免疫15humoralimmunity.ppt

The interactions of helper T cells and В cells in lymphoid tissues. CD4* helper T cells recognize processed protein antigens displayed by professional APCs and are activated to proliferate and differentiate into effector cells. These effector T cells begin to migrate toward lymphoid follicles. Naive В lymphocytes, which reside in the follicles, recognize antigens in this site and are activated to migrate out of the follicles. The two cell populations come together at the edges of the follicles and interact. 需学习的内容： 过程、生发中心、Th细胞的必要性、亲和力成熟的实质（抗体对抗原的亲和力提高）和意义。 Selection of high- affinity В cells In germinal centers. Some of the В cells that are activated by antigen, with help from T cells, migrate into follicles to form germinal centers, where they undergo rapid proliferation and accumulate mutations in their Ig V genes. The mutations generate В cells with different affinities for the antigen. Follicular dendritic cells (FDCs) display the antigen, and only В cells that recognize the antigen are selected to survive. FDCs display antigens by binding immune complexes to Fc receptors or by binding immune complexes with attached C3b and C3d complement proteins to C3 receptors (not shown). As more antibody is produced, the amount of available antigen decreases, so the В cells that are selected have to express receptors with higher affinities to bind the antigen. FDCs express CD40L (not shown), and germinal centers contain a few T cells that also express CD40L . CD40L may be the molecule that delivers survival signals to the В cells that recognize antigen on the FDCs. Affinity maturation is the process by which the affinity of antibodies produced in response to a protein antigen increases with prolonged or repeated exposure to that antigen. This increase in affinity is due to point mutations in the V regions, and particularly in the antigen-binding hypervariable regions, of the antibodies produced. Because of affinity maturation, the ability of antibodies to bind to a mi