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In this essay we will discuss about lymphocytes and its types.
Lymphocytes are a kind of granulocytes of leucocytes (WBCs) of blood. These form the main cells involved in immune system. These are formed by liver and thymus in the foetus and by the bone marrow and lymph nodes in adults. These form about 30% of leucocytes of blood.
There are two classes of lymphocytes:
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T-lymphocytes and B-lymphocytes. About 75 per cent of circulating lymphocytes are T-cells. Both types of lymphocytes are formed from the stem cells, called haemocytoblasts, in the bone marrow of adult persons but undergo further processing at different sites (Fig. 8.17).
T-lymphocytes are those lymphocytes which differentiate in the thymus to become immunologically competent in the presence of a local polypeptide hormone called thymosin. B-lymphocytes are those lymphocytes which differentiate in the gut-associated bursal lymphoid tissues especially the bursa of Fabricus, a mass of lymphoid tissue near the cloaca in most of vertebrates but in mammals these differentiate in lymphoid tissues of tonsils, Peyer’s patches and appendix so B- lymphocytes are independent of thymus. The mature lymphocytes then enter general circulation.
Both types of lymphocytes need stimulation by specific antigen to trigger them into action but they respond differently:
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1. Mode of Action of B-Lymphocytes:
There are thousands of antigen-specific B-cells in the body. Once B-Iymphocytes are stimulated by an antigen by contact they synthesize RNA, divide rapidly and differentiate into the RER-rich histologically distinctive plasma eel s (Fig. 8.18). A group of plasma cells is known as a clone.
These plasma cells produce antibodies at a high rate of about 2,000 molecules per second. These antibodies circulate in the lymph to fight the antigen so forming humoral immune system (humor means body fluid). The B-lymphocytes are short-lived and are replaced by new cells from the bone marrow after every few days.
Five classes of immunoglobulins are known, namely IgM, IgG, IgA, IgD and IgE. Immunoglobulin G is most abundant immunoglobulin in man.
2. Mode of Action of T-Cells:
Like B-lymphocytes, 1 -cells are also antigen specific, as each T-cell recognizes a specific antigen and different types of T-cells are stimulated by different types of antigens. When a T-cell IS stimulated by specific antigen, this immunologically competent T-lymphocyte divides rapidly to form a clone of T-cells, called lymphoblasts. In a clone of T-cells (collectively called committed T-cells), though the cells are morphologically similar but they are divided in three classes (collectively called effector cells) on the basis of their functions.
i. Natural Killer T or Cytotoxic-T Cells:
These migrate to the site of infection and release lymphokines which attract the macrophages by chemotaxis), lymphotoxins (which kill the foreign cells by secreting cytolysin (Fig. 8.19) or perforin which damages the cell membrane of microbes) and interferon (which inactivates the viruses). So the phagocytic activity is greatly enhanced at the site of infection. These also kill the cancer cells.
ii. Helper T-Cells:
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These are most numerous of T-cells (about 75% of T-cells). These secrete helper factors which stimulate B-cells to produce more antibodies and enhance the activity of killer T-cells. These also produce a variety of factors, called cytokines, which include lymphokines, interleukins, monokines and interferons. Some of these stimulate the growth and proliferation of B-cells and are called B-cell growth factors. InterIeukin-2 cytotoxin stimulates the growth and proliferation, of cytotoxic T-calls.
iii. Suppressor T-Cells:
These secrete suppressive factors which suppress the whole immune system to protect own body cells from its attack, called immune tolerance. Some of these change into memory or primed cells. These memory cells are stored in spleen and lymph nodes which produce the antibodies spontaneously when the same antigens invade the body. The memory cells also produce more effector cells and memory cells. Some of the memory cells lost for whole life. These T-cells survive for about 4 or 5 years or even more.
