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Animals have several ways of defending themselves against pathogens—harmful organisms and viruses that can cause disease. These defense systems are based on the distinction between self—the animal’s own molecules—
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The two general types of defense mechanisms are innate defenses and adaptive defenses.
Many animal groups have Toll-
All white blood cells originate from multipotent stem cells in the bone marrow.
In mammals, the major immune system proteins include antibodies, MHC proteins, T cell receptors, and cytokines.
The defensive response involves three phases:
Recognition phase. The organism must be able to discriminate between self and nonself.
Activation phase. The recognition event leads to a mobilization of cells and molecules to fight the invader.
Effector phase. The mobilized cells and molecules destroy the invader.
There are two general types of defense mechanisms:
Innate defenses, or nonspecific defenses, are inherited mechanisms that provide the first line of defense against pathogens. Innate defenses typically act very rapidly and include barriers such as the skin, molecules that are toxic to invaders, and phagocytic cells that ingest invaders. (Recall from Key Concept 6.5 that phagocytosis is a form of endocytosis, in which a cell engulfs a large particle or another cell.) The innate immune system recognizes broad classes of organisms and molecules, such as viruses, bacteria, fungi, protists, worms, and various toxins.
Adaptive defenses are aimed at specific pathogens and are activated by the innate immune system. For example, the adaptive immune system can make an antibody protein that will recognize, bind to, and aid in the destruction of a specific pathogen, if that specific pathogen ever enters the body. Adaptive defenses are typically slower to develop than the innate defenses and longer-
Immunity occurs when an organism has sufficient defenses to successfully avoid the effects of biological invasion by a pathogen.