Complement is a group of proteins present in blood plasma that interact with pathogens to mark them for destruction by phagocytes. Once the complement system is activated by binding to the surface of a pathogen it triggers very specific responses including; inflammation at the site of infection and release of factors that attract innate immune cells such as neutrophils. The complement proteins bound to the surface of the pathogen also identifies that cell as a “non-self” target for the phagocytic cells to engulf and kill.
There are numerous complement molecules, which participate in host defense. One example is complement 3b (C3b), which binds to the surface of pathogens. This process is know as opsonization and is defined as enhancing the binding of the pathogen for phagocytosis. Phagocytes (such as neutrophils) have a specific receptor for C3b, so that when they encounter a pathogen with C3b on its surface, they are triggered to bind, engulf, and kill the pathogen.
