The immune system is more effective than the nonspecific methods, and has a memory component that improves response time when an invader of the same type (or species) is again encountered.

Immunity results from the production of antibodies specific to a given antigen (antibody-generators, located on the surface of an invader). Antibodies bind to the antigens on invaders and kill or inactivate them in several ways. Most antibodies are themselves proteins or are a mix of protein and polysaccharides. Antigens can be any molecule that causes antibody production.

Lymphocytes

White blood cells known as lymphocytes arise from by mitosis of stem cells in the bone marrow. Some lymphocytes migrate to the thymus and become T cells that circulate in the blood and are associated with the lymph nodes and spleen. B cells remain in the bone marrow and develop before moving into the circulatory and lymph systems. B cells produce antibodies.

Macrophage Attacking E.coli (SEM x8,800). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Antibody-mediated (humoral immunity)

Antibody-mediated (humoral) immunity is regulated by B cells and the antibodies they produce. Cell-mediated immunity is controlled by T cells. Antibody-mediated reactions defend against invading viruses and bacteria. Cell-mediated immunity concerns cells in the body that have been infected by viruses and bacteria, protect against parasites, fungi, and protozoans, and also kill cancerous body cells.

Human T-lymphocyte (SEM x12,080). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

The cell-mediated immune responses. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Information provided by: http://gened.emc.maricopa.edu