block ii: complement

complement

a system of plasma proteins that protects against extracellular pathogens through opsonization, inflammation, and direct killing

three complement activation pathways

classical pathway, lectin pathway, alternative pathway

classical complement pathway

complement pathway that can be activated by antibodies bound to a pathogen

lectin complement pathway

complement pathway initiated by recognition of carbohydrates on microbial surfaces

alternative complement pathway

complement pathway involving spontaneous low-level C3 activation and amplification on microbial surfaces

point where complement pathways converge

formation of C3 convertase

C3 convertase

cleaves C3 into C3a and C3b

C3a

soluble complement fragment that promotes inflammation

C3b

complement fragment that binds microbial surfaces, acts as an opsonin, enhances phagocytosis, contributes to formation of C5 convertase

opsonization

coating a pathogen to make it easier for phagocytes to recognize and ingest

major complement opsonin

C3b

C5 convertase

cleaves C5 into C5a and C5b

C5a

potent inflammatory complement fragment and chemoattractant

C5b

complement fragment that initiates formation of the membrane-attack complex

membrane-attack complex (MAC)

terminal complement complex that forms pores in susceptible target membranes

components of MAC

C5b through C9

result of MAC formation

pore formation in the target membrane that can lead to lysis

three major complement effector functions

opsonization, inflammation and leukocyte recruitment, direct killing through MAC

complement fragment most associated with opsonization

C3b

complement fragments associated with inflammation

C3a and C5a

complement fragment especially associated with chemotaxis

C5a

complement fragment that begins MAC formation

C5b

why complement requires regulation

prevents complement from damaging normal host cells

C3b and phagocytosis

C3b coats a pathogen, phagocytes recognize the coated pathogen through complement receptors, phagocytosis becomes more efficient

classical pathway trigger

antibody bound to pathogen can initiate the classical pathway