There are several systems in the body that defend it against infection. Skin, mucus membranes, and antimicrobial agents in the blood all help identify and destroy infectious substances. Another system of the body that defends against infection allows a percentage of lymphocytes to become natural killer cells. These are present in the bloodstream, the spleen, lymph nodes, and bone marrow (Tortoria & Derrickson, 2013). Natural killer cells can destroy other cells in the body that are infected, and they can also kill some tumor cells. In general, they try to destroy any cells in the body that have defective proteins in their plasma membranes.
Natural killer cells complete the destruction of abnormal body cells by binding with the infected cell and then releasing toxic granules into the abnormal cell. Either perforin or granzymes are released. Perforin causes holes in the abnormal cell’s membrane, allowing extracellular fluid to enter the abnormal cell to burst it. Granzymes cause the abnormal cells to self-destruct. The microbes released by this self-destruction are then destroyed by phagocytes, an additional type of defensive cell. Phagocytes absorb or ingest the cellular debris and microbes from the self-destruction of granzymized cells.
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There are two distinct types of phagocytes: neutrophils and macrophages. The process of phagocytes ingesting the microbes and cellular debris of a killed infectious cell has five different stages. In the chemotaxis stage, phagocytes are chemically called to the damaged cells. During the adherence stage, a phagocyte sticks to the microbe or cellular debris with protein. In the ingestion stage, the phagocyte reaches into the microbe or cellular debris with pseudopods, fuses with the abnormal cell, and throws a phagosome covering around everything. In the digestion phase, the phagocyte merges chemically with the abnormal cell, and in the final stage, the chemical killing of the abnormal is completed.
- Tortoria, G.T. & Derrickson, B. (2013). Principles of anatomy & physiology. Hoboken, NJ: Wiley.
