PHAGOCYTOSIS
Phagocytosis is a nonspecific
defense mechanism in which
various phagocytes engulf and
destroy the microorganisms
of disease.
Phagocytes.
Among the important
phagocytes are the circulating white
blood cells called neutrophils
and monocytes. In the tissues,
the monocytes are transformed
into phagocytic cells called macrophages. The macrophages move through the
tissues of the body performing phagocytosis and destroying parasites. They are
part of the reticuloendothelial system. Phagocytes also initiate the processes
of the immune system.
The process of phagocytosis
begins with attachment and ingestion of microbial particles into a bubblelike
organelle called a phagosome. Once inside the phagocyte, the phagosome
containing the microorganism joins with a lysosome, which contributes enzymes.
The fusion of phagosome and lysosome results in a phagolysosome.
Microorganism are destroyed within
minutes, and the microbial debris is eliminated from the cell in the process of
egestion. In the immune process, chemical portions of the microorganism called
antigenic determinants are displayed on the surface of the phagocyte to
stimulate the immune process.
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Phagocytosis is enhanced by
products of the immune system called antibodies.These protein molecules bind to
microorganisms and encourage engulfing by phagocytosis.
The complement system.
The complement system is a
series of proteins that circulate
in the blood and encourage
phagocytosis or otherwise complete the defensive process. Many immune reactions
stimulate the complement system. The complement system operates in a cascade of
reactions. In the pathway, certain complement components react with one another
and produce new substances that induce other components to react. The results
of the myriad reactions are substances that induce other complement components
into action. The overall result is a number of substances toxic to microorganisms.
The substances encourage phagocytosis or bring about destruction of microbial
membranes.
Two general pathways for
complement activity exist. The classical pathway operates with the highly specific immune
system and is initiated when certain antibodies unite with antigens and
stimulate the complement system into action. The alternative pathway is nonspecific and is initiated by
tumors, cell wall components of bacteria, and various microorganisms. It is
sometimes called the properdin pathway because properdin is one of the proteins
operating in it. The alternative
pathway invokes a slower and less specific method for ridding the body of
parasites, particularly Gram-negative bacteria and viruses.
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