L20- the complement system

what is the complement system

major effector system of the humoral branch of the innate and adaptive immune response

• inflammation, phagocytosis and membrane attack

how do some activated proteins bind covalently to bacteria opsonising them

phagocytosed by cell with complement receptors

what do terminal components of the complement system generate

the membrane attack complex (MAC) → lysis of the pathogen

what is the classical pathway of activation

initiated by antigen: antibody complexes

what is the alternative pathway of activation

triggered by some pathogen surfaces

what is the lectin pathway of activation

initiated by acute phase proteins that bind glycoproteins or carbohydrates on micro-organisms

what do routes for the pathways all generate

C3 convertase

• leads to opsonisation of antigen, inflammatory response and production of the membrane attack complex

what is the first component of the classical pathway activation

C1

• C1q, C1r, C1s

• 2 molecules of C1r and C1s bind to each C1q

when is activation of the classical pathway initiated

when antibodies bind to the surface of an antigen or pathogen bound C1q

what is C3 convertase

C4b2a

what forms C3

C3a and C3b

what is C3b

opsonin

• also involved with C5 convertase

what is C5 convertase

C4b2a3b

what is C5 made of

C5a and C5b

how is the alternative pathway activated

• by action of the classical or lectin pathway

• spontaneous hydrolysis of C3

what does C3b bind to

factor B

what does factor D cleave into

Ba and Bb

what does factor P do

properdin

• stabilises

what receptors does the lectin pathway use

soluble (acute phase proteins)

what does MASP-2 cleave

C4 and C2

what complexes does the lectin pathway get

mannose binding lectin and MBL-associated serine proteases

• MASP-1 and MASP-2

what does C3 do

opsonisation, inflammation and cytolysis

what is the clinical consequence of C3

life-threatening infections with a range of bacteria

what is the function of C6, C7, C8 and C9

cytolysis

what is the clinical consequence of C6, C7, C8, and C9

problems with infection with Neisseria