lectin pathway
Summary
The lectin pathway is an alternative complement activation route triggered by mannose-binding lectin (MBL) and ficolins binding to pathogen carbohydrates. It bypasses C1 but converges with classical pathway at C4, leading to C3 convertase formation and downstream complement effects.
Detail
The lectin pathway represents one of three complement activation routes, functioning as an innate immune mechanism that recognizes pathogen-associated molecular patterns (PAMPs). Mannose-binding lectin (MBL) and ficolins serve as pattern recognition molecules that bind to mannose, N-acetylglucosamine, and other carbohydrate residues commonly found on bacterial, viral, and fungal surfaces but absent on healthy host cells. Upon binding, MBL associates with MBL-associated serine proteases (MASPs), particularly MASP-1 and MASP-2, forming active enzyme complexes. MASP-2 cleaves C4 and C2, generating C4b2a (C3 convertase), which then cleaves C3 to initiate the common complement cascade leading to membrane attack complex formation, opsonization, and inflammatory mediator release. This pathway is particularly important in early childhood before adaptive immunity fully develops and in immunocompromised states. MBL deficiency, affecting 3-10% of the population, increases susceptibility to recurrent infections, especially respiratory tract infections in children. The pathway provides evolutionary advantage by rapidly recognizing and eliminating pathogens without requiring prior exposure or antibody production.
Sources
- Janeway's Immunobiology
- Abbas Basic Immunology
- Robbins Basic Pathology
- First Aid for the USMLE Step 1
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