On activation, helper T cells express CD40 ligand (CD40L), which engages its receptor, CD40, on antigen-stimulated B cells at the T-B interface and induces subsequent proliferation and differentiation initially in extrafollicular foci and later in germinal centers (Fig. 11-10). CD40 is a member of the TNF receptor superfamily. Its ligand, CD40L (CD154), is a trimeric membrane protein that is homologous to TNF. CD40 is constitutively expressed on B cells, and CD40L is expressed on the surface of helper T cells after activation by antigen and costimulators. When these activated helper T cells interact physically with antigen-presenting B cells, CD40L recognizes CD40 on the B cell surface. CD40L binding to CD40 results in the conformational alteration of preformed CD40 trimers, and this induces the association of cytosolic proteins called TRAFs (TNF receptor-associated factors) with the cytoplasmic domain of CD40. The TRAFs recruited to
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