Epitope An epitope

An epitope, also known as antigenic determinant, is the part of a macromolecule that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that recognizes the epitope is called a paratope. Although epitopes are usually thought to be derived from nonself proteins, sequences derived from the host that can be recognized are also classified as epitopes.

Most epitopes recognized by antibodies or B cells can be thought of as three-dimensional surface features of an antigen molecule; these features fit precisely and thus bind to antibodies.

T cell epitopes are presented on the surface of an antigen-presenting cell, where they are bound to MHC molecules. T cell epitopes presented by MHC class I molecules are typically peptides between 8 and 11 amino acids in length, whereas MHC class II molecules present longer peptides, and non-classical MHC molecules also present non-peptidic epitopes such as glycolipids.

Epitopes are sometimes cross-reactive. This property is exploited by the immune system in regulation by anti-idiotypic antibodies. If an antibody binds to an antigen's epitope, the paratope could become the epitope for another antibody that will then bind to it. If this second antibody is of IgM class, its binding can upregulate the immune response; if the second antibody is of IgG class, its binding can downregulate the immune response.

Paratope

The paratope is the part of an antibody which recognises an antigen, the antigen-binding site of an antibody. It is a small region (of 15–22 amino acids) of the antibody's Fv region and contains parts of the antibody's heavy and light chains.

The part of the antigen to which the paratope binds is called an epitope. This can be mimicked by a mimotope.

1. Fab region

A mimotope is a macromolecule, often a peptide, which mimics the structure of an epitope. Because of this property it causes an antibody response identical to the one elicited by the epitope. An antibody for a given epitope antigen will recognize a mimotope which mimics that epitope. Vaccines utilizing mimotopes are being developed

Epitope mapping is the process of identification and characterization of the minimum molecular structures that are able to be recognized by the Immune System elements, mainly T and B cells. A collection of in vivo and in vitro methodologies are used for epitope mapping. Among the most used are binding assay, ELISPOT, HLA transgenic mice and prediction software. Epitope mapping of proteins is used for development of new vaccines and diagnostic approaches.

A conformational epitope is a sequence of subunits (usually, amino acids) composing an antigen that come in direct contact with a receptor of the immune system.

An antigen is any substance that the immune system can recognize as foreign. Since, antigens are usually proteins that are too large to bind as a whole to any receptor, only specific segments that form the antigen bind with a specific receptor. Such segments are called epitopes. Likewise, it is only paratope of the receptor that comes in contact with the epitope.

Polyclonal B cell response

Polyclonal response by B cells against linear epitopes

Examples of substances recognized as foreign (non-self)

Polyclonal B cell response is a natural mode of immune response exhibited by the adaptive immune system of mammals. It ensures that a single antigen is recognized and attacked through its overlapping parts, called epitopes, by multiple clones of B cell.

In the course of normal immune response, parts of pathogens (e.g. bacteria) are recognized by the immune system as foreign (non-self), and eliminated or effectively neutralized to reduce their potential damage. Such a recognizable substance is called an antigen. The immune system may respond in multiple ways to an antigen; a key feature of this response is the production of antibodies by B cells (or B lymphocytes) involving an arm of the immune system known as humoral immunity. The antibodies are soluble and do not require direct cell-to-cell contact between the pathogen and the B-cell to function.

Antigens can be large and complex substances, and any single antibody can only bind to a small, specific area on the antigen. Consequently, an effective immune response often involves the production of many different antibodies by many different B cells against the same antigen. Hence the term "polyclonal", which derives from the words poly, meaning many, and clones ("Klon"=Greek for sprout or twig); a clone is a group of cells arising from a common "mother" cell. The antibodies thus produced in a polyclonal response are known as polyclonal antibodies. The heterogeneous polyclonal antibodies are distinct from monoclonal antibody molecules, which are identical and react against a single antigen only, i.e., are more specific.

Although the polyclonal response confers advantages on the immune system, in particular, greater probability of reacting against pathogens, it also increases chances of developing certain autoimmune diseases resulting from the reaction of the immune system against native molecules produced within the host.