Basophil – Definition, Mechanism, Structure, Functions

Basophil is a type of white blood cell (WBC). It is present in blood in very small amount. It is the rarest type of leukocyte and form less than 1% of total circulating white blood cells.

Basophil is formed in the bone marrow. It is a granulocyte, because its cytoplasm contains granules. These granules are dark stained and filled with chemical substances.

The granules of basophil contain histamine and heparin. Histamine is used in inflammatory reaction. It causes swelling, redness and increase blood flow. Heparin is anticoagulant substance, which prevents quick clotting of blood.

Basophils are involved in immune defence of body. They help mainly against parasitic infection, such as worms and ticks. They also act during foreign body entry in blood.

During allergic reaction, basophil release histamine from its granules. This produces itching, inflammation, swelling and running nose. So, basophil is important cell for allergy and inflammatory response.

Characteristics of Basophils

The following are the characteristics of Basophils–

• Basophils are the rarest type of white blood cells. It constitute less than 1% of total circulating leukocytes in blood.
• Basophils are formed from hematopoietic stem cells in the bone marrow. The life span of basophil in circulation is short and it is about 60 hours.
• The size of basophil is about 5-16 μm in diameter. It is usually rounded cell having granules in cytoplasm.
• The nucleus of basophil is S-shaped or bilobed. It is mostly covered by the large granules, so the nucleus is not clearly visible.
• Basophils are included under granulocytes. Their cytoplasm contain large coarse granules.
• The granules take basic stain such as hematoxylin. So in microscope these granules appear deep blue or purple in colour.
• The granules of basophils contain histamine, heparin and leukotrienes. These are the important chemical substances of basophil.
• Histamine is concerned with inflammatory reaction. It causes swelling, itching and increase of blood flow.
• Heparin is an anticoagulant substance. It prevents rapid clotting of blood.
• The surface of basophil contain high affinity IgE receptor (FcεRI). These receptors bind with IgE antibody.
• Basophils take part in allergic reaction. When allergen combines with IgE, basophil release its granules.
• This process is called degranulation. Due to this histamine is released and symptoms like itching, swelling, inflammation and asthma are produced.
• Basophils also help in defence against parasites. They mainly act against large extracellular parasites such as ticks and helminths.
• Basophils secrete Interleukin-4 (IL-4) and Interleukin-13 (IL-13). These cytokines are important for Th2 type immune response and recruitment of other immune cells.

Structure of Basophils

The following are the structure of Basophils–

• Basophil is a small rounded granulocyte. Its size is about 10-15 μm in suspension, but in compressed blood smear it may appear about 5-10 μm.
• The cell is covered by a plasma membrane. Just below the membrane, there is a dense peripheral cytoplasmic layer of about 250 nm width.
• This sub-membranous layer contain Galectin-10 protein. It is present as a dense layer below the outer cell membrane.
• The nucleus of basophil is present near the centre of the cell. It is generally S-shaped or bilobed in form.
• The nucleus is not clearly visible in many basophils. This is due to presence of large dark staining granules over the nucleus.
• The cytoplasm of basophil contains many large secretory granules. These granules are membrane bound and about 0.5 μm in diameter.
• The granules are packed closely in the cytoplasm. Due to this arrangement, they almost hide the nucleus.
• The granules contain negatively charged mucopolysaccharides. So they take basic stain very strongly.
• In microscope, basophil granules appear deep purple, blue or blue-black colour with stains like hematoxylin or Wright-Giemsa stain.
• The internal matrix of the granule is acidic and rich in proteoglycans. It contain heparin, chondroitin sulphate and dermatan sulphate.
• This granule matrix acts as an electrostatic scaffold. It binds and holds histamine and other inflammatory substances in stable condition.
• The cytoplasm also contains abundant glycogen aggregates. But it lack highly arranged crystalline lattices which are seen in similar cells like mast cells.

Development of Basophils

The following are the step by step process of development of Basophils–

1. Origin from hematopoietic stem cells
   The development of basophil starts in the adult bone marrow. In this region, hematopoietic stem cells (HSCs) are present. These cells are self-renewing and multipotent cells, from which different blood cells are formed.
2. Formation of common myeloid progenitor
   In this step, HSCs become committed toward myeloid line. They differentiate into common myeloid progenitors (CMPs). These cells give rise to different myeloid cells.
3. Formation of granulocyte-monocyte progenitor
   The common myeloid progenitors (CMPs) further develop into granulocyte-monocyte progenitors (GMPs). This is an important stage for granulocytic cell development.
4. Formation of bipotent progenitor
   From GMPs, the progenitor cell may form common progenitor for basophil and mast cell. This cell is called basophil and mast cell progenitor (BMCP). In this step, C/EBPα is temporarily suppressed and GATA-2 helps the cell to enter this pathway.
5. Basophil lineage specification
   For formation of only basophil, the cell becomes basophil progenitor (BaP). In this stage, C/EBPα again increase with PLZF and P1-RUNX1. These factors help the cell to become committed to basophil line.
6. Suppression of mast cell pathway
   During this process, MITF and IKAROS are suppressed. These factors normally help in mast cell development. So their suppression prevent the progenitor cell from going to mast cell line.
7. Regulation by other transcription factors
   Other transcription factors like GATA-1 and PU.1 also take part in this development. They regulate gene expression needed for continued formation and maturation of basophils.
8. Cytokine driven growth
   The growth and differentiation of basophil precursor is controlled by cytokines. Interleukin-3 (IL-3) is the main cytokine for basophil development. Thymic Stromal Lymphopoietin (TSLP) also helps in expansion and differentiation of these precursor cells.
9. Terminal maturation in bone marrow
   The immature basophil precursor undergo complete maturation inside the bone marrow. It is different from mast cells, because mast cells complete their maturation in tissues. The maturation of basophil generally takes about seven days.
10. Release into blood circulation
    After maturation, mature basophils are released from bone marrow into peripheral blood. In blood, they circulate for short time. Their circulating life span is about 60 hours.

Mechanism of Basophils against Pathogens

The following are the step by step mechanism of Basophils against pathogens-

Step 1- Entry of pathogen
When large pathogen like helminths or ticks enter the body, their antigens are present in tissue. This usually occur in skin, lungs or gut.

Step 2- Recruitment of basophils
Basophils are recruited from blood to the infected tissue. They accumulate at the affected site. This is the first cellular response of basophil against such pathogens.

Step 3- Detection of antigen
The surface of basophil contain high affinity IgE receptor (FcεRI). IgE antibodies are already attached with these receptors. The antigen of pathogen bind with IgE.

Step 4- Cross linking of IgE
When antigen bind with more than one IgE antibody, cross linking occur. This activates the basophil. The cell then becomes ready for release of granule contents.

Step 5- Degranulation
Activated basophil undergo degranulation. During this process, granules release histamine, leukotrienes (LTC4) and other inflammatory mediators into the tissue.

Step 6- Formation of local inflammation
Histamine causes increase of blood flow and swelling of tissue. It also help in movement of fluid into infected site. This makes the area inflamed.

Step 7- Weep and sweep response
The inflammatory mediators produce weep and sweep response. In this response, fluid secretion, tissue swelling, smooth muscle contraction and epithelial shedding occur. These reactions help to wash out and expel parasites and their eggs.

Step 8- Cytokine secretion
After activation, basophils also secrete cytokines. The main cytokines are Interleukin-4 (IL-4) and Interleukin-13 (IL-13).

Step 9- Th2 immune response
IL-4 and IL-13 start Th2 type immune response. This response is important against large extracellular parasites. It also support allergic type inflammation.

Step 10- Recruitment of other immune cells
These cytokines recruit other immune cells such as eosinophils. They also help in formation of M2 macrophages.

Step 11- Removal of pathogen
Eosinophils, M2 macrophages and other cells act together. They trap the larvae, restrict their migration and help in killing or expulsion of remaining pathogen. In this way basophils control the parasitic infection.

Normal Level of Basophils

The following are the normal level of Basophils–

• Basophils are present in very low number in normal blood. It usually form less than 1% of total circulating white blood cells.
• The normal percentage of basophils is generally 0-2% of total leukocytes. In most condition it remain about 0.5-1%.
• The absolute count of basophils is about 0-200 cells/µL of blood. This is the usual normal range in many laboratory report.
• Some laboratory also write the count as 0.01-0.10 thousand cells/µL. It means the same basophil count in different reporting form.
• In adult, the normal absolute count may be written as 0.01 × 10⁹/L to 0.08 × 10⁹/L.
• So, basophil count is checked in two main way. One is percentage of total WBC and another is absolute basophil count in blood.

Diseases Related to High and Low Basophil Counts

The following are the diseases related to high and low basophil counts-

A. High Basophil Count (Basophilia)

• Myeloproliferative disorders – Basophilia is commonly seen in bone marrow disorders like chronic myeloid leukemia (CML), polycythemia vera, essential thrombocythemia, primary myelofibrosis and basophilic leukemia, where abnormal production of blood cells occur.
• Allergic diseases – High basophil count may occur in asthma, eczema, hay fever, food allergy and severe hives, because basophils take part in allergic reaction by releasing histamine.
• Autoimmune diseases – Rheumatoid arthritis, lupus, diabetes and other autoimmune conditions may increase basophils, due to long lasting immune reaction and inflammation in the body.
• Inflammatory bowel disease – Crohn’s disease and ulcerative colitis may show increased basophil count, because these are chronic inflammatory disease of intestine.
• Infections – Tuberculosis, influenza, chickenpox, fungal infections and some bacterial infections may cause high basophils, when body is reacting against invading organisms.
• Parasitic infection – Intestinal worms and other helminths may increase basophil count, because basophils are involved in defence against large extracellular parasites.
• Hypothyroidism – In hypothyroidism or myxoedema, basophil count may become high. It is related with underactive thyroid gland.
• Other causes – Iron deficiency, solid tumour and removal of spleen may also increase basophils in some cases.

B. Low Basophil Count (Basopenia)

• Acute infection – Basopenia may occur during acute infection, because basophils move from blood to infected tissue and blood count become low.
• Anaphylaxis – In severe allergic reaction or anaphylaxis, basophils may become low in blood, because they move into tissue and release histamine rapidly.
• Hyperthyroidism – Low basophil count may occur in hyperthyroidism or thyrotoxicosis, where thyroid gland become overactive.
• Stress condition – Emotional stress, physical stress, heart attack and pregnancy may reduce basophils, due to increased cortisol effect in body.
• Corticosteroid therapy – Long term use of corticosteroid drugs like prednisone may lower basophil count, because these drugs suppress immune cell activity.
• Cancer treatment – Chemotherapy and radiation therapy may cause low basophils, as these treatments affect the bone marrow and blood cell production.
• Cushing syndrome – In Cushing syndrome, excess cortisol is present in body and this may decrease basophil count.
• Other causes – Major hemorrhage and ovulation may also be associated with temporary low basophil count.

Causes of Low and High Count of Basophil

The following are the causes of low and high count of basophil–

A. Causes of High Basophil Count (Basophilia)

• Blood cancers – High basophil count is seen in blood cancers and myeloproliferative disorders. The important examples are chronic myeloid leukemia (CML), polycythemia vera, primary myelofibrosis and essential thrombocythemia.
• Allergic reaction – Basophilia may occur in allergic conditions like asthma, hay fever, eczema, food allergy and chronic hives. In these conditions basophils are increased due to allergic immune reaction.
• Autoimmune disease – Rheumatoid arthritis, lupus and diabetes may cause high basophil count. This is due to long continued immune reaction in body.
• Chronic inflammation – Inflammatory bowel diseases like Crohn’s disease, celiac disease and ulcerative colitis may increase basophils. The inflammation remain for long time in these disease.
• Infections – Some infections may cause increase of basophil count. It include parasitic infection, viral infection like influenza, chickenpox, Epstein-Barr virus and bacterial infection like tuberculosis.
• Hypothyroidism – In hypothyroidism, thyroid gland is underactive. This condition may cause increase in circulating basophils.
• Spleen removal – After removal of spleen, the basophil count may become high. This is called post-splenectomy condition.
• Other causes – Severe iron deficiency, solid tumour and exposure to some drugs or toxins may also cause high basophil count.

B. Causes of Low Basophil Count (Basopenia)

• Acute infection – Low basophil count may occur in acute and severe infection. In this condition basophils move from blood to the affected tissue.
• Anaphylaxis – Severe allergic reaction like anaphylaxis may cause low basophils in blood. The cells move into tissue and release histamine.
• Stress – Emotional stress and physical stress may decrease basophil count. It may occur during heart attack, pregnancy and other severe stress condition due to cortisol effect.
• Corticosteroid drugs – Long use of corticosteroids like prednisone may cause low basophil count. These drugs suppress immune cell activity and blood cell response.
• Cancer treatment – Chemotherapy and radiation therapy may reduce basophils. It occur because these treatments affect the bone marrow.
• Hyperthyroidism – In hyperthyroidism, thyroid gland is overactive. It may change immune cell distribution and cause low basophil count.
• Cushing syndrome – Cushing syndrome is related with excess cortisol. Due to high cortisol level, basophil count may decrease.
• Other causes – Major hemorrhage, ovulation and some inherited genetic features may also be responsible for basopenia.

Methods Used for Basophil Count

The following are the methods used for basophil count–

• Complete Blood Count (CBC) with Differential – CBC with differential is the common routine blood test used for counting basophils. In this test, the percentage of basophils and also the absolute number of basophils are measured with other white blood cells.
• Automated Hematology Analyzer – This method is used in most laboratory. The blood sample is passed through automated machine, and the machine count large number of blood cells. Basophils are detected by their electrical property and light scattering property.
• Radiofrequency and Light Method – In this method, the automated analyzer uses high electromagnetic radiofrequency (RF) and light signals. The different cells give different signals, and from this basophils are separated and counted.
• Automated Cytochemistry – In this method, a chemical dye is added to the blood sample. The dye react with the cell component, and the machine detect the colour change by light sensor. Then basophil number is counted.
• Manual Blood Smear Method – This method is used when machine result is abnormal or doubtful. A thin blood smear is prepared on glass slide and stained with Wright-Giemsa stain or hematoxylin. The granules of basophils stain deep blue or purple, so they can be identified under microscope.
• Flow Cytometry – Flow cytometry is an advanced method used for special counting of basophils. It uses laser light and fluorescent markers to identify and count basophils. It is useful in very low basophil count and in Basophil Activation Test (BAT).

Functions of Basophils

The following are the functions of Basophils–

• Allergic reaction – Basophils are concerned with allergic reaction. When allergen combines with IgE antibody on the surface of basophil, the granules are released. The released substances are histamine, leukotrienes and other inflammatory mediators.
• Inflammation – Basophils help in producing inflammatory reaction. Histamine causes dilation of blood vessels and more blood flow come to the affected area. Due to this swelling, redness and itching are produced.
• Defence against parasites – Basophils act against large extracellular parasites like ticks and helminths. They help in weep and sweep response, where tissue swelling, smooth muscle contraction and epithelial shedding occur. This helps in removal of parasites from the body.
• Immune regulation – Basophils produce cytokines such as Interleukin-4 (IL-4) and Interleukin-13 (IL-13). These are needed for Th2 type immune response. It also help in calling other immune cells at the affected site.
• Blood clot prevention – The granules of basophils contain heparin. Heparin is an anticoagulant substance. It prevents too fast clotting of blood and helps in normal flow of blood.
• Venom detoxification – Basophils contain enzymes like tryptase and Carboxypeptidase A3 (CPA3). These enzymes are used to neutralize venom of snake, scorpion and bee.
• Tissue repair – Basophils also help in repair of injured tissue. They release IL-4 and IL-13 at the damaged site. This helps in wound healing, removal of cell debris and formation of scar tissue.

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