Mechanisms of Cell and Tissue Damage

Cell damage has profound effects if it is the endothelial cells of small blood vessels that are involved. When bacteria invade tissues, they almost inevitably cause some damage, and this is also true for fungi and protozoa. Cell and tissue damage are sometimes due to the direct local action of the microorganism and microbial toxins. They either interfere with the transcription, translation, and DNA synthesis or change the permeability of the cell membrane. Some of the indirect damage brought about by these microbes is through inflammation and immune responses. Host cells are destroyed or blood vessels injured as a direct result of the action of microbes or their toxins. Inflammatory materials are liberated from necrotic cells, whatever the cause of the necrosis. Also many bacteria themselves liberate inflammatory products and certain viruses cause living infected cells to release inflammatory mediators. The expression of the immune response necessarily involves a certain amount of inflammation, cell infiltration, lymph node swelling, even tissue destruction. Sometimes they are very severe, leading to serious disease or death, but at other times they play a minimal part in the pathogenesis of disease. Other indirect mechanisms of damage include stress, hemorrhage, placental infection, and tumors.