Compare and contrast clonal deletion and clonal selection of B lymphocytes.
What will be an ideal response?
Both clonal deletion and clonal selection are screening processes in which the antigen specificity of the B cell receptors are scanned. These screening processes have very different end results.
Unactivated B lymphocytes produce their B cell receptor (BCR), which is a membrane-bound version of IgM or an IgD, while still in the bone marrow. Cells in the bone marrow present "self" antigen to the B lymphocytes, and those that respond are "deleted" (undergo apoptosis). Thus, clonal deletion prevents autoreactive B lymphocytes from completing their development.
Mature, unactivated B lymphocytes in lymphoid tissue undergo clonal selection. New antigen appearing in the body is bound by BCR specific for it, internalized, and processed for presentation with MHC II. If a Th2 lymphocyte specific for the same antigen detects the presented antigen, the B lymphocyte attached to it is "selected" for activation and clonal expansion (production of many daughter cells).
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