C5b-9, also known as the membrane attack complex (MAC), is a crucial component of the complement system, which is part of the immune response. It forms when proteins C5, C6, C7, C8, and C9 come together to create a structure that can insert into the membranes of target cells, leading to cell lysis or destruction. The formation of C5b-9 is triggered by various immune responses, including those against pathogens. By creating pores in the membranes of bacteria or infected cells, C5b-9 helps eliminate threats to the body, playing a vital role in maintaining health and fighting infections.