• Recognition of Foreignness : The immune system must be able to recognize bacteria, viruses, fungi, parasites, and foreign materials in order to locate and destroy them.
  
• Specificity : Immunity to one foreign organism or substance does not necessarily provide resistance to different ones, even if the two are highly similar.
  
• Memory : The ability to develop an accelerated, enhanced, and long-lasting immune response after the initial attack by an infectious disease, or initial development of cancerous tissue.

1. Production of antibodies and the stimulation of specialized cells, which destroy the cells or organisms and neutralize their toxic products.When aspects of the immune system involved in this function becomes too active, however, the result may be undesirable features, such as hyper-sensitivity allergic reactions. On the other hand, when the immune response is not working properly (immuno deficient) the individual may become more susceptible to repeated infections.
   
2. Removal of damaged or dying cells. This function may be misdirected, however, resulting in an immune response against the body's own cells or tissues, producing autoimmune disease. The function of the immune system most recently discovered is the system's ability to recognize and eliminate the abnormal cancer cells that frequently arise within the body. An immune system that is functioning properly can recognize and dispose of such cancer cells by means of a process called immune surveillance.

• phagocytes. These engulf (phagocytose) foreign bodies, responding in a non-specific manner, attacking everything of a foreign nature. They are chemically attracted to sites of inflammation, by the release of histamine and other messengers into the blood by exploding mast cells.
• neutrophils. Short-lived cells stored in the bone marrow and released in response to infection. They contain powerful digestive enzyme filled granules. After engulfing a micro-organism, the granules empty their enzymes, which kill the organism; after destroying a few, the neutrophil succumbs to its own enzymes. Neutrophils are incapable of reproduction.
• eosinophils. Specialized cells involved in destroying IgE-antibody-allergencomplexes. They are an intrinsic part of the allergic reaction.
• monocytes. Small cells circulating in the blood that can rapidly transform into a large and powerful known as macrophages, containing enzyme filled lysosomes. They are long lived and are capable of fusing together to combat large pathogens. Capable of reproduction, the number of circulating macrophages can become very large. Macrophages destroy micro-organisms, engulf ineffective red blood cells, dead and dying tissue, exhausted neutrophils, and other debris.
• basophils. Play an important role in inflammation and the allergic response. IgE- basophil-antigen interactions lead to the acceleration ofthe body's defense system.
• ymphocytes. White blood cells involved in specific immunity.Individual lymphocytes are genetically specific to specific targets. Initially undifferentiated, they develop into two major groups. Any substance that elicits activity from lymphocytes is called an antigen.
  • B Iymphocytes. (B-cells) Immunity from antibodies is provided by the B cells, which are produced in the bone and distributed to the lymphoidtissues of the body, such as the lymph nodes, spleen, tonsils, and Payer's patches in the small intestine. These proteins antibodies ([[gamma]] globulinsor immunoglobulins) have very specific targets. When some B-cells meet antigens they transform into plasma cells which produce vast amounts of the appropriate antibody. Other B-cells transform into rapidly multiplying memory cells which circulate throughout the body preparing it for the next encounter with this antigen. Important circulating immunoglobulins are:

• IgA. Found mainly in mucous membranes. Intercepts antigensin nose and throat.
  
• IgE. Binds to certain antigens to cause allergic reactions. Attachesto basophils and mast cells. When they encounter an antigen, configuration changes cause the mast cell to explode, releasing histamine and other chemical into the blood which attract other leucocytes.
  
• IgG. A common antibody, vital for proper immune system functioning.

• T Iymphocytes. (T-cells) Long-lived cells that transform intoactive and memory cells when encountering an antigen. T-cells are activeagainst bacterial infection, fungi, cancerous cells, transplanted tissue, wounded cells harboring pathogens, etc. T-cells produce lymphokines that draw macrophages, neutrophils and eosinophils toward sensitized T-cells, and activate the macrophages which then inhibit viral reproduction etc. Because the immunity associated with T cells does not involve the secretion of antibodies but requires direct physical contact with antigens, it iscalled cell-mediated immunity. T cells originate in the thymus and also become localized in lymphoid organs. A range of T-cells have been identified:

Killer T-cells. These attack antigens directly.
Helper T-cells. Enable the other T cells and most B cells to perform their functions. It is this cell that is destroyed by the HIV virus in AIDS, resulting in a depressed immune response that allows infection by a variety of micro-organisms and the growth of certain tumors. They activate B-cells, causing the appropriate ones to transform into plasma cells and flood the blood stream with antibodies.
Suppressor T-cells. They monitor and adjust the level of antibodies and counteract the action of Helper T-cells.

The Complement System
When an antibody binds to an antigen, a complementary set of chemical reactionsis initiated. The binding activates the complement proteins in the bloodstream. This set of nine proteins are transformed into active enzymes which attach to the surface of the pathogen and essentially burn a hole through the cell membrane causing the cell to disrupt. Other complement proteins directly attack viruses; others attach to antigens and make them a more susceptible target for phagocytes; some may act as phagocytes themselves.

The sequence of events involved in this kind of immune response can be summarizedin this series of steps :

• Invading micro-organisms initially encounter whatever macrophages, neutrophils, B and T cells might be present in the immediate vicinity.
• The resulting activation of basophils releases histamine.
• Pathogenic destruction of local phagocytes produces other chemical messengers.
• Inflammation is initiated.
• Chemical signals are sent to the bone marrow where the production ofphagocytes increases.
• More macrophages, neutrophils and lymphocytes are drawn to the siteof invasion.
• Any antibodies present at the site of infection lock onto their antigen, and the complement system is brought into action.
• The complement system directly kills some pathogens and makes others more susceptible to attack from phagocytes.
• Debris from this interaction produces strong attracting signals for phagocytes & lymphocytes.
• The killer T-cells attack antigens directly, and T-cells begin to producethe lymphokines which serve to intensify the battle.