Thursday, February 28, 2008

Immunity - Overview

Immune is from the latin immunitas refering to freedom from taxes or the exemption from service. We use the word to mean protection from disease, usually infectious disease.

Immunity is mediated by the immune system, a network of organs, tissues, cells and molecules. A brief introduction to the immune system can be found here.

The immune response is the collective and integrated response of the immune system to the introduction of a foreign antigen into the body.


Natural or innate immunity are the barriers and mechanisms that mediate nonspecific resistance or defense in the body. Skin, mucous membranes, phagocytic cells and various molecules mediate innate immunity.

Acquired or specific immunity are those immune responses that are specific for distinct antigens and can increase in magnitude and capabilities with every exposure to the same antigen. An animation of specific immunity in action can be seen here.

Active Immunity is the type of specific immunity that is induced when an individual is exposed to an infectious agent or foreign antigen. This is the immunity that is generated in response to vaccination.

Passive Immunity is that immunity that can be transferred using cells or serum obtained from an immune individual. This is a means of rapidly inducing resistance to an individual.

The adaptive immune system is often classified into:

Humoral immunity which is mediated by antibodies. Go here to learn how lymphocytes make antibodies. Watch this animation on an allergic reaction that is mediated by IgE antibodies.

Cell-Mediated immunity which is mediated by T lymphocytes.

The immune response has several important features:

Specificity: immune response is specific for the antigen that induced the immune response.

Diversity: the number of antigen that can be recognized by the immune system is estimated to be 10,000,000,000.

Memory: exposure of the immune system to an antigen results in enhanced capacity to produce more antibodies or T cells to subsequent exposure to the same antigen.

Tolerance: individuals typically do not make immune responses to their own tissues. This is because the immune system is capable of discriminating self from non-self.

Self-limitation: immune responses normally decline after a period of time. This usually correlates with elimination of the antigen that originally induced the immune response.

Learn more about the immune system here and here.

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