Complement activation without antibodies Polysaccharides on microbial surfaces can directly activate the complement system in the absence of antibodies via which pathway?

Introduction:
The complement system can be triggered by multiple pathways to enhance pathogen clearance. Not all pathways require antibodies; this question focuses on antibody-independent mechanisms on microbial surfaces.

Given Data / Assumptions:

• Microbial polysaccharides and other surface patterns can promote spontaneous C3 hydrolysis and deposition.
• The alternative pathway amplifies complement activation on susceptible surfaces without antibodies.
• The classical pathway usually requires antigen–antibody complexes.

Concept / Approach:

Spontaneous C3 “tickover” produces C3b that can covalently attach to microbial surfaces. Properdin and factor B/D stabilize convertases on non-host surfaces, leading to opsonization and membrane attack complex formation independently of antibodies.

Step-by-Step Solution:

Verification / Alternative check:

Immunology references describe alternative pathway activation on bacteria, fungi, and some viruses without pre-existing antibodies, contributing to early innate defense.

Why Other Options Are Wrong:

Option B/E: These incorrectly impose antibody requirements typical of the classical pathway.

Option C: “Metabolic pathway” is not a complement activation mechanism.

Option D: Incorrect because a valid pathway exists (alternative pathway).

Common Pitfalls:

Assuming complement always needs antibodies; overlooking lectin and alternative pathways that act early in infection.

Final Answer:

Antibody-independent activation via the alternative pathway