Which of the following initiates the lectin pathway by binding to carbohydrate patterns on pathogens?

• A. Mannose-binding protein
• B. C1q
• C. C3
• D. Factor B

Answer: (A)

The lectin pathway begins when a pattern-recognition molecule detects carbohydrate patterns on microbes. Mannose-binding protein is a collectin that binds to sugars like mannose on the surface of many pathogens. This binding activates MASP enzymes, which cleave C4 and C2 to form the C4b2a complex, the C3 convertase for the lectin pathway. From there, the cascade proceeds to opsonization and lysis similar to the classical pathway.

C1q, by contrast, initiates the classical pathway by recognizing antibodies or certain acute-phase proteins, not pathogen carbohydrate patterns. C3 is a central component in all pathways but does not by itself initiate the lectin pathway. Factor B is part of the alternative pathway, helping form the Bb-containing C3 convertase.