LYMPHOCYTES
Lymphocytes are a group of leucocytes. They are contained in blood, lymph and specialized lymphoid tissues. General number of lymphocytes in a human organism is 2·1012. Two classes of lymphocytes are distinguished. They are T-cells, which are developed in the thymus and responsible for cell immunity, and B-cells, which are developed independently of the thymus and produce antibodies.
Lymphocytes are formed in the central lymphoid tissue, then they migrate into peripherical lymphoid tissues, where they become T-lymphocytes or B-lymphocytes. In peripherical lymphoid tissues T-cells and B-cells react with alien molecules.
Every lymphocyte in the course of its development attains the ability to react with a particular antigen. Proteins-receptors, which specifically correspond to the given antigen, appear on the cell surface. Bonding the antigen with these receptors activates the cell, causes its multiplication and maturation of its decsendants. Thus, the alien antigen selectively stimulates only the clones of T- and B-cells carrying specific receptors (antigen-specific receptors), which are complementary to it.