The Immune Response

An antigen is a molecule that the immune system can recognize as either part of the body or as foreign to the body. Pathogens have antigens on the surface of their cells. These antigens, or marker molecules, signal the body's immune system that there is a foreign invader.

The immune system works in two ways. One type of white blood cell produces antibodies against specific antigens that lead to the destruction of the pathogens. Another type of white blood cell reacts directly against pathogens to destroy them.

Antibodies are proteins that bind to specific molecules called antigens, which are present on the cells of invading organisms. Once antibodies bind to antigens, the pathogen is marked for destruction. Some antibodies make it easier for macrophages to destroy the pathogens. A macrophage cell can engulf an invading pathogen and secrete enzymes to destroy it.

In the immune system’s response, the white blood cells that produce antibodies are called B lymphocytes. Each type of B lymphocyte produces only one kind of antibody. Antibodies help destroy and remove pathogens. For example, when you suffer from strep throat caused by bacteria, your body produces anti-strep antibodies that lead to the removal of the bacteria.

T lymphocytes are another type of white blood cell that reacts against pathogens through direct cell contact or by release of chemicals that are toxic to the pathogen. For example, T lymphocytes can attach to cancer cells and release chemicals that poke holes in and kill the cancer cells.

B and T lymphocytes are scattered together all over the body, especially in the lymph nodes and spleen, which function as focal points for fighting pathogens in the body.

Lymph nodes are small kidney bean shaped filtration structures rich in lymphocytes and macrophages. Lymph nodes monitor foreign materials, and are found throughout the body. For example, when you suffer from a sore throat, the lymph nodes below your chin become larger as the white blood cells work to fight off the bacteria or virus.

The role of the spleen is similar to a lymph node except that it filters blood, while lymph nodes filter other fluid. Like lymph nodes, the spleen is full of B and T lymphocytes and macrophages. These cells help clear the blood of foreign materials and old or abnormally-shaped red blood cells.