Monoclonal antibodies

Definition

Monoclonal antibodies are proteins produced in the laboratory from a single clone of a B-cell, the type of cells of the immune system that make antibodies.

Description

Antibodies, also known as immunoglobulins (Igs), are proteins that help identify foreign substances to the immune system, such as a bacteria or a virus. Antibodies work by binding to the foreign substance to mark it as foreign. The substance that the antibody binds to is called an antigen. All monoclonal antibodies of a particular type bind to the same antigen, which distinguishes them from polyclonal antibodies.

The structure of most antibodies can be divided into two parts: the section that binds the antigen and a section that identifies the type of antibody. This second region is called a constant region, because it is essentially the same within the same type of antibody. The most common type of antibody is IgG (immunoglobulin gamma), which is found in the blood and body fluids. For cancer treatments, monoclonal antibodies are often humanized. This involves using human sequences for the constant regions and using mouse or other animal-derived sequence for the binding region. Humanization reduces the immune reaction of the patient to the antibody itself.

When used as a treatment for cancer, there are three general strategies with monoclonal antibodies. One uses the ability of the antibodies to bind to the cancer cells having the tumor antigens on their surface. The immune system will see the cancer cells marked with bound antibodies as foreign and destroy them. A second strategy is to use the antibodies to block the binding of cytokines or other proteins that are needed by the cancerous cells to maintain their uncontrolled growth. Monoclonal antibodies designed to work like this bind to the receptors for the cytokine that are on the tumor cell surface. As doctors don't completely understand how monoclonal antibodies work as drugs, both strategies may help rid the body of the tumor cells.

A final strategy involves special antibodies that are linked (conjugated) to a substance that is deadly to the cancer cells. Both radioactive isotopes, like yttrium 90, and toxins produced by bacteria, like pseudomonas exotoxin, have been successfully conjugated to antibodies. The antibodies are then used to specifically destroy the tumor cells with the radioactivity or toxic substance. The use of monoclonal antibodies is a useful approach to cancer therapy and as scientists learn more about the function of the immune system and cancer, new antibodies and new strategies promise to become more and more effective.

Michelle Johnson, M.S., J.D.