<unk>  are Y-shaped proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses. They are made of a few basic structural units called chains; each antibody has two<unk> rge heavy chains and two small light chains. There are several different types of antibody heavy chain, and several different kinds of<unk> , which are grouped into different isotypes based on which heavy chain they possess. Five different antibody isotypes are known in mammals, which perform different roles, and help direct the appropriate immune response for each different type of foreign object they encounter.
Although the general structure of all<unk>  is very similar,  a small region at the tip of the protein is extremely variable, allowing millions of<unk>  with slightly different tip structures to exist. Each of these variants can bind to a different target, known as an antigen. This huge diversity of<unk>  allows the immune system to recognize an equally wide diversity of antigens. The unique part of the antigen recognized by an antibody is called an epitope. These epitopes fit precisely with their antibody, similar to a key fitting into a lock, in a highly specific interaction that allows<unk>  to identify and bind only their unique antigen in the midst of the millions of different molecules that make up an organism. Recognition of an antigen by an antibody tags it for attack by other parts of the immune system.<unk>  can also neutralize targets directly by, for example, binding to a part of a pathogen that it needs to cause an infection.
The<unk> rge and diverse population of<unk>  is generated by random combinations of a set of gene segments that encode different antigen binding sites (or paratopes), followed by random mutations in this area of the antibody gene, which create further diversity. Antibody genes also re-organize in a process called class switching that changes the base of the heavy chain to another, creating a different isotype of the antibody that retains the antigen specific variable region. This allows a single antibody to be used by several different parts of the immune system.<unk>  occur in two forms: a soluble form secreted into the blood and tissue fluids, and a membrane-bound form  attached to the surface of a B cell that is called the B cell receptor (BCR). The BCR allows a B cell to detect when a specific antigen is present in the body and triggers B cell activation.  Activated B cells differentiate into either antibody generating factories called plasma cells that secrete soluble antibody, or into memory cells that survive in the body for years afterwards to allow the immune system to remember an antigen and respond faster upon future exposures.<unk>  are, therefore, an essential component of the adaptive immune system that learns, adapts and remembers responses to invading pathogens. Production of<unk>  is the main function of the humoral immune system.
Antibodies
Autoantibody
Immune complex
Titer
Antigen
Immunoglobulin G
Antinuclear antibodies
Tumor antigen