagocytes, natural killer cells, inflammation, fever and different chemical signals are involved. These cells and reactions identify, engulf and destroy the invading pathogens.

Cytokines act as chemical signals during this process. They help in communication between immune cells and regulate the immune reaction. Thus, non-specific immune response is a fast, inborn and generalized defence mechanism of body.

Characteristics of Non-Specific Immune Response

Present from birth
Non-specific immune response is present in the body from birth. It is a natural and inborn defence system. It is not produced due to previous infection or vaccination.
Immediate response
It gives very fast protection against pathogens. It starts its action immediately or within few minutes to hours after entry of microbes. So, it acts as the first defence reaction of body.

Generalized response
This immune response is not specific for a particular antigen. It recognizes broad foreign patterns present on pathogens, such as PAMPs. It also recognize damage signals released from injured cells, called DAMPs.
No immunological memory
It does not form immunological memory. It does not learn from previous contact with microbes. Therefore same type of response is produced every time when pathogen enter the body.
Multilayered defence system
It has both external and internal defence mechanisms. Skin, mucous membrane, tears, saliva and stomach acid act as external barriers. Phagocytes, natural killer cells, inflammation, fever and complement system act as internal defence.

Evolutionarily ancient system
It is an old defence system found in many multicellular organisms. It is encoded by germline genes and works as first mechanism of host defence. It protects the body before the adaptive immune response becomes active.

Components of Non-Specific Immune Response

Physical barriers– These are the first protective covering of the body. They prevent the entry of pathogens into body.
- Skin– It acts as a strong physical shield and contains keratin.
- Mucous membrane– It produces sticky mucus which trap incoming pathogens.
- Cilia and body hairs– They catch and remove microbes from respiratory tract, nose and ear.
Chemical and biological barriers– These barriers kill, inhibit or compete with pathogens.
- Secretions with enzymes– Saliva, tears and mucus contain enzymes which break bacterial cell wall.
- Acidic environment– Stomach acid kills many swallowed pathogens and skin acidity prevent bacterial growth.
- Beneficial microorganisms– Harmless microbes compete with disease causing microbes for space and nutrients.

Internal cellular defenses– These are formed by different immune cells present inside the body.
- Phagocytes– Neutrophils, monocytes and macrophages engulf and digest microbes.
- Natural killer cells– NK cells destroy virus infected cells and cancerous cells by apoptosis.
- Mast cells– They release histamine and other inflammatory chemicals during injury or infection.

Internal chemical and protein defenses– These include chemical signals and protein molecules which support immune reaction.
- Pattern recognition receptors– PRRs such as Toll-like receptors (TLRs) detect broad microbial patterns.
- Cytokines– These are messenger molecules which regulate immune response, inflammation and fever.
- Interferons– These are released by virus infected cells and inhibit viral replication in nearby cells.
- Complement system– It is a group of plasma proteins which tag pathogens, attract immune cells and destroy microbes by pore formation.

Physical Barriers of Non-Specific Immune Response

Skin– Skin is the outer covering of the body and it works as first physical barrier. It is made up of closely arranged cells called keratinocytes. These cells contain tough fibrous protein keratin, which prevent the entry of microorganisms into the body.

Desquamation– The outer layer of skin is continuously removed from the body. This process is called desquamation. During this process many attached pathogens are also removed from the skin surface.
Mucous membranes– Mucous membranes are present in the body openings. They are found in respiratory tract, gastrointestinal tract and genitourinary tract. These membranes produce sticky mucus, which trap the microorganisms and prevent their entry into deeper tissues.
Cilia– Cilia are small hair like structures present on epithelial cells of upper respiratory tract. They show continuous beating movement. This movement pushes mucus containing trapped pathogens towards pharynx and mouth, from where it is thrown outside or swallowed into stomach.

Body hairs– Body hairs are present in nose, ear and other openings. They act as filter and catch dust particles and microbes. Thus large airborne particles and pathogens are stopped before entering inside the body.
Cellular junctions– Cellular junctions are present between epithelial cells. Tight junctions and desmosomes hold the cells together. They form a continuous barrier and prevent microbes from passing through the spaces between the cells.
Mechanical clearance reflexes– These are involuntary actions of the body. Sneezing, coughing, blinking and lacrimation are included in this type. They remove irritants, dust particles and pathogens from respiratory tract and eyes.

Chemical Barriers of Non-Specific Immune Response

Acidic environment– Acidic environment is one important chemical barrier of body. The stomach secretes hydrochloric acid (HCl), which kill most of the microorganisms entering with food and water. Skin surface is also acidic due to sebum and fatty acids, which inhibit growth of many pathogenic bacteria.
Lysozyme– Lysozyme is a destructive enzyme present in tears, saliva and nasal fluid. It breaks the bacterial cell wall by enzymatic action. Due to this, bacterial cell burst and destroyed.
Digestive proteins and bile acids– Digestive proteins are present in gastrointestinal tract. Pepsin denatures microbial proteins and make them inactive. Bile acids and fatty acids act like detergent and destroy the lipid membrane of many microbes.

Iron binding proteins– Lactoferrin and transferrin are iron binding proteins present in mucosal secretion. They bind free iron tightly. So pathogenic microbes do not get enough iron for growth and multiplication.
Antimicrobial peptides– Antimicrobial peptides are secreted by epithelial cells. Defensins and histatins are important examples. They attack pathogens directly on skin and mucosal surface and help in neutralizing them.
Pulmonary surfactants– Pulmonary surfactants are chemical secretions found in respiratory tract. They help to trap airborne pathogens entering into lungs. They also help in elimination of these pathogens from respiratory passage.

Biological Barriers of Non-Specific Immune Response

Beneficial microorganisms– Beneficial microorganisms are harmless microorganisms which normally live on body surfaces and inside lower digestive system. These include bacteria, archaea and fungi. They are also called normal flora.
Resource competition– The normal flora compete with disease causing pathogens for food, space and other nutrients. Due to this, harmful microbes do not get enough resources for growth. This is act as a biological shield of body.
Direct combat– Some beneficial microorganisms directly inhibit the growth of pathogens. They prevent the pathogens from colonizing on body surface and mucosal surface. Thus infection is not easily established in the host body.

Cells Involved in Non-Specific Immune Response

Neutrophils– Neutrophils are the most abundant white blood cells. These are the first cells which reach at the infected area. They engulf and digest the pathogens by phagocytosis. They also release toxic granules and form neutrophil extracellular traps (NETs) for destroying microbes.
Monocytes– Monocytes are circulating white blood cells present in blood. When they move into infected tissues, they become macrophages. So, these cells help in starting the tissue level defence reaction.
Macrophages– Macrophages are large eating cells present in tissues. They engulf foreign particles, microbes and dead cell debris. They also release cytokines, which call other immune cells at the infected site and help in tissue repair.

Mast cells– Mast cells are present in connective tissues and mucous membranes. They do not mainly circulate in blood. During injury or entry of large pathogens, they release histamine and other inflammatory chemicals. This starts inflammation and increase blood flow in that area.
Natural killer cells– Natural Killer cells (NK cells) are lymphocytes of innate immune system. They do not kill the free pathogens directly. They recognize virus infected cells and cancerous cells. Then they destroy these cells by apoptosis or programmed cell death.
Eosinophils– Eosinophils are granulocytic cells. They mainly act against multicellular parasites. They release highly toxic proteins and free radicals which damage the parasite body. They are also involved in allergic reactions.

Basophils– Basophils are granulocytes which are similar to mast cells in function. They release histamine during allergic response. They also take part in defence against parasites.
Dendritic cells– Dendritic cells are present in skin and mucosal linings. These are the places which come in contact with outside environment. They identify the pathogens, engulf them and then present antigen pieces to adaptive immune system. So, they act as a link between innate and adaptive immunity.
Innate lymphoid cells– Innate Lymphoid Cells (ILCs) are a newer group of innate immune cells. They do not have antigen specific receptors. They produce different cytokines and help in inflammation, tissue balance and control between normal microbiota and immune system.

Pattern Recognition Receptors (PRRs)

Pattern Recognition Receptors (PRRs) are conserved receptor molecules of innate immune system. These receptors work as primary alarm system of body. It is present on resident immune cells like macrophages, mast cells and dendritic cells. It is also present on non immune cells like epithelial cells.

PRRs do not recognize one particular antigen. They recognize common and fixed molecular patterns present on microbes and injured cells. So these receptors give early signal when pathogen enter into the body.

Pathogen Associated Molecular Patterns (PAMPs) are foreign molecular structures present on pathogens. Lipopolysaccharide (LPS), peptidoglycan, viral RNA and fungal sugars are some examples of PAMPs. These are not normally present in host cell.

Damage Associated Molecular Patterns (DAMPs) are danger signals released from damaged, stressed or dying body cells. Released DNA, ATP and uric acid are included under this group. It indicate that tissue damage has occurred.

The main families of PRRs are Toll-Like Receptors (TLRs), NOD-Like Receptors (NLRs), C-Type Lectin Receptors (CLRs) and RIG-I-Like Receptors (RLRs). These receptors are present on cell membrane, inside endosome, in cytoplasm or in soluble form.

Toll-Like Receptors (TLRs) are transmembrane receptors. It is present on the outer membrane of cell and also inside the endosome. They detect bacterial lipids, flagellin and viral nucleic acids.

NOD-Like Receptors (NLRs) are present inside the cytoplasm. It detect bacterial products and internal stress signals. Some NLRs like NLRP3 form large protein complex called inflammasome. It produces strong inflammation and cause pyroptosis.

C-Type Lectin Receptors (CLRs) recognize sugar and carbohydrate structure. These structures are found on bacteria, fungi and dead cells. Some CLRs are present on cell membrane and some are soluble proteins in blood, such as mannan-binding lectin (MBL).

RIG-I-Like Receptors (RLRs) are intracellular receptors. They mainly detect viral genetic material in cytoplasm. Viral double stranded RNA is important example detected by these receptors.

When PRRs bind with PAMPs or DAMPs, then signalling reaction starts inside the cell. Pathway like NF-κB pathway may become active. After this pro-inflammatory cytokines, interferons and chemokines are produced. These chemicals attract immune cells and start inflammation in the body.

Mechanism of Non-Specific Immune Response (Step-by-Step)

Pathogen entry– In this step pathogen enters into the body. It enters when first line barriers like skin, mucous membrane, mucus and stomach acid are crossed. Then the microbe reaches into the tissue.
Recognition of pathogen– The pathogen is first detected by tissue cells. Macrophages, dendritic cells and mast cells are present in tissue. These cells act as sentinel cells and recognize the pathogen.

Action of PRRs– These cells have Pattern Recognition Receptors (PRRs). These receptors identify common microbial pattern called PAMPs. It also identify danger signals from damaged cells called DAMPs.
Release of chemical signals– After recognition, the sentinel cells release chemical mediators. Histamine, prostaglandins and cytokines are released. TNF-α and IL-1 are important cytokines in this process.
Inflammatory reaction– These chemicals act on nearby blood vessels. The blood vessels become wider. This is called vasodilation. The wall of blood vessel also become more permeable.

Movement of fluid– Due to more permeability, fluid and plasma proteins come out from blood into tissue. So redness, heat, swelling and pain are produced. These are the main signs of inflammation.
Recruitment of leukocytes– The chemical signals attract white blood cells to the infected area. Neutrophils come first in large number. They stick to the inner wall of blood vessel.
Diapedesis– The white blood cells pass through the wall of blood vessel and enter into tissue. This process is called diapedesis or transmigration. After that they move towards the pathogen by chemotaxis.

Phagocytosis– Neutrophils and macrophages engulf the pathogens. The pathogen is enclosed inside a vesicle called phagosome. This is the main eating step of innate immune response.
Killing of microbes– The phagosome joins with lysosome and forms phagolysosome. Inside it, the pathogen is killed by enzymes and toxic substances. Reactive Oxygen Species (ROS) also help in killing.
NET formation– Some neutrophils release DNA like net outside the cell. This is called Neutrophil Extracellular Traps (NETs). It traps the microbes and helps in their destruction.

Complement activation– Along with cells, the complement system also become active. It is a group of plasma proteins present in blood. These proteins help in destruction of microbes.
Opsonization– In this step complement proteins coat the surface of pathogen. This is called opsonization. After coating, phagocytes can engulf the pathogen more easily.
Membrane attack complex– Some complement proteins form Membrane Attack Complex (MAC). It makes pore in the microbial membrane. Due to this, the microbe burst and destroyed.

Fever and systemic response– Some cytokines reach to hypothalamus of brain. It causes fever. Fever increases body temperature and helps to inhibit microbial growth.
Resolution and repair– After killing of pathogen, anti-inflammatory molecules are produced. Lipoxins stop further entry of immune cells. Macrophages remove dead cells and debris. Then tissue repair and healing starts.

Interaction Between Non-Specific and Specific Immune Response

Antigen presentation– Dendritic cells and macrophages are cells of non-specific immune response. They engulf the pathogens and break them into small antigen pieces. These antigen pieces are then presented to T cells and B cells. This process starts the specific immune response.

Cellular bridge– Dendritic cells act as important bridge between innate and adaptive immunity. It carries the information of pathogen from infected tissue to lymphoid organs. Then specific immune cells become activated against that pathogen.
Antibody triggered defence– B cells of specific immune response produce specific antibodies. These antibodies bind with the pathogen surface. After this, the classical complement pathway become active and helps in destruction of microbes.
Complement and B cell activation– Some complement proteins coat the pathogen surface. C3d is one important complement fragment. It binds with CR2 receptor present on B cells and makes B cell activation easier. Thus more antibody production and memory formation occur.

Cytokine regulation– Innate immune cells release different cytokines. These cytokines control and guide the response of T cells. They help in deciding what type of specific immune response will be formed.
Time giving function– Non-specific immune response starts immediately after infection. It gives a general attack on pathogens and control the spread of infection. During this time, the specific immune response gets time to become active.
Co-operative action– Both immune responses work together in the body. Non-specific immunity starts the early defence and gives signals to specific immunity. Specific immunity then produce targeted response, antibody formation and immunological memory.

Advantages of Non-Specific Immune Response

Immediate response– Non-specific immune response starts immediately after entry of pathogen. It does not need previous contact with the same microbe. It acts within few minutes to hours and gives first protection to the body.
Broad defence– It does not act against only one particular antigen. It recognize common patterns present on many microbes. So it can protect the body from different types of bacteria, virus, fungi and other foreign particles.
Prevention of infection– Skin, mucous membrane, mucus, tears, saliva and stomach acid prevent the entry of pathogens. These barriers stop many microbes before they enter into the tissue. Thus infection is prevented at the beginning.

Containment of pathogen– If pathogen enters inside the body, innate immunity acts quickly. Inflammation, phagocytosis, fever and complement system limit the spreading of microbes. During this time specific immune response get time to become active.
Self and non-self recognition– It can distinguish the foreign materials from the body own cells. Regulatory proteins and inhibitory receptors are involved in this process. So healthy host cells are not normally destroyed.
Bridge to specific immunity– Dendritic cells and macrophages capture and digest the pathogen. They present antigen pieces to T cells and B cells. In this way non-specific immunity helps to start the specific immune response.

Activation of adaptive response– Innate immune cells release cytokines and other chemical signals. These signals activate and guide the specific immune cells. It helps in formation of targeted immune response against pathogen.
Trained immunity– Some innate immune cells show improved response after first exposure. This is called trained immunity. It is due to metabolic and epigenetic changes in the cells. After this, the cells may give faster and stronger response in later infection.

Limitations of Non-Specific Immune Response

Lack of specificity– Non-specific immune response does not identify one particular antigen. It recognize only common molecular patterns of microbes. So it cannot produce highly special attack against a definite pathogen.

Can be overwhelmed– The barriers of innate immunity can be crossed by pathogens. It may enter through cuts, wounds, punctures or skin abrasions. When microbes are very large in number, then mucus, cilia and chemical barriers may not stop them properly.
Tissue damage– Some defence reactions of innate immunity are very strong. Neutrophils release Reactive Oxygen Species (ROS) and toxic proteases for killing microbes. These substances may also damage nearby healthy tissue.
Chronic inflammation– If innate immune response remains active for long time, then it becomes harmful. Continuous inflammation can damage tissues and organs. It may also take part in diseases like atherosclerosis and Alzheimer’s disease.

Uncontrolled reaction– Some innate immune reactions may become uncontrolled. Complement system and movement of leukocytes into tissue may become excessive. In severe condition like sepsis, immune cells and proteins may start damaging the host tissue.
Pathogen evasion– Many pathogens have special methods to escape innate immunity. Some microbes survive the attack of ROS by antioxidant systems. Some pathogens also avoid activation of complement pathway and continue their growth.
No strong memory– It does not form true immunological memory like specific immune response. The response is almost same when the same pathogen enter again. So it cannot give long lasting specific protection.

Deficiency causes severe infection– If any important component of innate immunity is absent or defective, then body becomes highly weak against infection. Complement protein deficiency and NADPH oxidase defect in Chronic Granulomatous Disease are examples. In these conditions recurrent bacterial and fungal infections may occur.

Clinical Significance of Non-Specific Immune Response

Chronic granulomatous disease– Chronic Granulomatous Disease (CGD) is an inherited disorder of phagocytic cells. It occurs due to defect in NADPH oxidase (NOX2) enzyme. Due to this defect, phagocytes cannot produce Reactive Oxygen Species (ROS) properly for killing pathogens.
Recurrent bacterial and fungal infection– In CGD, the patient suffer from repeated and life threatening infections. Aspergillus and Staphylococcus aureus are common organisms involved in this condition. Granuloma formation also occur excessively in tissues.

Complement deficiency– Complement system deficiency makes the body highly susceptible to infection. Deficiency of C3 protein causes severe recurrent bacterial infection. Defect in terminal complement proteins C5-C9 causes high susceptibility to Neisseria infection, which may cause meningitis and gonorrhea.
Leukocyte adhesion deficiency– Leukocyte Adhesion Deficiency (LAD) occurs due to defect in integrin molecules. In this condition white blood cells cannot attach properly to the wall of blood vessel. So they cannot move into infected tissue properly and recurrent bacterial infections and poor wound healing occur.
Mannose-binding lectin deficiency– Mannose-Binding Lectin (MBL) deficiency is a common immunodeficiency. In this condition pathogens are not properly opsonized and complement activation is also reduced. It increases the chance of systemic infection, mainly in early childhood.

Trained immunity and chronic disease– Sometimes innate immune cells show trained type of response. If this response become chronic or excessive, then harmful inflammation occur. It may take part in atherosclerosis and neuroinflammation of Alzheimer’s disease.
Complement overactivation– Overactivation of complement system can damage own cells of body. This may occur when complement regulatory proteins fail to control the reaction. It is associated with diseases like systemic lupus erythematosus (SLE), atypical hemolytic-uremic syndrome, age related macular degeneration and paroxysmal nocturnal hemoglobinuria.
Inflammasome driven diseases– Excess activation of NLRP3 inflammasome causes strong inflammatory reaction. It may cause pyroptosis, which is an inflammatory cell death. In diabetic retinopathy, this inflammation damages blood-retinal barrier and retinal blood vessels, leading to vision loss.

Barrier dysfunction– Defect in skin barrier and antimicrobial peptides can cause inflammatory skin diseases. Atopic dermatitis and psoriasis are related with such barrier problem. In gut, disturbance of Innate Lymphoid Cells (ILCs) is associated with Crohn’s disease and ulcerative colitis.
Sepsis– During severe systemic infection, innate immune response may become uncontrolled. Neutrophils release toxic oxygen radicals and proteases in large amount. These substances may destroy healthy tissues and this condition contribute to life threatening sepsis.
Hereditary angioedema– Hereditary angioedema occurs due to deficiency of C1-inhibitor. It causes uncontrolled formation of bradykinin and complement fragments. Due to this, severe fluid swelling or edema occur in different body parts and it may become fatal when it occurs in trachea.

Cancer surveillance– Natural Killer cells (NK cells) are important in defence against cancer cells. They detect tumour cells which reduce MHC class I molecules on their surface. This is known as missing-self recognition.
CAR-NK cell therapy– NK cells have strong cell killing ability. They also have lower chance of causing graft-versus-host disease than T cells. So these cells are used for making Chimeric Antigen Receptor NK cells (CAR-NK cells) for cancer immunotherapy.
Targeted biological treatment– Knowledge of innate immune signalling is used in treatment of inflammatory diseases. Recombinant cytokines and monoclonal antibodies are used for blocking innate inflammatory pathways. Anakinra, an IL-1 receptor antagonist, is used in autoinflammatory diseases, gout and rheumatoid arthritis.

Disorders Associated with Non-Specific Immune Response

Chronic granulomatous disease– Chronic Granulomatous Disease (CGD) is a genetic disorder of phagocytes. It occurs due to defect in NOX or NADPH oxidase enzyme complex. Due to this, phagocytes cannot form Reactive Oxygen Species (ROS) properly and ingested pathogens are not killed.
Severe bacterial and fungal infection– In CGD, the patient suffer from repeated bacterial and fungal infections. Infection may become severe and life threatening. Tissue granulomas are also formed in this disease.
Leukocyte adhesion deficiency– Leukocyte Adhesion Deficiency (LAD) is a genetic disease caused by defective integrin molecules. The β2 chain of integrin may be defective. Due to this, white blood cells cannot stop properly on blood vessel wall and cannot enter into infected tissue.

Impaired wound healing– In LAD, leukocytes fail to reach the infected area in proper number. So recurrent bacterial infections occur. Wound healing is also delayed and poor.
Complement system deficiency– Complement protein deficiency makes the body weak against infections. Deficiency of C3 or Factor B causes severe bacterial infections. Defect in terminal complement proteins C5-C9 causes more susceptibility to Neisseria infections.
Hereditary angioedema– Hereditary Angioedema occurs due to deficiency or functional defect of C1-inhibitor. This protein normally controls complement cascade and bradykinin formation. When it is defective, uncontrolled fluid swelling occurs in body.

Systemic lupus erythematosus– Systemic Lupus Erythematosus (SLE) is associated with defect in early complement components. C1, C4 and C2 are mainly involved. It causes autoimmune and inflammatory reactions in different tissues.
Glomerulonephritis– Glomerulonephritis is also related with complement defect and immune inflammation. Early complement components like C1, C4 and C2 may be involved. In this condition kidney glomeruli become inflamed.
Paroxysmal nocturnal hemoglobinuria– Paroxysmal Nocturnal Hemoglobinuria (PNH) occurs when protective proteins are absent on red blood cells. Decay Accelerating Factor (DAF) is one such protein. Without these proteins, red blood cells are destroyed by own complement system.

Atypical hemolytic-uremic syndrome– Atypical Hemolytic-Uremic Syndrome is caused by overactivation of complement system. It is mainly due to mutation in complement regulatory genes. Factor H and Factor I are important regulatory proteins involved in this disease.
Inflammatory bowel disease– Inflammatory Bowel Disease (IBD) is associated with defects in innate immune regulation. Mutation or polymorphism in NOX subunit genes may be present. Complement dysregulation also take part in chronic intestinal inflammation.
Diabetic retinopathy– Diabetic Retinopathy occurs due to long term high blood glucose. It activates NLRP3 inflammasome in retinal cells. This causes pyroptosis, damage of blood-retinal barrier and finally loss of vision.

Age-related macular degeneration– Age-Related Macular Degeneration (AMD) is associated with complement abnormality. Mutation in Complement Factor H gene is important. NLRP3 inflammasome activation also take part in this disease.
Sepsis– Sepsis is a severe life threatening condition. In this condition innate immune response become uncontrolled. Neutrophils release toxic oxygen radicals and proteases in large amount and healthy host tissues are damaged.
Neurodegenerative diseases– Some diseases of nervous system are related with uncontrolled innate immune activation. Complement system and NLRP3-mediated pyroptosis may cause inflammation in central nervous system. It is seen in conditions like Alzheimer’s disease and spinal cord injuries.

References

5.04: B. The innate immune system, PAMPs and DAMPs, and inflammation. (n.d.). Biology LibreTexts.
A two-signal model for NLRP3 inflammasome activation. The priming… (n.d.). ResearchGate.
An update on the regulatory mechanisms of NLRP3 inflammasome… (n.d.). PubMed Central (PMC).

Aristizábal, B., & González, Á. (2013). Innate immune system. In J. M. Anaya, Y. Shoenfeld, A. Rojas-Villarraga, et al. (Eds.), Autoimmunity: From bench to bedside. El Rosario University Press.
Brief review: Innate immune memory and the host response to… (n.d.). PubMed Central (PMC).
Complement system. (2026, May 21). In Wikipedia.
DAMP-driven trained immunity: Metabolic and epigenetic reprogramming in critical illness and chronic inflammation. (n.d.). PubMed Central (PMC).

Delves, P. J. (2026, April). Complement system. In Merck Manual Professional Version. Merck & Co., Inc.
Ditki Medical and Biological Sciences. (n.d.). Acute inflammation – Advanced.
Evolving our understanding of trained immunity. (n.d.). PubMed Central (PMC).

Gani, Z. (n.d.). Complement system. BiteSized Immunology. British Society for Immunology.
Getting leukocytes to the site of inflammation. (n.d.). PubMed Central (PMC).
Jaiswal, A., & Osborne, T. (2026). Key role of histone modifiers in coupling metabolism to epigenetic reprogramming associated with “trained immunity” in innate immune cells. In P. Dabas, R. T. Maia, C. Oliveira, G. V. Faheina-Martins, & M. De Araújo Campos (Eds.), Gene expression – From code to cure. IntechOpen. https://doi.org/10.5772/intechopen.1014522

Janeway, C. A., Jr., Travers, P., Walport, M., & Shlomchik, M. J. (2001). The complement system and innate immunity. In Immunobiology: The immune system in health and disease (5th ed.). Garland Science.
Khan Academy. (n.d.). Innate immunity.
Leukocyte extravasation. (2025, September 27). In Wikipedia.
Malmquist, S., & Prescott, K. (n.d.). 2.6 Innate immunity. In Human Biology. Pressbooks.
Massive Bio. (2026, April 16). Innate immunity.
Mechanism of NLRP3 inflammasome activation. (n.d.). PubMed Central (PMC).
Mechanisms governing inflammasome activation, assembly and pyroptosis induction. (n.d.). PubMed Central (PMC).
MedicTests. (n.d.). Inflammatory response – Pathophysiology.
Metabolic dysregulation of trained immunity in immune aging and… (n.d.). American Physiological Society Journals.
Moghadam, Z. M., Henneke, P., & Kolter, J. (2021). From flies to men: ROS and the NADPH oxidase in phagocytes. Frontiers in Cell and Developmental Biology, 9, 628991. https://doi.org/10.3389/fcell.2021.628991
Molecular mechanisms of the non-specific immune response: Barrier defense, humoral cascades, cellular effectors, and trained innate memory. (n.d.).
Molecular mechanisms and therapeutic target of NETosis in… (n.d.). PubMed Central (PMC).
Natural killer cell. (2026, June 7). In Wikipedia.
NETosis and NADPH oxidase: At the intersection of host defense, inflammation, and injury. (n.d.). PubMed Central (PMC).
Osinski, V. (n.d.). PAMPs and DAMPs: The differences and similarities between these molecules. Bio-Techne.
Pathogenesis of fever. (n.d.). PubMed Central (PMC).
Pattern recognition receptors and inflammasome: Now and beyond. (n.d.). PubMed Central (PMC).
Primer on the immune system. (n.d.). PubMed Central (PMC).
R&D Systems. (n.d.). NOD-like receptors and the inflammasome.
Role of trained innate immunity activation by metabolic reprogramming in immune response regulation against emerging viral infections. (n.d.). ResearchGate.
The phagocyte respiratory burst: Historical perspectives and recent advances. (n.d.). ResearchGate.
Trained immunity: A program of innate immune memory in health and disease. (n.d.). PubMed Central (PMC).
Trained innate immunity, long-lasting epigenetic modulation, and skewed myelopoiesis by heme. (n.d.). PNAS.
Zaru, R. (n.d.). Pattern recognition receptors (PRRs): Toll-like receptors. BiteSized Immunology. British Society for Immunology.
Zheng, X., Wan, J., & Tan, G. (2023). The mechanisms of NLRP3 inflammasome/pyroptosis activation and their role in diabetic retinopathy. Frontiers in Immunology, 14, 1151185. https://doi.org/10.3389/fimmu.2023.1151185

Non-Specific Immune Response – Definition, Types, Mechanism