Page 410 - Veterinary Immunology, 10th Edition

VetBooks.ir Superantigens

Fewer than 1 in 10,000 T cells can bind and respond to any specific
foreign antigen. However, some microbial molecules called

superantigens are unique in that they may stimulate as many as one
in five T cells. These molecules are not simply nonspecific mitogens.
Superantigens only activate T cells with specific TCR Vβ domains.
Unlike conventional antigens that must bind within the grooves of
both an MHC molecule and a TCR, superantigens directly link a

TCR Vβ domain to an MHC class II molecule on the antigen-
presenting cell. All superantigens come from microbial sources
such as streptococci, staphylococci, and mycoplasma, and from

viruses such as rabies. The responses to superantigens are not MHC
restricted (i.e., they do not depend on specific MHC haplotypes),
but the presence of MHC antigens is required for an effective
response since superantigens do not bind to the antigen-binding
groove of the MHC class II molecule but attach elsewhere on its

surface (Fig. 14.12). As a result, they bind the T cell and the antigen-
presenting cell together. Because of this strong binding,
superantigens trigger a powerful T cell response. Some

superantigens may stimulate the secretion of so many cytokines
that they trigger a toxic shock syndrome (Chapter 7).

410