Final answer:
The complement cascade assembles proteins on an enemy cell to form a membrane attack complex, causing the cell to lyse. This is part of the immune system's response to eliminate pathogens, facilitated through either classic or alternative complement activation pathways.
Step-by-step explanation:
Molecules formed by the reactions of the complement cascade assemble on the surface of an enemy cell, resulting in the formation of a structure known as the membrane attack complex (MAC). This complex creates pores in the pathogen's plasma membrane. Consequently, water rushes into the hyperosmotic cytoplasm of the pathogen, causing the cell to lyse, or burst, thereby eliminating the threat posed by the invading microorganism.
The complement system can be activated through two pathways: the classic pathway, which requires antibodies bound to the pathogen, and the alternative pathway, which does not. In both cases, the complement proteins bind in a sequence, leading to opsonization, inflammation, or cell lysis. C3b, a fragment from the complement protein C3, binds to the pathogen surface and further triggers the complement cascade, eventually leading to the assembly of the MAC.