Innate immunity: the complement system

• complement has four functions in innate immunity

• 1 local vasodilation

• 2 attraction of immune cells to the site of infection

• 3 opsonization of pathogens

• 4 synthesis of the membrane attack complex

Complement cascade

• C3 is at the center of the three complement pathways

• C3 is broken into C3b and C3a

  • C3a and C5a are the major chemokines and vasodilators

  • C3b is the main opsonin

• C5b through C9 assemble into a membrane-attack complex to poke holes in Gram-negative organisms

patients with inherited complement deficiency syndromes are subject to Gram-negative bacteria infections such as Neisseria meningitidis PDF 13

Neutrophil chemotaxis and migration to tissues

• neutrophils roll, bind tightly, diapedese, and migrate up a chemotactic gradient

Molecular basis of neutrophil chemotaxis

• neutrophils are able to migrate up a tiny chemotactic gradient

• ATP release and amplification of this ATP release leads to the directed movement of neutrophils up this gradient

• this is mediated by A3 receptors
  

Opsonization by complement (C3b) or specific antibody greatly enhances phagocytosis

• macrophages have Fc receptors and complement receptors

• without antibodies and complement, phagocytosis is very inefficient

Category: Microbiology Notes