Unit 4 - Complement Pathways

complement proteins are a part of

innate immune system

complement proteins consist of

more than 30 plasma proteins and membrane associated factors

zmogens

proteins circulate in inactive form

compliment proteins when activated

become protease enzymes and cleave and activate downstream elements

routes of complement activation

classical, mannose binding lectin, alternative

what complement responses can happen when activated

complement protein lyse cells

act as chemotactic agents

act as vasoactive cytokines

act as opsonins

classical complement pathway

C1 binds to Ab bound cells → C2 unfolds = reveals enzyme domain

C1 cleaves and activates C2 and C4 → fuses to form C3 convertase → cleaves C3 into C3a and b

C3b still bound to cell → binds C2a-C4b makes C5 convertase → cleaves C5 into C5a and b

C5b promotes binding of C6, 7, 8, and C9 → forms MAC in a complex making a hole in cell membrane

C3a

chemotactic agent

C5a

chemotactic and pro inflammatory agent

mannose binding lectin (MBL) pathway

recognizes mannose on bacterial cell surface

MBL pathway process

MBL+MASP1+MASP2 localize C2 and 4 to cell surface → C3 activation and MAC formation same as classical

complement not activated in MBL

C1

alternate pathway

C3b spontaneously produced Factor B and D binds to C3b = C3 convertase

another C3b binds to C3b-B complex = makes C5 convertase

follows rest of classical pathway from here

C3b

opsonisation

complement deficencies are developed by

genetic defects, overconsumption, or protein dysregulation

when are complement deficiency tests done

recurrent infections observed or when autoimmune disease is suspected

Neisseria infection is linked to

MAC defects

encapsulated microbe infection linked to

C1 defects

C1, C2, C4 deficiency

Ag-Ab complexes build up

increased infection by encapsulated bacteria

C3 deficiency leads to

severe recurrent bacterial infection

C5, C6, C7, C8 deficiency leads to

decrease in MAC opsonization

decrease in MBL leads to

increased risk of infection in toddlers without mature Ab response

Encapsulated Bacteria

resist phagocytosis unless opsonized

complement protein made in the

liver

deficiency in C1 esterase

leads to increased C1 activation = hereditary angioedema

CH50

measures functional ability of patients complement components in the CLASSICAL PATHWAY

tests ability to lyse sensitized cells

AH50

measures functional ability of patients complement components in ALTERNATIVE PATHWAY

how CH50 works

sheep RBC coated with rabbit anti sheep RBC Ab

extent of hemolysis = efficiency of MAC production by complement factors → hemolysis is good

if CH50 = 0

no lysis → any complement protein in classical can be affected

how AH50 works

Ca from patient serum is removed with EGTA

serum is added to rabbit RBC → centrifuged

if lysed = has complement = good

complement fixation

complement protein is used as a reagent to detect unknown Ab or Ag

complement fixation procedure

collect blood

heat serum to remove complement

add antigen → forms Ag-Ab complex if Ab is present in blood

add complement protein from rabbit → will complex with Ab-Ag

add Ab bound SRBC

complement fixation outcomes

Ab-Ag complex is absent → SRBCs are bound and lysed by complement = non reactive

Ab-Ag complex is present → complement protein titrated away from SRBC and bind with complex = no lysis/reactive