What is the Difference Between Classical Alternative and Lectin Pathway?
🆚 Go to Comparative Table 🆚The classical, alternative, and lectin pathways are three distinct biochemical mechanisms that activate the complement system, a part of the immune system that enhances the ability of the body to clear pathogens. Here are the main differences between these pathways:
Classical Pathway:
- Requires antigen-antibody complexes (immune complexes) for activation, typically involving IgM and IgG isotypes of antibodies.
- Initiated by a plasma protein called C1q, which detects antibodies bound to the surface of a microbe.
- More directly involved in the adaptive immune response.
Alternative Pathway:
- Does not depend on a pathogen-binding protein for its initiation; instead, it is initiated through the spontaneous hydrolysis of C3.
- Activated on microbial cell surfaces in the absence of antibodies.
- Can act as an amplification loop for all three pathways, as it is initiated by the binding of C3b.
Lectin Pathway (MB-Lectin Pathway):
- Triggered by a plasma protein called mannan-binding lectin (MBL), which is a collectin similar to C1q and binds specifically to mannose residues on microbes.
- Activated by C3 hydrolysis or antigens without the need for antibodies.
- Homologous to the classical pathway, using a protein very similar to C1q to trigger the complement cascade.
All three pathways ultimately generate homologous variants of the protease C3-convertase, which cleaves and activates component C3, creating C3a and C3b.
Comparative Table: Classical Alternative vs Lectin Pathway
Here is a table comparing the differences between the classical, alternative, and lectin pathways of the complement system:
Feature | Classical Pathway | Alternative Pathway | Lectin Pathway |
---|---|---|---|
Initiation | Activated by certain isotypes of antibodies bound to antigens | Activated on microbial cell surfaces in the absence of antibody | Activated by mannose-binding lectin binding to mannose residues on microbes |
Initiation Protein | C1s (C1q, C1r, C1s) | C3b | MASP-2 |
C4 and C2 Activation | Involved in the classical pathway | Absent in the alternative pathway | Involved in the lectin pathway |
Function | Acts as the effector arm of adaptive immunity | Part of the immune system with unique proteins for initiation | Activated by a plasma lectin that binds to mannose residues on microbes |
The classical pathway is initiated through the binding of antigen-antibody complexes, while the alternative pathway is initiated through the binding of C3b onto foreign surfaces. The lectin pathway is initiated through mannose-binding lectin binding to mannose residues on microbes.
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