Immunology Overview The immune system is the name given to a collection of molecules, cells and organs whose complex interactions form an emergent system which is generally able to protect an individual both from outside invaders and its own altered cells. This combination of components is the product of millions of years of evolution which has selected those organisms which are betterable to prevent their destruction by microorganisms or tumors. This text attempts clarify exactly what the immune system does,and more importantly, the mechanism of its action. It is difficult to clearly delineate the elements of the immune system from the rest of the body, or from each other. However, a traditional, if somewhat inadequate, method separates the immune system into two functionally distinct parts: those elements which are innate, and those which are acquired. It is becoming apparent however, that these two groups are in no way isolated,and that a complex dialog is constantly going on between them. As such, some attention will also be given to the"innate/acquired interphase." Innate Immunity Innate immunity refers to immune elements which are non-specific and non-adaptive. They are generally able to distinguish foreign tissues/organisms but are unable to recognize a particular invader. Similarly, they will respond to such an organism in the same manner despite repeated exposures to it - they do not adapt and improve their effectiveness against previously encountered antigens. Innate immunity can best be divided into three sub-groups based on both function and form: barriers,non-specific chemical agents, and non-specific effector cells. Barriers to Infection Perhaps the most often forgotten fact concerning the immune system is that it works best when it doesn't have to work at all.That is to say, if the infectious organism is unable to enter an individual's body, for whatever reason, no immune response need be mounted and the person is left completely healthy. For this reason, two anatomic features not normally associated with the immune system also serve important innate immune functions: the skin, and its close relative, the mucosa. While it seems elementary, the importance of skin in resisting infection can not be overemphasized. It is the location at which most microorganisms are stopped. To appreciate its importance, it is worthwhile to consider the relative frequency of lesion forming on healthy, intact skin as compared to infections at locations where the skin has been broken. The less effective, but close cousin of the skin is the mucosa: the tissue which covers our eyes, alimentary and genital/urinary tracts. As the higher frequency of mucosal infections indicate, it is not quite as effective at resisting the myriad of challengers to which it is subjected. Since this tissue is easier to penetrate than normal skin, it possesses a number of means to dissuade the frequent microscopic interlopers. Tears and saliva, as well as other mucus secretions act to wash away many potential invaders, and many also contain chemical elements which are effective microbicides Non-Specific Chemicals Mucosal secretions contain a number of chemical weapons to prevent infections. There is a great variety of such elements,including several well known enzymes, such as lysozymes, which serves to destroy the outer surface of many bacteria. Non-mucosal tissues are also rife with non-specific immune chemicals. One of the most important is a complex of chemicals known as complement. Acting alone, a complement fragment acts by attracting various immune cells to the surface of foreign organisms, and when multiple fragments are brought together they are able to form destructive pores in the membranes of microorganisms. While these chemicals are found floating in their various fluids, waiting to encounter a target,many other chemicals may be released when the cells producing them are stimulated. These include the kinins and histamine - chemicals which, when released in response to trauma or invasion, create an inflammatory response. The inflammatory response is immunologically important as it results in the attraction of other immune cells as well as increased temperature and blood flow to the threatened area. Some cytokines, including the interferons and chemokines, among others, are also included here as they appear to induce an anti-viral state in other cells encountering them. Other cytokines play an important role in regulating the interactions of the immune system.