Lymphocytes, Clonal Selection, And Antigen Recognition, Secondary Lymphoid Organs And Immune Responses, Immune Tolerance And Autoimmunity
The immune system provides the body with resistance to disease. Innate immunity is furnished by relatively nonspecific mechanisms, such as the rapid inflammation experienced shortly after injury or infection. In contrast to innate mechanisms that hinder the entrance and initial spread of disease, adaptive immunity is more selective in its activity, and upon repeated exposures to pathogens can often prevent disease. There are two kinds of adaptive immune responses. Humoral immune responses are effective against agents that act outside of cells, such as bacteria and toxins. During humoral immune responses, proteins called antibodies, which can bind to and destroy pathogens, are secreted into the blood and other body fluids. In contrast, cell-mediated immune responses are important in resisting diseases caused by pathogens that live within cells, such as viruses. During cell-mediated responses, immune cells that can destroy infected host cells become active. Furthermore, cell-mediated immunity may also destroy cells making aberrant forms or amounts of normal molecules, as in some cancers.
Numerous aspects of adaptive immunity differ substantially in aged individuals from what is seen in young adults. For example, aged individuals often have attenuated or otherwise impaired immune responses to various bacterial and viral pathogens. Indeed, this general trend forms the basis for recommended immunizations against infectious agents that younger individuals resist easily. Aged individuals often respond differently to vaccination, however, sometimes resulting in a lack of protective immunity. In addition, unto-ward immune phenomena, such as certain forms of autoimmunity, as well as cancers involving cells of the immune system, show increased incidence in aged individuals. A complete understanding of these age-associated changes in immune status and function remains elusive, requiring knowledge of the mechanisms underlying maintenance, activation, and control of the immune system.
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