Antibodies are proteins produced by B-lymphocytes (B-cells), cells of the immune system. In the body, antibodies bind to target molecules (antigens) present on foreign objects that enter the body, such as pollen or bacteria. The target molecules are often but not always proteins. Antibodies may also bind to antigens located on the surface of other cells, including cancer cells. The binding of an antibody to an antigen marks the target cell or foreign object for destruction and removal.⁽¹⁾

The interaction between antibody and antigen is very precise and is analogous to the interaction between a lock and key. Any given B-cell can produce only one type of antibody but our cells contain millions of different B-cells. Because of this, there are many different kinds of antibodies in our bodies. In fact, our bodies have the ability to respond to each of the many thousands or even millions of different possible antigens to which we may be exposed during our lifetime. When a particular B-cell encounters the appropriate antigen it reproduces to create a large number of cells. Some of the resulting cells are very long-lived. They can live for years after the initial 'challenge' with antigen and provide the immune system with a form of memory. A second exposure to the same antigen leads to to a much more rapid and vigorous response. This long term memory is the basis of vaccinations. The rest of the cells are short-lived cells that produce large amounts of antibodies.⁽²⁾⁽¹⁾

As shown below, antibodies are composed of four proteins, two larger chains and two smaller chains, which are represented in purple. They are shaped like Ys and each side is identical so that an antibody is able to bind to two copies of its target, one per each short arm of the Y.

antibody

Any given antigen can lead to the activation of many different B-cells and the production of many different antibodies, all directed against different portions of the same target. A mixture of different antibodies is said to be polyclonal.
In the laboratory, it is possible to produce and purify large amounts of a single type of antibody, produced by a single type of B-cell. These antibodies are said to be monoclonal. The antibody based treatments developed so far utilize monoclonal antibodies.⁽²⁾⁽¹⁾

This section contains content on

The use of antibodies in cancer treatment.

A list of antibody-based cancer treaments with links to more information.

A Closer Look at Antibodies

Antibody structure
All antibodies (Immunoglobulins or Ig) share some common features. Each contains two smaller proteins (light chains) and two larger proteins (heavy chains). The four proteins that make up the antibody are held together by disulfide (S-S) bonds between cysteine amino acids. Each of the four chains of the antibody contains a constant region and a variable region. The constant (Fc) region contains domains that allow other cells of the immune system to recognize that antibody. The small red pieces represent the light chains and the blue pieces represent the heavy chains. On the left arm of the antibody the variable regions are colored in green (there is also a variable region on the opposing arm in the same location). The disulfide (S-S) bonds that hold the molecule together are shown.

Types of antibody
There are five major kinds of antibody, called isotypes. The isotype of antibody is determined by its constant region. The five types of antibody are IgG, IgD, IgA, IgE, and IgM; each isotype is associated with a distinct immune response. For example, antibodies of the IgG isotype often recruit cytotoxic (killer) T-cells, that destroy target cells. The antibody shown in the figure above is of the IgG1 isotype.

The variable regions of antibodies recognize and bind to specific antigens. It is possible for antibodies produced by more than one type of B-cell to recognize a particular target. For example, a bacterial invader would have many target proteins on its surface and would be recognized by many different antibodies within the body. Even a single protein (or other type of antigen) may be recognized by many different antibodies.

Antibody responses
Polyclonal( )antibodies arise when the body is presented with an antigen and many different B-cells are activated to produce antibodies. The result is a mixture of antibodies that would all recognize different parts of the antigen.
Monoclonal antibodies are those that are derived from a single type of B cell. Monoclonal antibodies are currently utilized as anti-cancer drugs.

Introduction to Antibodies

A Closer Look at Antibodies