Three Functions for C3 Cleavage Products

The cleavage products of complement C3 protein, namely C3a and C3b, play diverse roles in the immune response, functioning as anaphylatoxins, opsonin proteins, and initiators of the membrane attack complex (MAC). Here’s an explanation of how these cleavage products contribute to each of these activities:

1. Anaphylatoxins (C3a):
   • Function: C3a is released during the cleavage of C3, and it acts as an anaphylatoxin. Anaphylatoxins are molecules that induce rapid, localized inflammatory responses.
   • Mechanism:
     • C3a binds to its receptor (C3aR) on the surface of various immune cells, including mast cells and basophils.
     • Binding triggers the release of histamine and other inflammatory mediators from these cells.
     • This results in vasodilation, increased vascular permeability, and the recruitment of immune cells to the site of infection or injury.
   • Outcome: The inflammatory response helps in the recruitment and activation of immune cells to combat pathogens.
2. Opsonin Proteins (C3b):
   • Function: C3b serves as an opsonin, a molecule that enhances the phagocytosis of pathogens by phagocytic cells such as macrophages and neutrophils.
   • Mechanism:
     • C3b binds covalently to the surface of pathogens or immune complexes.
     • The presence of C3b enhances the recognition of the pathogen by phagocytic cells through complement receptors on their surfaces.
     • Phagocytes recognize the opsonized pathogen more efficiently, leading to engulfment and destruction.
   • Outcome: Improved phagocytosis and elimination of pathogens.
3. Initiating the Membrane Attack Complex (MAC) (C3b):
   • Function: C3b is a critical component in the formation of the membrane attack complex (MAC), which is responsible for lysing bacterial and viral membranes.
   • Mechanism:
     • C3b, when bound to a pathogen surface, serves as a platform for the assembly of the C5 convertase (C4b2a3b) of the classical and lectin pathways.
     • C5 convertase cleaves C5 into C5a and C5b.
     • C5b associates with C6, C7, C8, and multiple molecules of C9 to form the MAC, which creates pores in the pathogen’s membrane, leading to lysis.
   • Outcome: Destruction of the pathogen’s membrane, preventing its survival and replication.

In summary, the cleavage products of complement C3, C3a, and C3b, exhibit diverse functions in the immune response. C3a acts as an anaphylatoxin, inducing inflammation, while C3b serves as an opsonin and plays a central role in initiating the formation of the membrane attack complex for pathogen lysis. These activities collectively contribute to the efficient elimination of pathogens and maintenance of immune homeostasis.

Next Topic: How Do Phagocytes Eat Opsonized Cells?

Source: ChatGPT response prompted and edited by Joel Graff.

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