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Immunity is a biological phenomenon, providing long-term autosupport of the genetic "self" and "not-self" balance in the body under foreign surroundings. The immune system develops and releases certain mechanisms to provide this support. Understanding the basis of immunology is achieved through the study both of the protective mechanisms challenged by "not-self", and of regulatory processes which form internal homeostasis of "self" in the body. Various tests are available to help doctors investigate the status of an individual's immune system. Failure of any one component may lead to immunodeficiencies and loss of host defense. Dysregulation of the immune system may result in autoimmune diseases, allergies, infections or tumors. The immune system works closely with the central nervous system, the endocrine system and the liver to maintain homeostasis.
Function
A full blood count and a differential white cell count tell us whether there are enough white blood cells of each required type present. However, this does not tell us how efficiently these cells will respond to a pathogenic challenge: It is necessary to look at their capacity to react to a challenge. There is more than one way in which to determine a white blood cell's reaction capacity. One can stain the cells for cell markers that indicate an activated status - these are molecules expressed on the cell surface whenever a cell commits itself to react to a challenge. Alternatively, one can measure the concentrations of the substances the cells secrete in response to a pathogen or stimulus.
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