Humans can deter the infectious microorganism by mounting an immune response. The immune system protects the body from potentially harmful substances by recognizing and responding to antigens.
Antigens are large molecules (usually proteins) on the surface of cells, viruses, fungi, or bacteria.
Substances that contain these antigens are recognized and destroyed by the immune system.
There are two general types of immunity: non-specific and specific. Non-specific (innate) immunity
Almost all humans are born with innate immunity. Innate immunity includes:
Barriers such as intact skin, stomach acid, mucous membranes, cough, tears, and skin oils can keep microorganisms from entering the body.
Certain blood chemicals and proteins, such as complement and interferon, directly attack and kill microorganisms if they make it through the various barriers.
Chemicals such as histamine, serotonin, and bradykinin are released from tissues and certain white blood cells when they are injured by microorganisms. These chemicals cause blood vessels to leak fluid into the tissues, causing swelling. This helps isolate the foreign substance from further contact with body tissues. This chemical reaction is called inflammation. Inflammation also attracts white blood cells that "eat" microorganisms and dead or damaged cells. The process in which these white blood cells surround, engulf, and destroy foreign substances is called phagocytosis.
Specific (acquired) immunity
Acquired immunity develops when the body is exposed to various antigens and builds a defense that is specific to that antigen. When exposed to an antigen, T cells and B cells (types of lymphocytes) become reactive. Specific B cells and T cells are formed and a few of those cells will multiply and provide "memory". When exposed to the antigen again, the “memory cells” will quickly reactivate and destroy the antigen, thus preventing the development of the disease.
For example, an individual who has had rubella is immune to getting rubella again.
Acquired immunity can occur naturally or via vaccination.
Specific (passive) immunity
Passive immunity involves antibodies that are produced in someone's body other than your own.
Infants have passive immunity because they are born with antibodies that are transferred through the placenta from the mother. These antibodies disappear between 6 and 12 months of age.
Gamma globulin is another form of passive immunity that occurs via an injection. For example, gamma globulin high in antibodies to hepatitis A may be injected into someone who has been exposed to hepatitis A. Its protection is also temporary.
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