An antibody, also called immunoglobulin, is a protective protein produced by the immune system in response to the presence of a foreign substance, called an antigen. Antibodies recognize and attach to antigens to remove them from the body. A wide range of substances is regarded by the body as antigens, including disease-causing organisms and toxic materials such as insect venom.

How antibodies work

When a foreign substance enters the body, the immune system can recognize it as foreign because the molecules on the surface of the antigen differ from those found in the body. To eliminate the invader, the immune system resorts to several mechanisms, including one of the most important: the production of antibodies. Antibodies are produced by specialized white blood cells called B lymphocytes (or B cells). When an antigen binds to the surface of the B cell, it stimulates the B cell to divide and mature into a group of identical cells called a clone. Mature B cells, called plasma cells, secrete millions of antibodies into the bloodstream and lymphatic system.

As the antibodies circulate, they attack and neutralize antigens that are identical to the one that triggered the immune response. Antibodies attack antigens by binding to them. The binding of an antibody to a toxin, for example, can neutralize the poison simply by changing its chemical composition; these antibodies are called antitoxins. By attaching to some invading microbes, other antibodies can immobilize these microbes or prevent them from entering cells in the body. In other cases, the antibody-coated antigen is subjected to a chemical chain reaction with complement, which is a series of proteins found in the blood. The complement reaction can trigger lysis (burst) of the invading microbe or it can attract microbe-killing scavenger cells that ingest or engulf the invader. Once started, antibody production continues for several days until all antigen molecules are removed. Antibodies remain in circulation for several months, providing prolonged immunity against that particular antigen.

Antibodies and B cells

B cells and antibodies together provide one of the most important functions of immunity, which is to recognize an invading antigen and produce a host of protective proteins that travel through the body to eliminate all traces of that antigen. Collectively, B cells recognize an almost unlimited number of antigens; however, individually, each B cell can bind only one type of antigen. B cells distinguish antigens through proteins, called antigen receptors, found on their surfaces. An antigen receptor is basically an antibody protein that is not secreted but instead is attached to the B cell membrane.

All of the antigen receptors found on a particular B cell are identical, but the receptors located on other B cells differ. Although their general structure is similar, the variation lies in the area that interacts with antigen: the antigen-binding or antibody-combining site. This structural variation between antigen-binding sites allows different B cells to recognize different antigens. The antigen receptor does not actually recognize the entire antigen; instead, it binds to only a portion of the antigen’s surface, an area called the antigenic determinant or epitope. The binding between the receptor and the epitope occurs only if their structures are complementary. If they are, the epitope and the receptor fit together like two pieces of a puzzle, an event that is necessary to activate the production of antibodies by B cells.

Structure and classes of antibodies.

Each antibody molecule is essentially identical to the antigen receptor on the B cell that produced it. The basic structure of these proteins consists of two pairs of polypeptide chains (lengths of amino acids linked by peptide bonds) that form a flexible Y. The stem of the Y consists of one end of each of two identical heavy chains, while each arm is made up of the remaining portion of one heavy chain plus a smaller protein called a light chain. The two light chains are also identical. Within particular classes of antibodies, the stem and lower arms are quite similar and are therefore called the constant region. The tips of the arms, however, have a highly variable sequence. It is these tips that bind to the antigen.

Therefore, each antibody has two identical antigen-binding sites, one at the end of each arm, and the antigen-binding sites vary greatly between antibodies. Antibodies are grouped into five classes based on their constant region. Each class is designated by a letter attached to an abbreviation for the word immunoglobulin: IgG, IgM, IgA, IgD, and IgE. Antibody classes differ not only in their constant region but also in their activity. For example, IgG, the most common antibody, is present primarily in blood and tissue fluids, while IgA is found in the mucous membranes lining the respiratory and gastrointestinal tracts.