- Microorganisms interact with humans such as settlement of innocuous resident bacteria and virulent infectious diseases.
- Bacteria are prokaryotes with cell walls, whereas fungi are eukaryotes. Viruses are important as pathogens but are not living organisms in a strict sense.
- The pathogenic factors of bacteria include such toxins as colonization factors, invasion factors, exotoxins, and endotoxins.
- Viruses proliferate by utilizing the proteins of host cells. Their virulence is manifested through the destruction of cells by the proliferated viruses or through the immune responses by the hosts.
- The immune system of vertebrates distinguishes between self and nonself and then responds to eliminate nonself infectious parasites, transplanted tissues, and allergens.
- This whole system is being operated by a mechanism to discern structures derived from evolutionarily distant organisms, as well as by a mechanism to recognize each and every structure absent in self.
- Bone marrow-derived immune cells such as macrophages, dendritic cells, and lymphocytes are widely distributed in the body. They dexterously operate the immune system by utilizing unique proteins whose functions are solely dedicated to the activities of the immune system.