T cellindependent B cell stimulation

Two types of T cell-independent antigens (types I and II) stimulate B cells without involving MHC] class II molecules, i.e. in a non-MHC-restricted fashion, to proliferation and to maturation into antigen-secreting cells. Memory to these antigens is normally not induced; the responses remain mainly IgM response and little, if any, hypermutation of IgV genes is observed. The stimulated B cells remain short lived. Typical type I antigens are those connected to lipopolysaccharide (LPS), while type II antigens are often polysaccharides.

T cell-independent antigens usually consist of repeating determinants with the capacity to crosslink slg, probably without ever being taken up and processed by the B cells. Three restriction points control the cell cycle of a B cell activated by a T cell-independent antigen: 1) antigen binds to slg early in the G( phase; 2) at entry into S phase CR2 receptors are ligated; and 3) at entry into mitosis IL-2 or IL-5 bind to their corresponding receptors. T cell-inde-pendent antigens of type I do not need IL-2 or IL-5 to stimulate B cells at the third restriction point. Other cytokines, such as IL-1, IL-4, IL-6 and interferon y (IFNy) have positive and negative modulating functions for proliferation and/or maturation of B cells.

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