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Home >> Immune System, Antibodies, Interferons and Vaccines >>Lymphoid System B Cells ans T Cells
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Lymphoid System B Cells ans T Cells -
The immune system of the body is monitored with the help of white blood cells called lymphocytes, which circulate between blood and the lymph system (lymph is a colourless fluid and is circulated like blood with the help of vessels that make a lymphatic system just like blood circulation system).

The lymphocytes are of two types: B cells and T cells. The B cells synthesize antibodies causing agglutination of unwanted antigens, while T cells carry out cellular immune responses, where cells having unwanted antigens on their surface are attacked without any mediation by antibodies.

The former response is called humoral immunity and the latter the cellular immunity. The T cells interact with antigen through cell surface proteins called T cell receptors, which are specially attached to the cell membrane. B cells, and T cells, both originate in bone marrow, which is the primary source of all lymphocytes.

In mammals, B, cells mature within bone marrow, but T cells migrate out and, mature in special organs like thymus. Once they mature, B cells leave bone marrow and T cells leave the organ like thymus and travel to secondary lymphoid organs like lymph nodes, tonsils and spleen. In these organs they can be activated to produce antibodies.

These cells then pass to blood system and back again to the lymph. T cells can be

(i) cytotoxic T lymphocytes (CTLs), which destroy infected cells of the host;

(ii) helper T cells that help B cells and the cytotoxic T cells to mature and perform their immune response related function

(iii) suppressor T cells, which block Band T cell responses, whenever required.

On the surface of cytotoxic T cells are found specific cell surface proteins called T cell receptors, which are analogous to antibodies in structure, having C (constant) and V (variable) regions with enormous variability for recognition of a large variety of antigens.

The T cell receptors, like antibodies, have been identified to be glycoproteins (mw 80-90 k d), each being a dimer consisting of alpha (α) and beta (β) chains (each 40-50 k d), which are linked with the help of a disulphide bond and other non-covalent bonds.

While B cells can be stimulated by the free antigens in the circulating system of the body, T cells respond to only those antigens which arc bound to cell surface and that too only in the presence of another determinant protein called major histocompatibility complex or MHC.

This helps the T cells in recognizing only the virus infected cells of the host to which the T cells belong, and does not allow the recognition of the cells which are either normal uninfected cells of the same organism or the infected cells of another organism to which the T cells do not belong.

This dual specificity of T cell receptors is dependent on large variety of receptors available just like antibodies. Since they resemble antibodies in structure and variability, their genes though distinct are similar to the Ig genes meant for the synthesis of immunoglobulins or antibodies.

 
 

 
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