harmful to the human host. This is what can happen in superantigen infections like toxic shock syndrome. In such cases, the fever can be life-threatening.
PATHOGEN RECOGNITION AND PHAGOCYTOSIS Phagocytosis happens in phagocytes. It is the process of engulfing and killing pathogens as a way of nonspecifically killing off the infection. It starts with the extravasation or diapedesis of white blood cells, which involves the cells leaving the blood vessels through gaps in the endothelial lining because of complement factor C5a and the release of cytokines. This process is also called transendothelial migration. Extravasation only happens in the capillaries because they have very thin walls and low levels of turbulence. The white blood cells need to adhere to the endothelium before they can get through the capillary walls. Remember that opsonization occurs because of complement factors, lectins, and antibodies that bind to the pathogen. This leads to pathogen recognition, necessary for the phagocytic process. Not all organisms need to be opsonized before they are recognized as pathogens. There are some parts of pathogens that are inherently seen as pathogenic. These are called pathogen-associated molecular patterns or PAMPs. Some PAMPs, which are automatically recognized as pathogenic, include peptidoglycan cell wall material, lipopolysaccharide from gram-negative bacteria, bacterial lipopeptides, flagellin seen in bacterial flagella, and bacterial or viral nucleic acid fragments. There are specialized structures on phagocytic cells that specifically recognize PAMPs. These are called pattern recognition receptors. One type of these is called a toll-like receptor, which binds to certain PAMPs to cause phagocytosis. Some are on the cell membrane, while others are on internal organelle membrane. The binding of a pattern recognition receptor and a PAMP will activate the phagocyte so that it becomes ready to engulf the pathogen. More cytokines are released to enhance the inflammatory response and bring in more phagocytes. This leads to a larger response than could happen with just one or a few phagocytes. In addition, proliferation of the phagocyte occurs as part of the process. 217
