Classical, Alternative, and Lectin Complement Pathways

Core Idea

The three complement pathways differ in activation triggers and initial components but converge at C3 convertase formation. The classical pathway is triggered by IgG or IgM bound to antigen. The alternative pathway is activated by microbial polysaccharides and involves factor B. The lectin pathway is initiated by mannan-binding lectin binding to carbohydrates on pathogens.

How It's Best Learned

Use flow diagrams tracking C3 and C5 convertase formation across all three pathways. Practice labeling each pathway's unique early components and identifying convergence points.

Common Misconceptions

The classical pathway is not the evolutionary ancestor; all three coevolved. The alternative pathway is not a backup—it is constitutively active at low levels and provides a first-line defense.

Explainer

From the complement system overview, you know that complement is a cascade of plasma proteins that, when activated, opsonize pathogens, recruit inflammatory cells, and directly lyse microbes through the membrane attack complex. The key question now is: how does the cascade get started? There are three distinct activation pathways — classical, lectin, and alternative — each triggered by different molecular signals, but all converging on the same critical step: formation of a C3 convertase that cleaves the abundant plasma protein C3 into C3a (an inflammatory mediator) and C3b (an opsonin that coats pathogen surfaces).

The classical pathway links complement to the adaptive immune system. It begins when the C1 complex (C1q, C1r, C1s) binds to the Fc regions of IgG or IgM antibodies that are already bound to an antigen on a pathogen surface. C1q has six globular heads that must engage multiple antibody Fc regions simultaneously — this is why IgM (a pentamer with five Fc regions) is so efficient at activating complement, while IgG activation requires multiple antibodies clustered closely together on the same surface. Binding activates C1r, which cleaves C1s, which then sequentially cleaves C4 and C2 to form the classical pathway C3 convertase, C4b2a. The beauty of requiring antibody binding first is specificity: the classical pathway only fires where adaptive immunity has already identified a target.

The lectin pathway achieves a similar outcome without antibodies. Instead of C1q, it uses mannose-binding lectin (MBL) or ficolins — soluble pattern recognition molecules that bind carbohydrate structures commonly found on bacterial and fungal surfaces but rare on mammalian cells. MBL associates with serine proteases called MASPs (MBL-associated serine proteases), which function analogously to C1r and C1s: upon MBL binding to a pathogen surface, MASP-2 cleaves C4 and C2, generating the same C4b2a convertase as the classical pathway. The lectin pathway is essentially an innate version of the classical pathway — it recognizes pathogen surface patterns directly rather than waiting for antibody production.

The alternative pathway is fundamentally different in its logic. Rather than being triggered by a specific recognition event, it relies on constitutive low-level activation through spontaneous hydrolysis of C3 in plasma (called "tick-over"). The resulting C3(H₂O) binds factor B, which is cleaved by factor D to generate a fluid-phase C3 convertase. The C3b generated by this convertase deposits randomly on nearby surfaces. On host cells, regulatory proteins (factor H, DAF, MCP) rapidly inactivate deposited C3b. On pathogen surfaces, which lack these regulators, C3b persists, binds more factor B, and generates surface-bound alternative pathway C3 convertase (C3bBb), stabilized by properdin. This creates a powerful amplification loop: each C3 convertase generates more C3b, which forms more convertase. The alternative pathway thus acts as both a first-line sensor and an amplifier for the other two pathways — once any pathway deposits C3b on a surface, the alternative pathway loop massively amplifies the response.

All three pathways converge at C3 convertase, and from there the cascade proceeds identically: C3b associates with either convertase to form a C5 convertase, which cleaves C5 into C5a (a potent inflammatory chemoattractant) and C5b (which initiates assembly of the membrane attack complex, C5b-C9). The three pathways thus represent three different surveillance strategies — adaptive antibody-dependent, innate carbohydrate-recognizing, and constitutive surface-sampling — all feeding into a single effector cascade.