ABSTRACT
The major histocompatibility complex is a large group of closely linked genes that encode cell surface molecules involved in the induction and regulation of both innate and adaptive immune responses. First shown to be the main barrier to tissue transplantation, the major histocompatibility complex was later revealed to play a fundamental role in determining immune tolerance to self and immune responsiveness to non-self or altered self. This chapter describes the major histocompatibility complex's hallmark characteristic of genetic polymorphism and its impact on molecular structure and function. Allele-specific binding and presentation of self and non-self peptides to T cells underlie the seemingly disparate associations between the major histocompatibility complex and transplantation, pathogen-specific immunity, and autoimmunity. As such, the major histocompatibility complex provides an elegant example of the role of genetic polymorphism on individual and population-wide immune responses.
