Immunity is defined as the resistance exhibited by the host towards injury caused by the microorganisms and their products.

Protection against the infectious agents is only one of the consequences of the immune response. But in true sense, immunity involves the defensive response, when a host is invaded by foreign organisms or other foreign substances (pollen, insect venom, transplanted tissue). Body cells that become cancerous are also recognized as foreign and may be eliminated.

Pathogenic microorganisms are endowed with special properties that enable them to cause disease, if given the right opportunity. If microorganisms never encounter resistance from the host, we would constantly be ill and would eventually die of various diseases. But in most cases, our body defenses prevent them of happening so. In some instances, the body does not allow the organisms to enter. In others, even if they enter, are eliminated by different mechanisms. In still others, even if they remain inside, the defenses combat with them. Our ability to ward off disease in general is called resistance (immunity). Vulnerability or lack of resistance is susceptibility.



Innate immunity is an invariable, hereditary response—an inborn defense. It is independent of previous exposure to disease causing agents and foreign substances. The innate immunity depends on the non-specific mechanisms, molecular defenses and the activity of the phagocytic cells. Innate immunity may be non-specific, when it indicates a degree of resistance to infection in general or specific, when resistance to particular pathogen is concerned.

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The resistance an individual acquires during life is called acquired immunity.

This protect from pathogens develops as we go through life. As we are exposed to diseases or get vaccinated, we build up a library of antibodies to different pathogens. This is sometimes referred to as immunological memory because our immune system remembers previous enemies.

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