In fact, production of an antibody is begun in response to the presence of its complementary antigen. 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.
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 .