Complement System Pathways Flashcards

Questions and Answers

What is the difference between the classical pathway and the alternative pathway?

The classical pathway requires an antibody to combine with an antigen to form an antibody-antigen complex for activation of C1 protein cascade, whereas the alternative pathway does not.

When is the alternative pathway triggered?

Even in the absence of antibody-antigen complexes.

How is the alternative pathway triggered?

Protein C3 can spontaneously convert into C3b.

What is the function of C3b in the alternative pathway?

C3b binds to healthy cell membranes and is inactivated by inhibitory membrane proteins, but reacts with Factor B on pathogens to form the C3b-Bb complex. Signup and view all the answers

What is the function of the C3b-Bb complex?

All of the above Signup and view all the answers

How is the complement system regulated?

Through proteins like Factor I, Factor H, and C1-inhibitor. Signup and view all the answers

What is the function of Factor I?

Inactivates C3b so it cannot combine with Factor B to form the C3b-Bb complex. Signup and view all the answers

What is the function of Factor H?

Removes the Bb protein from the C3b-Bb complex. Signup and view all the answers

What is the function of the C1 inhibitor?

Binds to the C1 complex of the classical pathway and inhibits the activation of C2 and C4, preventing C3 activation. Signup and view all the answers

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.