When a macrophage ingests a pathogen, it attaches parts of its proteins to a class II MHC protein. This complex is moved to the outside of the cell membrane, where it can be recognized by a T lymphocyte, which compares it to similar structures on the cell membrane of a B lymphocyte. If it finds a matching pair, the T lymphocyte activates the B lymphocyte, which starts producing antibodies. A B lymphocyte can only produce antibodies against the structure it presents on its surface.
Antibodies that recognize viruses can block these directly by their sheer size. The virus will be unable to dock to a cell and infect it, hindered by the antibody.
Antibodies that recognize bacteria mark them for ingestion by macrophages. Together with the plasma component complement, antibodies can kill bacteria directly.
"Designed" polyclonal antibodies are a potential weapon against cancer.
Applications in biochemistry
In biochemistry, antibodies are used for immunological identification of proteins (Western blot). Fluorescent antibodies are also used to locate proteins within a living cell.