When I was an undergraduate biology student, one of my favorite topics was the complement system in immunology. The complement cascade is an array of sequentially interacting proteins that serve a vital role in innate immune responses. The complement cascade can be activated via interactions with antibody-antigen complexes. Proteins involved in the complement cascade react with one another and with components of the target cell, marking pathogen cells for recognition by phagocytes or inducing cell membrane damage, leakage of contents, and cell lysis. 

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