The variable domains can also be referred to as the F V region. This variant terminology fell out of use due to the correspondence being inexact and due to confusion with γ (gamma) heavy chains which characterize the IgG class of antibodies. In an electrophoresis test of blood proteins, antibodies mostly migrate to the last, gamma globulin fraction.Ĭonversely, most gamma-globulins are antibodies, which is why the two terms were historically used as synonyms, as were the symbols Ig and γ. In between them is a hinge region of the heavy chains, whose flexibility allows antibodies to bind to pairs of epitopes at various distances, to form complexes ( dimers, trimers, etc.), and to bind effector molecules more easily. Structurally an antibody is also partitioned into two antigen-binding fragments (Fab), containing one V L, V H, C L, and C H1 domain each, as well as the crystallisable fragment (Fc), forming the trunk of the Y shape. Light chains consist of one variable domain V L and one constant domain C L, while heavy chains contain one variable domain V H and three to four constant domains C H1, C H2. These domains are usually represented in simplified schematics as rectangles.
#Fragment synonym biology series
Įach chain is a series of domains: somewhat similar sequences of about 110 amino acids each. In humans and most mammals, an antibody unit consists of four polypeptide chains two identical heavy chains and two identical light chains connected by disulfide bonds. Glycans in the Fc region are shown in black.Īntibodies are heavy (~150 k Da) proteins of about 10 nm in size, Īrranged in three globular regions that roughly form a Y shape.
Structure Ī more accurate depiction of an antibody (3D structure at RCSB PDB). The terms antibody and immunoglobulin are often used interchangeably, though the term 'antibody' is sometimes reserved for the secreted, soluble form, i.e. The soluble Y-shaped units can occur individually as monomers, or in complexes of two to five units.Īntibodies are glycoproteins belonging to the immunoglobulin superfamily. Soluble antibodies are released into the blood and tissue fluids, as well as many secretions.īecause these fluids were traditionally known as humors, antibody-mediated immunity is sometimes known as, or considered a part of, humoral immunity. Initially, all antibodies are of the first form, attached to the surface of a B cell – these are then referred to as B-cell receptors (BCR).Īfter an antigen binds to a BCR, the B cell activates to proliferate and differentiate into either plasma cells, which secrete soluble antibodies with the same paratope, or memory B cells, which survive in the body to enable long-lasting immunity to the antigen. They occur in two forms: one that is attached to a B cell, and the other, a soluble form, that is unattached and found in extracellular fluids such as blood plasma. Together with B and T cells, antibodies comprise the most important part of the adaptive immune system. The class hence determines the function triggered by an antibody after binding to an antigen, in addition to some structural features.Īntibodies from different classes also differ in where they are released in the body and at what stage of an immune response.
The constant region at the trunk of the antibody includes sites involved in interactions with other components of the immune system. It only occurs in a few variants, which define the antibody's class or isotype: IgA, IgD, IgE, IgG, and IgM. In contrast, the remainder of the antibody is relatively constant. To allow the immune system to recognize millions of different antigens, the antigen-binding sites at both tips of the antibody come in an equally wide variety. Using this binding mechanism, an antibody can tag a microbe or an infected cell for attack by other parts of the immune system, or can neutralize it directly (for example, by blocking a part of a virus that is essential for its invasion). Each tip of the "Y" of an antibody contains a paratope (analogous to a lock) that is specific for one particular epitope (analogous to a key) on an antigen, allowing these two structures to bind together with precision. The antibody recognizes a unique molecule of the pathogen, called an antigen. Each antibody binds to a specific antigen an interaction similar to a lock and key.Īn antibody ( Ab), also known as an immunoglobulin ( Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses.
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