tailieunhanh - Medical Microbiology made memorable - part 3

Hầu hết các trang web được bảo vệ bằng cách tiết ra chất nhầy, mà cái bẫy vi sinh vật và ngăn chặn họ gắn bó trực tiếp đến tế bào biểu mô, và các chất chẳng hạn như lysozyme (một mạnh mẽ degradative enzyme) và lactoferrin (một protein sắt ràng buộc mà làm cho việc mua lại thiết yếu của sắt | M E D I C A L MICROBIOLOGY Innate host defences to i nfectious disease The skin and mucous membranes are physical barriers that form the first defence against infection Fig. . They are characterised by high cell turnover with superficial cells which may have become colonised by pathogens over time constantly being shed from the surface. Most sites are further protected by secretions of mucus which trap micro-organisms and prevent them sticking directly to epithelial cells and substances such as lysozyme a powerful degradative enzyme and lactoferrin an iron-binding protein which makes the essential acquisition of iron by bacteria difficult . Environments may be made more hostile still by an acid pH such as in the stomach vagina or urine or by alkaline pH such as in the duodenum. It is also difficult for micro-organisms to rest on epithelial surfaces because of the peristalsis of gut contents periodic flushing of the urethra with urine and the muco-ciliary escalator of the respiratory tract. The normal bacterial flora at many of these sites may also be protective against incoming pathogens because of scavenging of all available nutrients the maintenance of hostile conditions . the metabolic activity of Lactobacillus spp. contributes to the acid pH of the vagina natural production of antibiotics ensuring fitness of specific immunity by subclinical stimulation at mucosal sites. Within tissues there is a series of phagocytic cells that ingest micro-organisms into phagosomes which then merge with intracytoplasmic granules lysosomes containing toxic reagents to form phagolysosomes where the micro-organisms will be killed by oxygen-dependent and oxygen-independent mechanisms Fig. . These antigen-presenting cells then channel the breakdown products from the phagocytosed micro-organisms onto their surface where they are made available for stimulation of passing cells from the specific immune system. Natural killer cells are cytotoxic cells they are related to T