How Do Phagocytes Eat Opsonized Cells?

Opsonins are molecules that enhance the phagocytosis of pathogens by phagocytic cells, and C3b plays a crucial role in the opsonization process.

Roles of C3b as an Opsonin

• Opsonization: C3b can covalently bind to the surface of pathogens or immune complexes.
• Enhanced Recognition: The presence of C3b on the pathogen’s surface enhances the recognition of the pathogen by phagocytic cells, such as macrophages and neutrophils.
• Phagocytosis: Phagocytes have receptors, including complement receptors, that recognize and bind to opsonized pathogens more efficiently.
• Engulfment and Destruction: The recognition and binding of opsonized pathogens by phagocytes lead to their engulfment, followed by destruction within the phagolysosome.

Why Opsonization Increases Phagocytosis

Opsonization increases the likelihood of phagocytosis because it facilitates the interaction between the phagocyte and the pathogen. The opsonin-coated pathogen becomes a more recognizable target for phagocytic cells, improving the efficiency and speed of the phagocytic process.

Roles of Complement Receptors (CR1, CR2, and CR3)

1. Complement Receptor 1 (CR1, also known as CD35):
   • Expression: Found on erythrocytes, macrophages, and other immune cells.
   • Function:
     • Binds to C3b and C4b that are deposited on immune complexes.
     • Enhances the clearance of immune complexes from the bloodstream.
     • Acts as a cofactor for the degradation of C3b and C4b by factor I.
2. Complement Receptor 2 (CR2, also known as CD21):
   • Expression: Predominantly on B cells.
   • Function:
     • Binds to C3d fragments that are covalently attached to antigens.
     • Plays a role in enhancing B cell activation when antigen-antibody complexes are opsonized with C3d.
3. Complement Receptor 3 (CR3, also known as Mac-1 or integrin alphaM-beta2):
   • Expression: Found on macrophages, neutrophils, and other phagocytic cells.
   • Function:
     • Binds to iC3b (inactive form of C3b) on opsonized pathogens.
     • Mediates phagocytosis of opsonized pathogens.
     • Contributes to the clearance of complement-coated particles.

Formation of Membrane Attack Complexes

Source: ChatGPT response prompted and edited by Joel Graff.

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