Immunology - Innate Immunity (Complement System Overview)

Summary

In this video, the complement system is explored as a crucial component of the innate immune response. It details how complement proteins, produced by the liver and circulating in the bloodstream, activate in response to pathogens, performing vital roles like opsonization, enhancing inflammation, and forming the membrane attack complex to destroy invaders. The video covers the three complement pathways: classical, lectin, and alternative, explaining how each activates C3, leading to pathogen destruction. A thorough understanding of these processes emphasizes the complement system's importance in defending the body against infections.

Takeaways

• 😀 The complement system is a crucial part of the innate immune system, made up of over 30 proteins that help protect the body from pathogens.
• 😀 Complement proteins circulate in the bloodstream in an inactive form and only activate when they encounter a pathogen.
• 😀 The main functions of the complement system include opsonization, the formation of the membrane attack complex (MAC), and enhancing inflammation.
• 😀 Opsonization is the process where complement proteins coat pathogens, making them easier for phagocytes, like macrophages, to engulf.
• 😀 The membrane attack complex (MAC) is formed when complement proteins create holes in the pathogen’s membrane, causing it to lyse (self-destruct).
• 😀 The complement system includes three main activation pathways: the classical pathway, the lectin pathway, and the alternative pathway.
• 😀 The classical pathway is activated when antibodies bind to antigens on a pathogen's surface.
• 😀 The lectin pathway is similar to the classical pathway but is triggered by proteins binding to carbohydrates on the pathogen's surface.
• 😀 The alternative pathway amplifies the other two pathways and is activated when C3b binds to the pathogen's surface.
• 😀 C3 is a central protein in the complement system, and when it is activated into C3a and C3b, it plays a key role in both inflammation and pathogen destruction.
• 😀 C5a enhances inflammation by stimulating histamine release, while C5b helps initiate the formation of the MAC, leading to pathogen destruction.

Q & A

What is the primary function of complement proteins in the immune system?

The primary function of complement proteins is to help the immune system destroy invading pathogens. They perform opsonization (coating pathogens to make them easier to engulf), form membrane attack complexes (which create holes in pathogen membranes), and enhance inflammation.

What is opsonization and how does it help the immune system?

Opsonization is the process where complement proteins coat the surface of a pathogen. This makes it easier for phagocytes, such as macrophages, to engulf the pathogen because the macrophages have receptors that recognize these complement proteins.

How does the membrane attack complex (MAC) work?

The membrane attack complex (MAC) is formed when complement proteins create holes in the pathogen's membrane. This disrupts the osmotic balance of the pathogen, causing it to burst and die.

Where are complement proteins produced and how are they activated?

Complement proteins are produced by the liver and circulate in the bloodstream in an inactive form. They become activated when they encounter a pathogen, initiating a cascade of events that help eliminate the threat.

What are the three main complement pathways?

The three main complement pathways are the classical pathway, the alternative pathway, and the lectin pathway. These pathways involve different proteins but all lead to the activation of C3, which is crucial for immune defense.

What role does C3 play in the complement system?

C3 is a central protein in the complement system. It is activated into C3a and C3b, where C3a enhances inflammation, and C3b plays a key role in opsonization and the formation of the membrane attack complex.

How is the classical pathway activated?

The classical pathway is activated when antibodies bind to antigens on the surface of a pathogen. Complement proteins C1q, C1r, C1s then bind to the antibody, triggering a cascade of events that lead to the activation of C3.

What is the difference between the classical and lectin pathways?

Both the classical and lectin pathways activate the C3 convertase (C4b2a complex), but they are initiated differently. The classical pathway is triggered by antibodies binding to antigens, while the lectin pathway is triggered by lectins binding to carbohydrates on the pathogen's surface.

What is the function of C5a in the immune response?

C5a is an important complement protein that enhances inflammation by stimulating mast cells to release histamine. Histamine increases vascular permeability, allowing white blood cells (leukocytes) to more easily pass through blood vessel walls to reach the site of infection.

How does the alternative pathway contribute to complement activation?

The alternative pathway is enhanced by the activation of C3b from the classical or lectin pathways. C3b then binds to the surface of the pathogen and helps form additional C3 convertases, amplifying the immune response.