| Περίληψη: | Lymphocytes play a significant role in the defense of the organism against infections and other noxious agents. These cells derive from stem cells which leave very early the marrow environment and move towards the lymph nodes, the spleen and the thymus in order to differentiate into a multitude of distinct lymphoid cell populations with precisely determined functions each. The latter are recognized through an array of immunologic markers and antigen receptors which are expressed on the cellular membranes. Main categories are the B, T and NK lymphocytes. The B lymphocytes recognize antigens against which they synthesize the respective immunoglobulins, which they then secrete in the plasma following their transformation into plasmacytes. The T lymphocytes also recognize a multitude of antigens and modify accordingly their membrane receptors; in this way, at a subsequent stage, when T lymphocytes meet other cells carrying an identifiable antigen they can readily destroy them through direct toxicity. NK cells act I an almost similar way. Antigen recognition requires antigen processing by the dendritic cells present in the marrow and the lymphoid organs and “presentation” of selected immunogenic peptides on the cellular surface along with other molecules facilitating the recognition. Complement plays a significant role in these processes.
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