Final answer:
The Membrane Attack Complex (MAC) is a product of complement activation where complement proteins form a pore in a pathogen's membrane, causing lysis; it is part of the innate immune response.
Step-by-step explanation:
The Membrane Attack Complex (MAC) refers to a structure formed from complement proteins C6 through C9 that penetrates the cell membranes of targeted pathogens, leading to cell lysis and death. This formation is a result of complement activation, which can be initiated via the classical or alternative pathway. The classical pathway is antibody-dependent and begins when antibodies attached to the surface of a pathogen activate the complement system. The alternative pathway, however, can be activated spontaneously. Both pathways lead to the splitting of the C3 protein, of which the larger fragment C3b activates the rest of the cascade, culminating in the creation of MAC.
Specifically, a fragment from C5 joins with proteins C6, C7, C8, and C9 to form MAC. This complex creates a pore in the pathogen's plasma membrane. Water then rushes into the hyperosmotic cytoplasm, causing the cell to swell and eventually lyse, effectively killing the pathogen.