Phagocytosis Cure Cancer

When the hyphae are too large for phagocytosis, activated neutrophils can still destroy them by secretion of an array of proteolytic molecules into the local microenvironment. Attachment of neutrophils to the hyphae ensures that the fungi are within "range" of these metabolites.

In addition to their role in B-cell activation, cytokines secreted by activated CD4+ T cells play a significant role in immunity to Candida species. IFNy augments the production of reactive oxygen intermediates, nitric oxide, and reactive nitrogen intermediates by phagocytes. The role of CD4+ cytokines is underscored by the fact that individuals who lack T cells with antigen specific T-cell receptors recognizing Candida peptide/class II MHC have recurrent candidiasis. This disorder is termed chronic mucocutaneous candidiasis and results from a so-called "hole" in the T-cell repertoire.

Summary

Microbes penetrate the normally protective physical and chemical defenses. Having breached host barriers, they face the onslaught of both the innate and adaptive immune systems. Different components of the immune system are recruited during immune responses, depending on the organism and its mode of infectivity (Fig. 11.8). Viruses infect and replicate within cells. Intracellular bacteria also infect cells, often thriving within macrophages because they have evolved mechanisms to evade the proteolytic degradation that occurs in the phagocytic vacuole. Extracellular bacteria are not sequestered, and many have become pathogenic because they have a polysaccharide capsule that hinders phagocytosis.

Extracellular bacteria thrive within the host environment, but are not sequestered within any cell. These microbes are vulnerable to soluble mediators of the immune system. Phagocytosis is the primary vehicle by which extracellular bacteria are eliminated. The process of phagocytosis is greatly enhanced by molecules generated following the activation of B cells, T cells, and complement. The deposition of molecules termed opsonins allows phagocytes to bind bacteria via receptors that are specific for the opsonin. Activated macrophages also secrete cytokines, some of which enhance the inflammatory response, while others influence the course of naive CD4+ T-cell differentiation. Antibodies contribute to the destruction of extracellular bacteria by activating the classical pathway of complement, and by serving as opsonins for opsonin mediated phagocytosis. Antibodies may also play a protective role in host defense by binding to, and hence neutralizing, toxins secreted by some bacteria.

Immune responses to viruses vary according to the site where the virus is detected and whether the infection is primary or secondary. When viruses are found within blood, lymph, or interstitial fluid, they may be phagocytosed. However viruses are found only transiently in fluids during the initial infection and when they are released from infected cells. Thus while phagocytosis of viral particles does occur, this is not a primary mechanism for the eradication of viral infections. Viruses are sequestered within cells, protected from the immune system. Destruction of the infected cell, a veritable "viral factory", is mediated by NK cells, and by

Relative Contibution in Host Defense to Extracellular Bacteria

Innate Immunity

Phagocytes NK cells

Adaptive Immunity

B cells CD4+ T cells CD8+ T cells

Increasing contribution ■

Relative Contibution in Host Defense to Viruses

Innate Immunity

Phagocytes NK cells

Adaptive Immunity

B cells CD4+ T cells CD8+ T cells

Increasing contribution ■

Relative Contibution in Host Defense to Intracellular Bacteria

Innate Immunity

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