Complement System Part 1 - Overview and Actions to Fight Infections

Summary

The complement system is a key part of the body's innate immune response, made up of over 30 proteins produced by the liver. These proteins circulate in an inactive state and, upon activation, trigger a cascade of reactions that help fight infections. The complement system aids in inflammation, enhances phagocytosis (eating of bacteria), and causes bacterial cell lysis. It activates in stages, starting with C3, which leads to the activation of other proteins, including C5, ultimately forming the membrane attack complex to destroy bacteria.

Takeaways

• 😀 The complement system is a crucial part of the body's innate immune response, consisting of over 30 proteins produced by the liver.
• 😀 Complement proteins circulate in an inactive state and are activated through a cascade reaction where one protein activates another.
• 😀 The complement system assists the immune response by enhancing inflammation, opsonization (phagocytosis), and causing bacterial lysis (cytolysis).
• 😀 Complement proteins are labeled with a 'C' followed by a number (e.g., C3, C5), and when activated, they are split into fragments labeled 'A' and 'B' (e.g., C3a, C3b).
• 😀 Activation of C3 splits it into C3a and C3b, where C3b acts as an opsonin, making it easier for phagocytes to recognize and engulf bacteria.
• 😀 C3a functions as an anaphylatoxin, causing mast cells to release histamine, which contributes to inflammation by dilating blood vessels and allowing immune cells to enter tissues.
• 😀 C3b also activates C5, which splits into C5a and C5b, where C5a is another anaphylatoxin that triggers histamine release and acts as a chemotactic factor for phagocytes.
• 😀 C5b, through its interaction with C6, C7, C8, and C9, forms the membrane attack complex (MAC), which creates a pore in bacterial membranes leading to bacterial lysis.
• 😀 The MAC (formed by C5b-C9) disrupts bacterial cell membranes by allowing extracellular fluid to enter, causing the bacteria to rupture and die.
• 😀 The complement system's amplification cascade significantly enhances the body's ability to fight infections by rapidly increasing the immune response.
• 😀 The video concludes part one of the complement system series and invites viewers to continue learning about the initial activation steps of the system in part two.

Q & A

What is the complement system and what is its role in the body?

The complement system is a series of more than 30 proteins produced by the liver that circulate in the blood in an inactive state. It plays a crucial role in the body's innate immune response by helping to fight infections.

Why is the complement system named 'complement'?

The complement system is named so because it helps or 'complements' the body's immune system in fighting infections.

How are complement proteins activated?

Complement proteins are activated through a cascade reaction, where one activated protein triggers the activation of another. This process amplifies the immune response and leads to more complement proteins being formed.

What are the three main ways in which the complement system helps to destroy microbes?

The complement system assists in destroying microbes through three primary methods: inflammation, enhancing phagocytosis (also called opsonization), and causing bacterial lysis (cytolysis).

What is opsonization and how does it work in the complement system?

Opsonization is the process by which complement proteins, like C3b, coat bacteria or foreign material to make it easier for phagocytes to recognize and ingest them. It's compared to spreading jam or honey on toast to make it more appealing to eat.

What role does C3 play in the complement system?

When C3 is activated, it splits into C3a and C3b. C3b acts as an opsonin, making it easier for phagocytes to recognize and consume bacteria, while C3a acts as an anaphylatoxin, triggering histamine release from mast cells.

What are the effects of histamine release in the immune response?

Histamine causes vasodilation (increasing blood flow to the area) and endothelial retraction (allowing immune cells to leave the blood and enter the tissue), both of which enhance inflammation and the immune response.

How does C5 contribute to the activation of the complement system?

C5, when activated, splits into C5a and C5b. C5a acts as an anaphylatoxin, similar to C3a, and also serves as a chemotactic factor, attracting phagocytes to the site of infection. C5b starts the activation of other complement proteins involved in bacterial lysis.

What is the membrane attack complex (MAC), and how is it formed?

The membrane attack complex (MAC) is formed when complement proteins C5b, C6, C7, C8, and C9 work together to insert into a bacterial membrane. This forms a channel that allows extracellular fluid to enter the bacteria, leading to its lysis.

What is the significance of the complement system in fighting bacterial infections?

The complement system plays a vital role in fighting bacterial infections by promoting inflammation, enhancing phagocytosis, and directly causing bacterial cell lysis, all of which help the body eliminate harmful pathogens efficiently.