[Imunologia] 4 - Sistema do Complemento

Summary

In this video, the presenter explains the complement system, a cascade of proteins in the plasma that plays a crucial role in immune defense. The complement system aids in pathogen destruction, facilitates phagocytosis through opsonization, and triggers inflammation. The presenter details the activation of the complement system through three main pathways: classical, alternative, and lectin. Each pathway converges on the cleavage of C3, leading to immune responses like opsonization and membrane attack complex formation. The video emphasizes the system's importance in combating infections and enhancing the immune response, setting the stage for further discussions on inflammation.

Takeaways

• 😀 The complement system is a cascade of proteins in plasma that, when activated, can destroy infected cells and pathogens, and trigger an inflammatory response.
• 😀 Complement proteins facilitate the phagocytosis of pathogens through a process called opsonization, where molecules mark the pathogen for easier identification and destruction.
• 😀 Opsonins, which include complement proteins and antibodies, help immune cells like macrophages and dendritic cells recognize and engulf pathogens.
• 😀 The complement system also plays a role in removing immune complexes, formed when antigens bind to antibodies.
• 😀 Complement activation results in necrosis, not apoptosis, which releases intracellular contents that can further stimulate inflammation.
• 😀 The complement system can be activated through three pathways: classical, alternative, and lectin, but they all converge at the cleavage of C3 into C3a and C3b.
• 😀 C3b acts as an opsonin, marking pathogens for destruction, while C3a triggers the inflammatory response.
• 😀 The C5 protein, once cleaved into C5a and C5b, also plays a key role in inflammation and contributes to the formation of the membrane attack complex (MAC).
• 😀 The alternative pathway begins with the spontaneous cleavage of C3, which is enhanced when a pathogen is present for C3b to opsonize.
• 😀 In the classical pathway, complement activation starts with the binding of C1 to an antigen-antibody complex, which leads to the cleavage of C3.
• 😀 The lectin pathway, similar to the classical pathway, begins when lectin binds to mannose on the surface of a pathogen, starting a complement cascade similar to the classical pathway.
• 😀 All three pathways eventually form C3 convertase and C5 convertase, which catalyze further complement cleavage and the formation of the membrane attack complex, causing cell lysis via osmotic imbalance.

Q & A

What is the complement system?

The complement system is a cascade of proteins present in plasma that, when activated, help eliminate pathogens, trigger inflammatory responses, and assist in immune cell functions like phagocytosis.

What are the main functions of the complement system?

The complement system primarily promotes the death of infected cells and pathogens, triggers inflammation, and enhances phagocytosis by marking pathogens for recognition.

What is opsonization in the context of the complement system?

Opsonization refers to the process where molecules like C3b from the complement system attach to pathogens, marking them for easier engulfment by immune cells like macrophages and dendritic cells.

What are opsonins and which molecules are considered opsonins in the complement system?

Opsonins are molecules that mark pathogens for phagocytosis. In the complement system, C3b and antibodies are the main opsonins.

How does the complement system activate an inflammatory response?

C3a and C5a fragments released during the complement cascade are anaphylatoxins that trigger the recruitment of immune cells like neutrophils to the site of infection, thereby promoting inflammation.

How does the complement system cause cell lysis?

The complement system causes cell lysis through the formation of the membrane attack complex (MAC), where proteins like C5b recruit other proteins (C6, C7, C8, C9), which form pores in the pathogen's membrane, causing osmotic imbalance and cell death.

What are the three pathways through which the complement system is activated?

The complement system can be activated through three pathways: the classical pathway, the alternative pathway, and the lectin pathway.

What is the key event that occurs after activation of any of the complement system's pathways?

Regardless of the pathway, all three converge on the cleavage of the C3 protein into C3a and C3b, which initiate downstream effects like opsonization and inflammation.

What is the difference between the C3 convertase and C5 convertase in the complement system?

C3 convertase is responsible for cleaving C3 into C3a and C3b, while C5 convertase cleaves C5 into C5a and C5b. Both enzymes play essential roles in triggering inflammation and the formation of the membrane attack complex.

How does the alternative pathway of complement activation begin?

The alternative pathway begins with the spontaneous cleavage of C3 in the plasma. If a pathogen is present, C3b binds to it, triggering the continuation of the complement cascade.

What distinguishes the lectin pathway from the classical pathway in complement activation?

The lectin pathway begins when a lectin protein binds to mannose residues on the surface of a pathogen, whereas the classical pathway starts when C1 binds to an antigen-antibody complex.