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  Section: General Zoology » Activity of Life
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Susceptibility and Resistance

Susceptibility and Resistance 
Innate Defense Mechanisms
  - Physical and Chemical Barriers
  - Cellular Defenses: Phagocytosis
Acquired Immune Response in Vertebrates 
  - Basis of Self and Nonself Recognition
  - Recognition Molecules 
  - Cytokines
  - Inflammation
  - Acquired Immune Deficiency Syndrome (AIDS)
  - Blood Group Antigens ABO Blood Types
Blood Group Antigens 
  - Rh Factor
Immunity in Invertebrates 

Susceptibility and Resistance

A host is susceptible to a parasite if the host cannot eliminate the parasite before the parasite can become established. The host is resistant if its physiological status prevents establishment and survival of the parasite. Corresponding terms from the viewpoint of a parasite would be infective and noninfective.

These terms deal only with the success or failure of infection, not with mechanisms producing the result. Mechanisms that increase resistance (and correspondingly reduce susceptibility and infectivity) may involve either attributes of a host not related to active defense mechanisms or specific defense mechanisms mounted by a host in response to a foreign invader. It is important to remember that these terms are relative, not absolute; for example, one individual organism may be more or less resistant than another, and a single individual may be more or less resistant at different times of its life, depending on age, health, and environmental exposure. The term immunity is often used as synonymous with resistance, but it is also associated with the sensitive and specific immune response exhibited by vertebrates. However, because many invertebrates can be immune to infection by various agents, a more concise statement is that an animal demonstrates immunity if it possesses tissues capable of recognizing and protecting the animal against nonself invaders. Most animals show some degree of innate (nonspecfic) immunity, a mechanism of defense that does not depend on prior exposure to the invader. In addition to having innate immunity, vertebrates—and invertebrates to a lesser extent—develop acquired (specific) immunity, which is specific to a particular nonself material, requires time for its development, and occurs more quickly and vigorously on secondary response. Many innate mechanisms discussed in the next section are dramatically influenced and strengthened in vertebrates as a consequence of acquired immune responses.

Frequently resistance conferred by immune mechanisms is not complete. In some instances a host may recover clinically and be resistant to a specific challenge, but some parasites may remain and reproduce slowly, as in toxoplasmosis , Chagas’ disease , and malaria . This condition is called premunition.


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