C3b is a protein that plays a crucial role in the immune system, specifically in the complement system. It is produced from the cleavage of the complement component 3 (C3) protein and helps mark pathogens, such as bacteria and viruses, for destruction by immune cells. This process is known as opsonization, which enhances the ability of phagocytes to engulf and eliminate these invaders. In addition to opsonization, C3b also participates in the formation of the membrane attack complex (MAC), which can directly lyse target cells. By facilitating these immune responses, C3b is essential for maintaining the body's defense against infections and ensuring proper immune function.