Final answer:
When C3 splits into C3a and C3b, they are active components of the complement system, with C3b binding to pathogens and C3a attracting phagocytes.
Step-by-step explanation:
When the complement protein C3 becomes C3a and C3b, this means they are active. The larger fragment, C3b, binds to the surface of the pathogen, while C3a diffuses outward from the site of activation and attracts phagocytes to the site of infection. This process is a part of the body's immune response, known as complement activation, where antibodies attached to the surface of a pathogen cell can activate the complement system, enhancing the immune system's ability to fight off infection. Surface-bound C3b interacts with other components, such as factor P, factor B, and factor D, which in turn helps form the membrane-attack complex (MAC) that can destroy pathogens.