White Blood Cells and Its Function

White blood cells also called leukocytes are cells that are involved with immune mechanism of the body that provide defense against bacteria, virus, fungi, parasites, and cancer cells. They also remove foreign bodies and dead cells and debris from the blood. Leukocytes are found in the blood and lymphatic system throughout the body. The WBCs will destroy and ingest bacteria. It also produces antibodies that attack the antigen and chemicals, and mediate inflammation. Leukocytes are produced from the hematopoietic stem cells in the bone marrow. The lifespan of leukocytes is about 3 to 4 days.

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The size of WBCs is around 7 to 21 micrometer in diameter. During centrifugation of blood, the WBCs form a white layer between the layers of sedimented RBCs and the blood plasma. The normal number of leukocytes in the blood comes around 4,500-10,000 WBCs per micro liter. The increase in number of the leukocytes is called leukocytosis and it indicates the evidence of a disease. It may be due to conditions such as anemia, bone marrow diseases, infectious diseases, inflammatory diseases, leukemia, severe emotional and physical stress, tissue damage etc. Decrease in number of leukocytes is called leucopenia and can occur in bone marrow disorders, infection, tumor, collagen vascular diseases, diseases of the liver or spleen, and radiation therapy or exposure etc.

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There are several different types of leukocytes. They differ in many aspects but resembles in some common characteristics. The most common characteristic of leukocytes is the presence of granules. The presence of differently staining granules in the cytoplasm of the leukocytes help to characterize under different staining and viewed under a microscope. These granules are membrane bound enzymes that act primarily in digestion of endocytosed particles.

​​Leucocytes are classified into
A. Granulocytes

Granulocytes are further divided into

1. Neutrophils

2. Eosinophils

3. Basophils

​B. Aggranulocytes or mononuclear cells

Aggranulocytes are further divided into

1. Lymphocytes
Lymphocytes again further divided into

a. T lymphocytes

b. B lymphocytes

2. Monocytes

3. Macrophages

Neutrophil:

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It comprises 54%-64% of WBCs. It has a diameter of 10-12 microns and they have multilobed nucleus. The shape is multilobed and it helps in emigration during inflammation. Its lifespan is 6 hours to a few days. Neutrophils contain granules. There are 2 types of granules, primary granules or lysosomal granules and secondary granules. Neutrophils attack​ bacteria, engulf the bacteria by phagocytosis, digest them by lysosomal granules. The primary granules contains different chemicals which kill the bacteria such as hydrolase, myeloperoxidase, cationic proteins, lysozyme etc. They also contain defensins, which is a broad spectrum antimicrobial agent. These agents kill the microbes by producing oxidants like H2O2, superoxide, etc. The secondary granules contain lactoferrin, vitamin B12 binding protein and some type of receptor molecules. These secondary granules modify inflammation. The most important function of leukocyte is attacking and destroying the invading bacteria and fungi. It takes part in inflammation and inflammatory process.

Eosinophil:

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​Eosinophils comprises 1%-6% of the WBCs. It has a diameter of 10 to 12 microns and has a bilobed nuclei. The cytoplasmic granules are coarse eosiniphilic, which is orange red or pink in color under Leishman's stain. The lifespan of eosinophil is about 8 to 12 days. Their function is to attack large parasites and act as a modulator for allergic inflammatory responses. The eosinophil count increases when there is parasitic infestation such as filariasis, hook worm, round worm etc., allergic conditions such as asthma, hay fever etc., and idiopathic hypereosinophilic syndrome.

Basophils:

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​Basophils comprises of less than 1% of WBCs. It has a diameter of 12 to 15 microns and has a bilobed or trilobed nucleus. The nucleus is S-shaped. The cytoplasm contains coarse basophilic granules. Nucleus stains blueish in color. The lifespan is a few hours to a few days. The functions of the basophils is release of histamine for inflammatory responses.

Lymphocyte:

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​The lymphocyte comprises about 28% to 33% of the WBCs. It is 7 to 8 microns in diameter. The nucleus is deeply staining and eccentric in nature. The granules are NK cells and cytotoxic T cells. The lifespan of lymphocytes are many weeks to many years. There are 2 types of lymphocytes, T lymphocytes and B lymphocytes. About 80% of the blood lymphocytes are T lymphocytes and 15% are B lymphocytes and the rest 5% are null cells, which is neither T cells nor B cells. There are also NK cells or natural killer cells. T cells are further grouped into CD4 and CD8. The function of B lymphocyte includes release of antibodies and assists in activation of T cells. The function of T lymphocyte involves CD4 and T helper cells activating and regulating T and B cells. CD8 and cytotoxic T cells act against virus infected and tumor cells. Regulatory T cells returns the functioning of immune system to normal after an infection and it prevents autoimmunity. T cell lymphocytes chiefly destroy viruses and some types of bacteria by different methods. B cell lymphocytes become plasma cells and it produces chemicals called immunoglobulins or antibodies and neutralizes the antigens, toxins, and foreign bodies.

Monocyte:

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Monocyte comprises 2% to 10% of the WBCs. It is 7.72-9.99 microns in diameter. The nucleus is kidney shaped. It does not stain on any of the stains. The lifetime of monocyte is many hours to many days. The monocytes leave the blood and enter the tissues where they are known as tissue macrophages. Monocytes of the blood and tissue macrophages are together known as reticuloendothelial cells. The function of monocyte is that they migrate from the blood stream to other tissues and differentiate into tissue resident macrophages such as the kupfer cells in the liver. ​They phagocytose and scavenge any dead cells, dead RBCs, bacteria, foreign bodies etc. They secrete chemical substances that mediate inflammation in conditions such as bacterial infection, septicemia, etc. They secrete chemical substances like interleukin 1 (IL-1), TNFalfa, transferrin, lysozyme, proteases, acid hydrolase etc.​

​Macrophage:

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Macrophages are present in blood. The shape of a macrophage varies. It is 21 microns in size. It is a monocyte derivative. The lifespan of macrophage lasts for many days. The immunity ​lasts from months to years. Functions include ​phagocytosis which includes eating and digestion of substances and cell remains, ​and stimulation of ​​lymphocytes and other immune cells which respond to pathogenic bacteria and foreign bodies.​

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