Complement system

what are the 3 pathways of complement?

• Classical

• Alternative

• Lectin

what is the complement system?

Defensive system made up of over 30 proteins produced by the liver and found in circulating blood plasma and within tissues.

what are the complement proteins designated the letter C

C1-C9 in the order they were discovered. Other proteins are referred to as factors B,D and P.

How does the complement system act as a cascade?

One reaction triggers another reaction, which in turn triggers another reaction.

What do lowercase a and b indicate in complement?

Activated fragments of C1-C9

eg: C3 becomes activated to fragments C3a and C3b

what is the classical pathway activated by?

Binding of specific antibody of the IgG or IgM class to surface antigens.

The antigen antibody complex binds to and activates C1. Eventually C3 is activated and splits into C3a and C3b.

what is the role of C3b in classical pathway?

• It binds to the surface of a microbe and receptors on phagocytes attach to C3b.

• It therefore enhances phagocytosis by neutrophils and macrophages by a process called opsonization

ALSO

• initiates a series of reactions that bring about cytolysis/lysis by first splitting C5.

how does C5b bring about cell lysis?

• C5b fragment binds to C6 and C7, which attach to the plasma membrane of an invading microbe.

• then C8 and C9 molecules join other complement proteins and together form a cylinder shaped membrane attack complex.

• this membrane attack complex inserts into the plasma membrane causing leakage of its contents and death.

what does the binding of C3a and C5a to mast cells cause?

Release of histamines = increases blood vessel permeability during inflammation

what protein of complement promotes chemotaxis?

C5a - attracts phagocytes to the site of inflammation

what is the alternative pathway of complement?

• doesn’t involve antibodies. It is initiated by an interaction between lipid-carbohydrate complexes on the surface of microbes and complement factors B,D and P.

• this interaction activates C3

what is the lectin pathway of complement?

• macrophages that digest microbes release chemicals that cause the liver to produce lectins.

• Lectins bind to carbohydrates on the surface of microbes, ultimately causing the activation of C3.

which hypersensitivity reactions involve the complement system?

Type II and type III

What activates complement in type II hypersensitivity reactions?

Antibody - antigen complexes

what activates the complement system in a type III hypersensitivity reaction?

Build up of immune complexes in the basement membrane under endothelium of blood vessels, causing inflammation.