Complement System and Its Biological Effects

Study Notes

Amplifies humoral response: Complement enhances the effectiveness of antibody-mediated defenses.
Lyses cells: The membrane attack complex (MAC) directly destroys pathogen cell membranes.
Inflammatory response: Complement components trigger inflammation, attracting immune cells to the site of infection.
Opsonisation: Complement proteins (e.g., C3b, C4b) coat pathogens, making them more readily recognized and engulfed by phagocytic cells.
Clearance of immune complexes: Complement helps remove immune complexes from circulation to prevent tissue damage.

MAC targets a broad spectrum of cells: The MAC is effective against a wide range of pathogens involved in infections.
Gram-positive bacteria resistance: Gram-positive bacteria often have thicker cell walls (peptidoglycan), offering protection against MAC formation.
Evasion mechanisms: Certain pathogens have evolved strategies to evade complement-mediated killing.

Some pathogens have mechanisms to obstruct the complement cascade, such as altering the composition of their cell walls to prevent MAC insertion or producing enzymes that inactivate complement components.
Details of mechanisms by which specific pathogens can limit complement activation are listed.

Anaphylatoxins: Complement components (e.g., C3a, C5a) activate mast cells and basophils, resulting in inflammation.
Vascular permeability: Complement activation enhances the permeability of blood vessels to allow immune cells to enter the infected tissue.
Chemotaxis: Complement components attract immune cells to the site of infection.

C3b and C4b mediate phagocytosis: These complement components act as opsonins, enhancing phagocytosis by macrophages and neutrophils.
Phagocytosis is a critical process of removing pathogens to limit infection and inflammation.

Complement activation can neutralize viruses by interfering with their ability to infect cells.
The mechanisms range from binding to viral surfaces to lysing virus-infected cells.
This helps prevent the spread of viral infections and reduce the potential for viral replication.

Immune complex buildup: Accumulation of immune complexes can trigger tissue damage.
C3b coating of complexes: C3b coats harmful immune complexes.
Removal by RBCs: Red blood cells assist in transporting and removing immune complexes to prevent harm.
Liver and spleen clearance: The liver and spleen filter and remove immune complexes from the blood.
Complement deficiency: Deficiencies in complement proteins can impair the clearance of immune complexes, potentially leading to complications.

Complement receptors mediate complement effector mechanisms: These receptors on immune cells recognize and bind to complement-coated pathogens.
Types of receptors are identified and their specific roles are described.

The activated complement system significantly boosts the immune response.
Direct cell death occurs by forming a MAC.
Inflammation is induced to attract immune cells to infection sites.
These mechanisms collectively strengthen the immune system's ability to eliminate pathogens and maintain homeostasis.