IT is generally agreed that B-cell surface immunoglobulin (Ig) functions as the receptor for antigen, and so accounts for the specificity of the immune response by clonal selection. It is not known, however, how the combination of surface Ig with antigen stimulates the cell to proliferation and/or differentiation to secrete antibody at a high rate1. A signal could be delivered directly through the receptors, either by an allosteric transition in the receptor itself or by cross-linkage or aggregation of receptor Ig in the fluid cell membrane, analogous to antigen-induced or anti-IgE-induced histamine release from mast cells2. Alternatively, the receptor may act passively as an antigen-specific 'address' for the delivery of an antigen-associated nonspecific (that is, potentially polyclonal) signal to some other site on the cell surface3.
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