B细胞的功能

B细胞的功能 After B cells are selected in the germinal center for those bearing mIg that binds with high affinity to antigen displayed on follicular dendritic cells, some B cells differentiate into plasma cells and others become memory B cells. Differentiation of mature B cells into plasma cells requires a charge in RNA processing so that the secreted form of the heavy chain rather than the membrane form is synthesized. Naive cells express only IgM and IgD; as a consequence of class switching, however, memory B cells express additional isotypes, including IgG, IgA, and IgE The presence of antibody can suppress the response to antigen. Antibody-mediated suppression can be explained in two ways. The first explanation is that the circulating antibody competes with antigen-reactive B cells for antigen. The second explanation is that binding of antigen-antibody complexes by Fc receptors on B cells reduces signaling by the B-cell receptor complex. B- and T-cell activation share many parallels, including compartmentalization of function within receptor subunits, activation by membrane-associated protein tyrosine kinases activity, and recruitment of several signal transduction pathway. The B-cell co-receptor can intensify the activating signal resulting from cross-linkage of mIg; this may be particularly important during the primary response to low concentrations of antigen. The cell membrane-associated molecule CD22 can act as a negative regulator of B-cell activation. Protein tyrosine phosphatases docked to the ITIMs of the cytoplasmic tail of CD22 deactive the BCR-associated signaling complexes by removing phosphates added by activating protein tyrosine kinases. Activation induced by TD antigens requires contact-dependent help delivered by interaction between CD40 on B cells and CD40L on activated T cells.H The properties of the primary and secondary antibody responses differ. The primary responses has a long lag peroid, and IgM is the first antibody class produced, followed by a gradual switch to other classes.The secondary response has a short lag time, and the responses lasts longer. IgG and other isotypes are the main products generated in the secondary response rather than IgM, and the average affinity of antibody produced is higher. Germinal centers, sites of somatic hypermutation of rearrangement immunoglobulin genes, form within a week or so of exposure to a TD antigen. Germinal centers are the sites of affinity maturation, formation of memory B cells, class switching and formation of plasma cells.