Complement System Pathways Flashcards

Study Notes

Complement System Pathways

Classical vs. Alternative Pathway
- Classical pathway necessitates an antibody-antigen complex for activation of the C1 protein cascade.
- Alternative pathway activates independently, without the need for antibody presence.
Triggering the Alternative Pathway
- Initiated in the absence of antibody-antigen complexes, allowing for immediate immune response.
Activation Process
- C3 protein can spontaneously convert to its active form, C3b, initiating the pathway.

Role of C3b

Interaction with Healthy Cells
- In healthy circumstances, C3b binds to healthy cell membranes and is inactivated by inhibitory membrane proteins.
- On pathogen surfaces, the absence of inhibitory proteins allows C3b to remain active.
Formation of C3b-Bb Complex
- C3b interacts with Factor B to create the C3b-Bb complex, which is crucial for the alternative pathway.

C3b-Bb Complex Functions

C3 Convertase Functionality
- Acts on additional C3 molecules to create the C3b-Bb-C3b complex, facilitating activation of C5.
- C5 contributes to the formation of the membrane attack complex, enabling lysis of infected cells.
- Promotes the generation of more C3b, amplifying the response.

Regulation of the Complement System

Key Regulatory Proteins
- Factor I: Inactivates C3b, preventing its combination with Factor B, thus halting the pathway.
- Factor H: Eliminates Bb protein from the C3b-Bb complex, regulating the cascade.
- C1 Inhibitor: Binds to the C1 complex of the classical pathway, preventing the activation of C2 and C4, and thereby C3.

Functions of Regulatory Proteins

Factor I
- Essential for regulating complement activation by inactivating C3b to impede the formation of the C3b-Bb complex.
Factor H
- Plays a critical role in controlling the complement pathway by removing Bb from complexes, thereby preventing overactivation.
C1 Inhibitor
- Prevents activation of the classical pathway components (C2 and C4), crucial for maintaining immune balance.

Description

This quiz explores the differences between the classical and alternative pathways of the complement system. It outlines the activation mechanisms and conditions under which each pathway is triggered. Perfect for students studying immunology and the complement system.