<unk> ch<unk>  binds to a specific antigen; an interaction similar to a lock and key. An<unk>  <unk> , 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. The<unk>  recognizes a unique molecule of the pathogen, called an antigen.<unk> ch tip of the "Y" of an<unk>  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. Using this binding mechanism, an<unk>  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). To allow the immune system to recognize millions of different antigens, the antigen-binding sites at both tips of the<unk>  come in an equally wide variety. In contrast, the remainder of the<unk>  is relatively constant. It only occurs in a few variants, which define the<unk> 's class or isotype: IgA, IgD,<unk> , IgG, or IgM. The constant region at the trunk of the<unk>  includes sites involved in interactions with other components of the immune system. The class hence determines the function triggered by an<unk>  after binding to an antigen, in addition to some structural features.<unk> es from different classes also differ in where they are released in the body and at what stage of an immune response. Together with B and T cells,<unk> es comprise the most important part of the adaptive immune system. 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. Initially, all<unk> es are of the first form, attached to the surface of a B cell – these are then referred to as B-cell receptors (BCR). After an antigen binds to a BCR, the B cell activates to proliferate and differentiate into either plasma cells, which secrete soluble<unk> es with the same paratope, or memory B cells, which survive in the body to enable long-lasting immunity to the antigen. Soluble<unk> es are released into the blood and tissue fluids, as well as many secretions. Because these fluids were traditionally known as humors,<unk> -mediated immunity is sometimes known as, or considered a part of, humoral immunity. The soluble Y-shaped units can occur individually as monomers, or in complexes of two to five units.<unk> es are glycoproteins belonging to the immunoglobulin superfamily. The terms<unk>  and immunoglobulin are often used interchangeably, though the term<unk> ' is sometimes reserved for the secreted, soluble form, i.e. excluding B-cell receptors.
Antibody
Microantibody
Heterophile antibody
Antigen
Isoantibodies
Immune complex
Autoantibody
Immunogen