T and B Cell.swf

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Lymphocytes are one of the five kinds of white blood cells or leukocytes), circulating in the blood. [More]

Although mature lymphocytes all look pretty much alike, they are extraordinarily diverse in their functions. The most abundant lymphocytes are:

•     B lymphocytes (often simply called B cells) and
•     T lymphocytes (likewise called T cells).

Each B cell and T cell is specific for a particular antigen. What this means is that each is able to bind to a particular molecular structure.

The specificity of binding resides in a receptor for antigen:

•     the B cell receptor (BCR) for antigen and
•     the T cell receptor (TCR) respectively.

Both BCRs and TCRs share these properties:

•     They are integral membrane proteins.
•     They are present in thousands of identical copies exposed at the cell surface.
•     They are made before the cell ever encounters an antigen.
•     They are encoded by genes assembled by the recombination of segments of DNA.
•     How antigen receptor diversity is generated.
•     They have a unique binding site.
•     This site binds to a portion of the antigen called an antigenic determinant or epitope.
•     The binding, like that between an enzyme and its substrate depends on complementarity of the surface of the receptor and the surface of the epitope.
•     The binding occurs by non-covalent forces (again, like an enzyme binding to its substrate).
•     Successful binding of the antigen receptor to the epitope, if accompanied by additional signals, results in:
•         stimulation of the cell to leave G0 and enter the cell cycle.
•         Repeated mitosis leads to the development of a clone of cells bearing the same antigen receptor; that is, a clone of cells of the identical specificity.

BCRs and TCRs differ in:

•     their structure;
•     the genes that encode them;
•     the type of epitope to which they bind.