How does the complement system enhance immunity, and what are the three pathways?

• A. It opsonizes pathogens, recruits inflammatory cells, and forms the membrane attack complex; pathways: classical, lectin, and alternative.
• B. It only lyses bacteria via membrane attack complex.
• C. It is activated solely by antibodies.
• D. It functions only in innate immunity and not in adaptive.

Answer: (B)

The complement system boosts immunity by three main actions: it tags pathogens for destruction (opsonization) using C3b to enhance phagocytosis, it releases inflammatory signals (C3a and C5a) that recruit and activate other immune cells, and it can directly kill certain microbes by forming the membrane attack complex (MAC) that creates pores in the pathogen’s membrane.

There are three activation routes that start this process. The classical pathway is triggered when antibodies bound to antigens are recognized, linking the complement system to the adaptive immune response. The lectin pathway is activated when mannose-binding lectin attaches to sugars on microbial surfaces, a mechanism that does not require antibodies. The alternative pathway is continuously activated at low levels and amplifies the response on microbial surfaces, again without needing antibodies. All paths converge to activate C3, generate C3a and C3b, proceed to C5 activation, and eventually lead to MAC formation.

So the best description is that complement enhances immunity through opsonization, inflammation, and MAC-mediated lysis, with three activation pathways—classical, lectin, and alternative. The idea that it only lyses via MAC misses the important roles of opsonization and inflammation and ignores the non-antibody triggers of the other pathways. The notion that it’s activated solely by antibodies is incorrect because two pathways do not require antibodies, and the system also functions beyond strictly innate immunity by interfacing with adaptive responses.