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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.
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1. Fab region
2. Fc region
3. Heavy chain
4. Light chain with one variable (VL) and one constant (CL) domain
5. Fv region with paratope
6. Hinge regions
Mimotope
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
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.
Epitope mapping is the process of matching surface proteins to antibodies that will bond to
them.
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Conformational epitope
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
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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.
Along with this draw the diagram of B and T cell.
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