The principal functions of mononuclear phagocytes in body defenses result from the changes that take place in these functions when the macrophage is activated (Box1). Macrophages carry out the fundamental function of ingesting and killing invading microorganisms such as intracellular parasites, M. tuberculosis, and some fungi. In addition, macrophages remove and eliminate such extracellular pathogens as pneumococci from the blood circulation. The macrophage also has the capacity to phagocytize particulate and aggregated soluble materials. This process is enhanced by the presence of receptors on the surface of the Fc portion of IgG and C3. The ability to internalize soluble substances supports the increased microbicidal and tumoricidal ability of activated macrophages. Activation of macrophages or monocytes can result in the release of parasiticidal media tors and in receptor-mediate phagocytosis during malaria infection. The most likely location for this innate immune response is within the spleen which is crucial for development of immunity to malaria.

Box1. Functions of Mononuclear Phagocytes

Another important phagocytic function of macrophages is their ability to dispose of damaged or dying cells. Macrophages lining the sinusoids of the spleen are particularly important in ingesting aging erythrocytes. They are also involved in removing tissue debris, repairing wounds, and removing debris as embryonic tissues replace one another.

Phagocytic activity increases when there is tissue damage and inflammation, which releases substances that attract macrophages. Activated macrophages migrate more vigorously in response to chemotactic factors and should enter sites of inflammation (e.g., locations of infection or cancer) more efficiently than resting macrophages. Migration of monocytes into different body tissues appears to be a random phenomenon in the absence of localized inflammation. An essential factor in the protective function of monocytes is the capacity of the cell to move through the endothelial wall of blood vessels (diapedesis) to the site of microbial invasion in tissues. The attracting forces for monocytes, chemotactic factors, include complement products and chemoattractants derived from neutrophils, lymphocytes, or cancer cells.

The activity of mononuclear phagocytes against cancer cells in humans is less well understood than the phagocytosis of microorganisms. Phagocytes are thought to suppress the growth of spontaneously arising tumors. The ability of these cells to control malignant cells may not involve phagocytosis but may be related to secreted cellular products such as lysosomal enzymes, oxygen metabolites (e.g., H₂O₂), proteinases, and TNF-α (cachectin). The proteolytic enzymes present on the surface membrane of monocytes also may play a role in tumor rejection.

مواضيع ذات صلة

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Activation of Mast Cells and Secretion of Mediators

Activation of Th2 Cells and Production of IgE Antibody

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Prevention and Treatment of Graft Rejection

Mononuclear Phagocyte System

Immune Mechanisms of Graft Rejection

Induction of Immune Responses Against Transplants

Transplantation Antigens

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Immune Responses Against Transplants

Cancer Immunotherapy : Stimulation of Host Antitumor Immune Responses by Vaccination With Tumor Antigens