A subsequent exposure results in a secondary response. IgM antibodies are produced slowly, and the person is likely to get sick. During the secondary response IgG antibodies are produced quickly, and the person is able to resist the pathogen. In active immunizations, the person is exposed to pathogens of attenuated virulence that have the same antigenicity as the virulent pathogen. The secondary response is believed to be due to the development of lymphocyte clones as a result of the antigen-stimulated proliferation of appropriate lymphocytes.
Passive immunity is provided by the transfer of antibodies from an immune to a nonimmune organism.
Monoclonal antibodies are made by hybridomas, which are formed artificially by the fusion of B lymphocytes with multiple myeloma cells. The thymus processes T lymphocytes and secretes hormones that are believed to be required for an effective immune response of T lymphocytes throughout the body. There are three subcategories of T lymphocytes. Killer T lymphocytes kill victim cells by a mechanism that does not involve antibodies but that does require close contact between the killer T cell and the victim cell. The T lymphocytes secrete a family of compounds called lymphokines that promote the action of lymphocytes and macrophages.