Читать книгу Immunology - Richard Coico - Страница 112

In contrast to B cells, in order for antigens to activate T cells to stimulate immune responses, interactions with MHC molecules expressed on antigen‐presenting cells (APCs) must occur (see Chapter 9). APCs must first degrade the antigen through a process known as antigen processing (enzymatic degradation of antigen) before they can express antigenic epitopes on their surface. Epitopes are also known as antigenic determinants. They are the part of an antigen that is recognized by the immune system and are the smallest unit of antigen that is capable of binding with antibodies and T‐cell receptors. Once degraded and noncovalently bound to MHC, these epitopes stimulate the activation and clonal expansion of antigen‐specific effector T cells.

A protein antigen’s susceptibility to enzymatic degradation largely depends on two properties: (1) it has to be sufficiently stable so that it can reach the site of interaction with B cells or T cells necessary for the immune response, and (2) the substance must be susceptible to partial enzymatic degradation that takes place during antigen processing by APCs. Peptides composed of D‐amino acids, which are resistant to enzymatic degradation, are not immunogenic, whereas their L‐isomers are susceptible to enzymes and are immunogenic. By contrast, carbohydrates are not processed or presented and are thus unable to activate T cells, although they can directly activate B cells.

In general, a substance must have all four of these characteristics to be immunogenic; it must be foreign to the individual, have a relatively high molecular weight, possess a certain degree of chemical complexity, and be degradable.