Complement simplified

Page 1: Alternative Pathway Initiation

• Formation and Action of Soluble C3 Convertase (iC3Bb)

  • iC3Bb initiates the alternative pathway.

  • Components involved:

    • C3 (with parts labeled C3a, C3b)

    • Ba and Bb components

  • Thioester bond plays a critical role.

  • Diagram illustrates interaction with pathogen surface.

Page 2: C3 Convertase Formation

• C3 Convertase (C3bBb)

  • Formation of C3 convertase at the pathogen surface.

  • Components: C3, Bb, and C3b.

  • Function: Cleaves additional C3 to amplify immune response.

  • Diagram depicts the process of convertase formation.

Page 3: Classical Pathway Initiation

• Classical Pathway

  • Initiated by antibodies (IgM or IgG).

  • Binding of antibodies leads to activation of C1q.

  • C1r is activated triggering the cleavage of C1s.

  • C1s cleaves C4 and C2 similar to MASP2 in the lectin pathway.

  • Formation of C3 convertase = C4b2a which leads to further complement activation.

Page 4: Lectin Pathway

• Comparison of Pathways

  • No distinct lectin C3 convertase identified.

  • C3 Convertases:

    • Alternative: C3bBb

    • Classical: C4b2a

  • Highlights homology between the two types of C3 convertases.

Page 5: C5 Activation

• Activation via Alternative C5 Convertase

  • C5 convertase formed from:

    • C5b = C3b2Bb

  • Initiates the next stage of the immune response at the pathogen surface.

  • Diagram shows the conversion process involving C5.

Page 6: Membrane-Attack Complex

• Formation of the Membrane-Attack Complex (MAC)

  • Components involved:

    • C5b, C6, C7, C8, C9

  • C5b initiates the assembly of the MAC.

  • Proteins in serum and their functions include:

    • C6: Stabilizes C5b, allows C7 binding.

    • C7: Binds to C5b6, attaches to cell membrane.

    • C8: Inserts into cell membrane.

    • C9: Polymerizes to form the complex that disrupts the cell integrity.

  • Leads to potential cell death by forming a membrane-spanning channel.