Monoclonal Antibodies

Antibodies (secreted by B lymphocytes) are proteins produced by vertebrate animals as a defense against infection. They are unique among proteins because they are made in millions of different forms, each with different markers that specifically recognize antigens (molecules foreign to the body). In fact, production of an antibody is begun in response to the presence of its complementary antigen.

The precise "fit" (specificity) of an antibody for its antigen makes the use of antibodies a very powerful tool for biomedical researchers. Labeled with fluorescent dyes, antibodies can be used to locate the presence of specific molecules in cells. They can also be used to identify specific proteins in a mixture outside the body, making them a useful tool for biochemists.

B lymphocytes have a limited life-span in culture. For this reason, individual B lymphocytes from mice immunized with an antigen are fused with cells derived from an "immortal" B lymphocyte tumor. From the resulting mixture of hybrid cells, researchers select hybrids that have the ability to make the antibody and to multiply indefinitely in culture. These so-called hybridomas are propagated as individual clones, each of which provides a permanent and stable source of a single monoclonal antibody. The hybridoma technique allows monoclonal antibodies of a single specificity to be obtained in virtually unlimited amounts.

Proteins inside living cells cannot be reached by antibodies added externally because the plasma membrane of the cell is impermeable to large molecules. However, it is possible to introduce antibodies into the cytoplasm of cells by a technique known as microinjection. In principle, monoclonal antibodies can be made against any cellular protein or other macromolecule and can be used to localize and purify the molecule and sometimes even to analyze its function.

Because of their uniform specificity for a given antigen, monoclonal antibodies have enormous advantage over conventional antibodies, which recognize a variety of different markers on an antigen.  Because of their ability to detect and localize specific biological molecules, these proteins have become one of the most important research tools in biomedicine.