An antibody ( or immunoglobulin) is a large Y-shaped protein produced by B-cells and used by the immune system to identify and neutralize foreign objects (e.g., bacteria, viruses). The antibody recognizes a unique part of an antigen (foreign object). Each tip of the "Y" of an antibody contains a paratope (a structure analogous to a lock) that is specific for one particular epitope (similarly analogous to a key) on an antigen, allowing these two structures to bind together with precision (Figure 1). Using this binding mechanism, an antibody can neutralize its target directly, or tag it for attack by other parts of the immune system.
Production of antibodies is the main function of the humoral immune system. Antibodies are produced by plasma cells (white blood cells), and can occur in two physical forms—a soluble form secreted from the cell, and a membrane-bound form attached to the surface of a B cell (the B cell receptor or BCR). The BCR is found only on the surface of B cells, and facilitates the activation of these cells (and their subsequent differentiation) into either antibody factories called plasma cells, or memory B cells that will survive in the body and remember that same antigen; thusly, the B cells can respond faster upon future exposure. In most cases, interaction of the B cell with a T helper cell is necessary to produce full activation of the B cell and thereby antibody generation following antigen binding. Soluble antibodies are released into the blood and tissue fluids, along with other secretions to continue surveying for invading microorganisms.
Antibodies are glycoproteins belonging to the immunoglobulin superfamily, and are typically made of basic structural units—each with two large heavy chains and two small light chains. There are several different types of antibody heavy chains and several different kinds of antibodies, each grouped into different isotypes based on which heavy chain they possess. Five different antibody isotypes are known in mammals: IgA, IgD, IgE, IgG, and M. These perform different roles and help direct the appropriate immune response for each type of foreign object encountered.
IgG: In its four forms, provides the majority of antibody-based immunity against invading pathogens. The only antibody capable of crossing the placenta to give passive immunity to fetus.
IgM: Expressed on the surface of B cells (monomer) and in a secreted form (pentamer) with very high avidity. Eliminates pathogens in the early stages of B cell mediated (humoral) immunity before there is sufficient IgG.
Activated B cells differentiate into either antibody-producing cells called plasma cells, or memory cells that survive in the body years afterward in order for the immune system to remember an antigen and respond faster at future exposures. At the prenatal and neonatal stages of life, the presence of antibodies is provided by passive immunization from the mother. Early endogenous antibody production varies for different kinds of antibodies, usually appearing within the first years of life. Since antibodies exist freely in the bloodstream, they are said to be part of the humoral immune system. Circulating antibodies are produced by clonal B cells that specifically respond to only one antigen. Antibodies contribute to immunity in three ways: preventing pathogens from entering or damaging cells by binding to them; stimulating removal of pathogens by macrophages and other cells by coating the pathogen; and triggering destruction of pathogens by stimulating other immune responses such as the complement pathway.
Activation of Complement
Antibodies that bind to surface antigens (e.g., on a bacterium) attract the first component of the complement cascade with their Fc region, and initiate activation of the "classical" complement system. This results in the killing of bacteria in two ways. First, the binding of the antibody and complement molecules marks the microbe for ingestion by phagocytes in a process called opsonization; these phagocytes are attracted by certain complement molecules generated in the complement cascade. Secondly, some complement system components form a membrane attack complex to assist antibodies to kill the bacterium directly.
Activation of Effector Cells
To combat pathogens that replicate outside cells, antibodies bind to pathogens to link them together, causing them to agglutinate. Since an antibody has at least two paratopes, it can bind more than one antigen by binding identical epitopes carried on the surfaces of these antigens. By coating the pathogen, antibodies stimulate effector functions against the pathogen in cells that recognize their Fc region. Those cells that recognize coated pathogens have Fc receptors which, as the name suggests, interacts with the Fc region of IgA, IgG, and IgE antibodies. The engagement of a particular antibody with the Fc receptor on a particular cell triggers the effector function of that cell; phagocytes will phagocytose, mast cells and neutrophils will degranulate, natural killer cells will release cytokines and cytotoxic molecules—resulting ultimately in destruction of the invading microbe. The Fc receptors are isotype-specific, lending greater flexibility to the immune system, and invoking only the appropriate immune mechanisms for distinct pathogens.