C5b is a component of the complement system, which is part of the immune response in the body. It is formed when the protein C5 is cleaved by the enzyme C5 convertase. This process is crucial for the activation of the complement cascade, which helps to enhance the ability of antibodies and phagocytic cells to clear pathogens. Once formed, C5b initiates the assembly of the membrane attack complex (MAC) by binding to other complement proteins, specifically C6, C7, C8, and C9. The MAC creates pores in the membranes of target cells, leading to cell lysis and death, thereby contributing to the immune defense against infections.