Antibodies are proteins produced in vertebrates in response to certain substances. Antibodies are an important part of humoral immunity (which is mediated by antibodies). They are part of the gammaglobulin fraction and are also called immunoglobulins (Ig).

Antibodies are produced by the body when B cells come into contact with a corresponding antigen. The B lymphocyte is then activated and differentiates into a plasma cell which secretes large amounts of antibodies. These antibodies can bind specifically to the antigen that causes B lymphocyte differentiation.

Antibodies have a common basic structure: each is composed of two heavy polypeptide chains and two identical light polypeptide chains, linked by disulfide bridges. Heavy and light chains include constant regions and variable regions. The variable regions of the two chains determine the binding site (paratope) of a specific antigenic determinant (epitope). The epitope is the part of the antigen that is recognized by the antibody. In addition, the constant regions of the heavy chains determine the category of immunoglobulin to which the antibody belongs (IgM, IgG, IgD, IgA or IgE). The category of immunoglobulin is also called isotype.

Each antibody molecule consists of two heavy chains (in blue) and two light chains (in yellow), linked by disulfide bonds. These also form what is referred to as the hinge region which connects the two heavy chains.

Heavy and light chains are made up of individual domains. Both light chains and heavy chains include constant domains (CL, CH) and variable domains (VL, VH). The variable domains of the two types of chains determine the specificity of the antigen binding site. The constant domains of the heavy chain determine the category to which the antibody belongs.

Different categories of antibodies are found in different compartments of the body. For example, while IgA is present in saliva, IgG and IgM are present in blood. In addition, membrane bound antibodies are also found (eg, IgE on mast cells or IgD on B cells).

As part of an immune response, antibodies play a variety of roles. They capture and block invading antigens so that they cannot exert their harmful effect, or they prevent the antigen from interacting with body cells (e.g., to prevent entry of viruses and bacteria. bacteria in body cells).
Antibodies can also opsonize pathogens. In this case, the antibody constitutes a marker, eg, phagocytes. Phagocytes interact with constant regions of the antibody in contact and are induced to take up and digest the pathogen (eg, bacteria).

Antibodies can also bind to body cells, with NK cells (natural killer cells) then selectively destroying the cell. This can be useful when a cell is infected with a pathogen (eg a virus). Another important role is the activation of various types of cells of the immune system and the complement system.

Antibodies are used in medicine for diagnostic and therapeutic purposes (eg, serological testing, diagnosis and monitoring of infectious diseases, passive immunization, etc.). They can also be used in various ways (ELISA, FACS, Western blot, immunohistochemistry, etc.) for research purposes. To do this, polyclonal antibodies, monoclonal antibodies or recombinant antibodies are used.

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