𝐀𝐜𝐭𝐢𝐯𝐚𝐭𝐢𝐨𝐧 𝐎𝐟 𝐂𝐨𝐦𝐩𝐥𝐞𝐦𝐞𝐧𝐭 𝐒𝐲𝐬𝐭𝐞𝐦 𝐈𝐦𝐦𝐮𝐧𝐨𝐥𝐨𝐠𝐲 [𝐇𝐃 𝐀𝐧𝐢𝐦𝐚𝐭𝐢𝐨𝐧 ]

Summary

The complement system is a group of serum proteins that play a crucial role in immune responses by triggering inflammation, destroying cells, and facilitating opsonization. It can be activated through the classical or alternative pathways. In the classical pathway, complement proteins C1, C2, and C4 form a protease complex that activates C3. In the alternative pathway, C3 is activated through interactions with foreign molecules, forming C3 convertase. Both pathways lead to the production of C3a and C3b, which stimulate inflammation and aid in phagocytosis. The cascade culminates in the formation of the membrane attack complex (MAC), which causes cell lysis.

Takeaways

• 😀 The complement system consists of serum proteins that activate inflammation, destroy cells, and participate in opsonization.
• 😀 Complement can be activated by various foreign molecules, triggering a cascade of reactions.
• 😀 The major complement components are C1 through C9, named in the order they were discovered, not in their functional order.
• 😀 The complement cascade can be activated through the classical or alternative pathways.
• 😀 In the classical pathway, C1 is activated when it binds to an antigen-antibody complex, leading to the cleavage of C2 and C4.
• 😀 The activated C1 cleaves C2 and C4, forming C3 convertase, which subsequently cleaves C3 into C3a and C3b.
• 😀 The alternative pathway involves antigens like endotoxins and cell wall components reacting with C3b to form a C3 convertase.
• 😀 Both pathways ultimately result in the cleavage of C3, with C3a promoting inflammation and C3b enhancing opsonization.
• 😀 C3b binds to microorganisms, marking them for phagocytosis and enhancing their susceptibility to immune attack.
• 😀 The formation of C5 convertase leads to the cleavage of C5, resulting in C5a (inflammation and chemoattraction) and C5b, which forms the membrane attack complex (MAC) that lyses cells.

Q & A

What is the complement system and what role does it play in the immune response?

The complement system is a group of serum proteins that activate inflammation, destroy cells, and participate in opsonization. It is an essential part of the immune response that helps to fight infections.

How are complement proteins activated?

Complement proteins are activated by foreign molecules, triggering a cascade of reactions. These proteins respond sequentially, and the major components are named C1 through C9 in the order they were discovered.

What are the two pathways that can activate the complement cascade?

The complement cascade can be activated by the classical pathway or the alternative pathway.

How does the classical pathway activate the complement system?

In the classical pathway, C1 becomes activated when it binds to an antigen-antibody complex. This activated C1 then cleaves C2 and C4, resulting in the formation of C3 convertase, which cleaves C3 into C3a and C3b.

What role does C3 convertase play in complement activation?

C3 convertase cleaves C3 into C3a and C3b. C3a stimulates inflammation, while C3b facilitates opsonization and enhances phagocytosis of microorganisms.

How does the alternative pathway differ from the classical pathway in activating complement?

The alternative pathway is activated by antigens such as endotoxins, polysaccharides, or cell wall components, leading to the formation of C3 convertase through interactions with factor B, factor D, and properdin.

What is opsonization, and how does it relate to C3b?

Opsonization is the process of coating microorganisms with C3b, making them more susceptible to phagocytosis. Phagocytes have binding sites for C3b, and when it is bound to microorganisms, it enhances their recognition and destruction.

What happens after C3 is cleaved into C3a and C3b?

After C3 is cleaved, C3a stimulates inflammation, and C3b forms more C3 convertase, leading to the continuous amplification of the complement cascade and aiding in opsonization and phagocytosis.

How does the formation of C5 convertase contribute to the complement cascade?

The addition of properdin into C3 convertase leads to the formation of C5 convertase, which cleaves C5 into C5a and C5b. C5a enhances inflammation and acts as a chemoattractant, while C5b plays a role in the formation of the membrane attack complex.

What is the membrane attack complex and how does it affect cells?

The membrane attack complex (MAC) is formed when C5b binds to C6, C7, C8, and C9. This complex forms a pore in the cell membrane, leading to cell lysis by disrupting the membrane integrity.