The Classical Pathway - The Complement System (Part IV) (FL-Immuno/16)

Summary

This video explains the classical pathway of the complement system, beginning with the role of antibodies in activating the pathway. The antibodies, produced by plasma cells, have a Y-shaped structure with antigen-binding sites and an FC region. The complement protein C1, a complex of C1q, C1r, and C1s, binds to the FC region of the antibody, initiating the classical pathway. This leads to the cleavage of C4 and C2, ultimately forming the C3 convertase complex (C4b2a), which is crucial in immune responses against pathogens. The process shares similarities with the lectin pathway, aiding in the understanding of immune activation.

Takeaways

• 😀 The classical pathway is a crucial part of the complement system, which helps in immune defense.
• 😀 Antibodies, which are soluble proteins produced by plasma cells, play an essential role in activating the classical pathway.
• 😀 The Y-shaped structure of an antibody has arms (antigen-binding sites) and a stem (FC region).
• 😀 The classical pathway begins when antibodies bind to specific antigens on the surface of a pathogen, forming antigen-antibody complexes.
• 😀 The C1 complex, made up of C1q, C1r, and C1s, binds to the FC region of the antibody once antigen binding sites are occupied.
• 😀 C4 is cleaved into C4a and C4b, with C4b binding to the microbial surface as part of the classical pathway.
• 😀 C2 is cleaved into C2a and C2b, with C2a binding to C4b to form the C3 convertase (C4b2a complex).
• 😀 The C3 convertase (C4b2a) is a key component in the activation of further complement proteins in the pathway.
• 😀 The process described in the classical pathway is similar to the lectin pathway, which also involves complement protein activation.
• 😀 Understanding the classical pathway's steps, including C1's role and the formation of C3 convertase, is vital in immunology for immune response activation.

Q & A

What is the classical pathway in the complement system?

The classical pathway is one of the three pathways in the complement system, which plays a crucial role in immune defense. It is activated when antibodies bind to specific antigens on a pathogen's surface.

How are antibodies structured in the classical pathway?

Antibodies have a Y-shaped structure. The arms of the Y represent antigen-binding sites, and the stem (or stock) represents the FC region, which is important for activating the classical pathway.

What is the role of C1 in the classical pathway?

C1 is a complex of proteins that plays a central role in the classical pathway. It consists of the large subunit C1q and two proteases, C1r and C1s, which are involved in cleaving other proteins in the cascade.

What initiates the activation of the classical pathway?

The activation begins when antibodies bind to specific antigens on the surface of a pathogen. This forms antigen-antibody complexes, which are critical for initiating the complement cascade.

What happens after antibodies bind to antigens on a pathogen's surface?

Once antibodies bind to their specific antigens, the FC region of the antibody becomes available for interaction. This is where the C1 complex binds, leading to the activation of the complement cascade.

What does the C1 complex bind to in the classical pathway?

The C1 complex binds to the FC region of the antibody, which becomes exposed after the antibody binds to its specific antigen.

How is the classical pathway similar to the lectin pathway?

Both pathways share similar steps in their activation process. In both, a complement protein binds to an antibody or other molecules to trigger the cleavage of subsequent complement proteins, leading to the formation of C3 convertase.

What happens to complement protein C4 during the classical pathway?

C4 is cleaved into C4a and C4b. C4b then binds to the surface of the pathogen, marking it for further complement activation.

What is the role of C2 in the classical pathway?

C2 is cleaved into two fragments, C2a and C2b. C2a binds to C4b, forming the C4b2a complex, which is known as C3 convertase, an important enzyme in the complement cascade.

What is the C4b2a complex in the classical pathway?

The C4b2a complex is the C3 convertase formed by the binding of C2a to C4b. This complex is responsible for the cleavage of C3 into C3a and C3b, leading to further amplification of the immune response.