THE IMMUNE SYSTEM
The immune system is a set of organs, tissues, cells and molecules that protect the body from disease caused by microorganisms and multicellular parasites . Immunologyis the study of the immune system and how it works. Unraveling of the mechanisms of immune defense and the use of products of the immune system in investigating and treating disease has revolutionized the biomedical sciences in a way that is comparable to that of molecular biology in recent years.
The immune system has to distinguish between the cells and macromolecules that make up the body that is, self, and those that are foreign or nonself. Immunological defenses are usually classified as nonspecific and specific. Nonspecific defenses constitute a first line of defense and are available immediately any foreign material, including substances such as wood splinters as well as microorganisms, enters the body. Nonspecific defenses include responses such as inflammation, a rapid immediate response to tissue damage, and the acute phase response, a relatively rapid response to infection. In specific immunological defense, cells of the immune system recognize not just individual microorganisms, but also the particular proteins or glycoproteins found on that microorganism. This type of defense may take several days to become effective, depending on whether the immune system was previously exposed to that specific microorganism but, once activated, results in a long-lasting immunity to it. This immunity can be humoral, in that it involves the production of antibodies and/or cell-mediated, which involves the production of cells that kill or recruit other cells to kill the infected cells. Humoral immunity is effective against microorganisms that do not invade cells and this includes most bacteria and multicellular parasites . Cell-mediated immunity is effective against intracellular parasites, including viruses and some bacteria. However, these two types of specific immunity are not mutually exclusive and usually both types are activated on exposure to the infectious agent.