The most important TI antigens are polysaccharides, gly-colipids, and nucleic acids, all of which induce specific antibody production in T cell-deficient animals. These antigens cannot be processed and presented in association with MHC molecules, and therefore they cannot be recognized by CD4+ helper T cells. Most TI antigens are multivalent, being composed of repeated identical anti-genic epitopes. Such multivalent antigens may induce maximal cross-linking of the BCR complex on specific B cells, leading to activation without a requirement for cognate T cell help. In addition, many polysaccharides activate the complement system by the alternative pathway, generating C3d, which binds to the antigen and is recognized by CR2, thus augmenting B cell activation (see Fig. 11-5 and Chapter 7). Membrane proteins at a high density on a microbial surface may be functionally multivalent and may function in a T-independent as well as in a T-dependent manner. As mentioned earlier, TI responses may also be facilitated by additional signals derived from microbial products that activate TLRs on B cells.
TABLE 11-2 Properties of Thymus-Dependent and Thymus-Independent Antigens
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