Innate Immune Effector Cells

The primary effector cells of the innate immune response are neutrophils and macrophages.[5,7] Resident tissue macrophages are one of the first cells to detect and become activated by the presence of an infectious agent. Once the pathogen is detected, macrophages and other host tissue cells release chemical messengers called chemoattractants that cause the directed migration of neutrophils to the site of infection. Potent chemoattrac-tants for neutrophils include leukotriene B4, IL-1, IL-2, and IL-8. Other proteins generated during inflammation, such as the complement cleavage product C5a, also attract neutrophils. Migration of neutrophils into tissues provides the first immunological line of defense against bacteria that penetrate the physical barrier of the skin. Neutrophils express a number of functionally important receptors on their surface, including l-selectin and p2-integrin adhesion molecules, both of which facilitate neutrophil-binding to and migration through the vascular wall. Membrane receptors for the Fc component of the IgG2 and IgM classes of immunoglobulins and complement component C3b mediate neutrophil phagocytosis of invading bacteria.

The most prominent characteristic of the neutrophil is the multilobulated nucleus. The multilobulated nucleus is important because it allows the PMN to line up its nuclear lobes in a thin line, allowing for rapid migration between endothelial cells. Macrophages, on the other hand, have a large horseshoe-shaped nucleus that makes migration between endothelial cells more difficult. Thus, the PMN is the first newly migrated phagocytic cell to arrive at an infection site. Activated macrophages and neutrophils are both sources of proinflammatory cyto-kines, as well as bactericidal proteases and toxic oxygen radicals. As mentioned earlier, neutrophils are also a primary source of defensins and BPI, both of which are directly bactericidal.

The first events in the process of phagocytosis are contact and recognition between the phagocyte and bacterium.[7] Opsonins, including antibodies and complement components, facilitate phagocyte recognition and engulfment of the bacterium. In the absence of specific opsonins, neutrophils are able to bind and ingest certain species of bacteria. After contact and recognition, pseu-dopods form around the microbe. Fusion of the engulfing pseudopods results in the formation of a phagocytic vacuole or phagosome. Cytoplasmic granules migrate toward the phagosome where the membrane surrounding the granules fuses with the internalized plasma membrane that lines the phagosome, creating the phagolysosome. As a result of this, bactericidal contents of the granule are then emptied into the phagolysosome where digestion of the microbe occurs.

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