Explain the role of isotype exclusion in pro-B cells.
Introduction:- In a given lymphocyte, either or light (L) chain, but not both, can combine with a heavy (H) chain to form a complete Ig molecule; this is called L chain isotypic exclusion.
Light chains show isotypic exclusion, since an individual cell or molecule has only k or l chains. k and l are not represented equally on B cells or serum Igs. In humans k is favored over l by 65% to 35%. In mice, serum Ig is 95% k , while in cats it is 95% l. The ratio of k to l reflects the relative numbers of V region segments in each isotype and the relative efficiency of their recombination into functional L chain genes.
Once the B cell leaves the marrow, its survival appears to depend on further signals thought to be delivered in the lymphoid follicles of secondary lymphoid tissue. Competition between newly created B cells and older B cells for these signals probably maintains B cell homeostasis. For example, survival of injected transgenic B cells (whose unique receptor can be identified by flow cytometry) has been shown to depend on depletion of the host's normal B cells by irradiation.
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