Innate immunity

Innate immunity is a primitive system common to all animals. It is not specific in its operation. In other words, natural immunity works against any agent that enters or attempts to enter the body rather than against a specific agent. For example, as part of the natural immune system, the skin itself acts as a physical barrier to many agents that could be harmful. Certain chemicals and cells in the body will also attack any foreign substance and attempt to destroy the substance.

Adaptive immunity

The second method is adaptive, or acquired, immunity. Only human beings and other animals with backbones (a class referred to as vertebrates) enjoy this method of protection. With this kind of immunity, certain cells in the body can spot and destroy specific invading agents. This action is called the immune response. The substance that provokes such a response is called an antigen.

One other property of acquired immunity that separates it from innate immunity is a kind of memory. Once it comes into contact with a specific invader, such as the virus that causes measles, for example, the system is able to “remember” that contact. The next time the same type of invader is met, the immune system has an even stronger response than it did the first time.

Certain white blood cells called lymphocytes make up the heart of the acquired immune system. Like all blood cells, lymphocytes are produced in the bone marrow and circulate throughout the body in the bloodstream. Lymphocytes also travel to selected organs and tissues that are part of the immune system, such as the spleen, tonsils, and lymph nodes.

Each lymphocyte has receptors that allow it to recognize a specific antigen. The receptors contain grooves that fit into patterns on the antigen—like a key fitting into a lock. This allows the lymphocyte to attach to the antigen. Once a lymphocyte attaches to an antigen the lymphocyte creates copies of itself, including its receptors. This way, if the body encounters the antigen again in the future there will be many lymphocytes to attack the antigen.

There are two different kinds of lymphocytes—so-called B cells and T cells. B cells release products called antibodies that attack antigens. T cells are divided into helper cells and cytotoxic, or killer, cells. The helper cells produce substances called interleukins that help both T and B cells grow. Cytotoxic T cells destroy cells infected with viruses and other pathogens and may also destroy cancerous cells.