Complement System

Question 1

What is a COMPLEMENT

Answer 1

-Group of serum proteins that play a key role in association with antibody in eliminating pathogens
->30 different proteins
-produced by macrophages & hepatocytes
-triggered enzyme system - inactive in the circulation until triggered - highly regulated
-activation generated split products that perform biological functions

Question 2

Complement functions

Answer 2

-defence against microorganisms: promotes phagocytosis, lysis of cells - bacteria and some viruses
-contributes to inflammatory response facilitating leukocyte recruitment activation
-elimination of waste (immune complexes, apoptotic cells)
-interface between innate and adaptive immune system

Question 3

Opsonisation

Answer 3

Promote phagocytosis

Question 4

Cell Lysis

Answer 4

Kill pathogen

Question 5

Chemotaxis

Answer 5

Cell activation & migration

Question 6

Products of enzyme reactions

Answer 6

C3b, C3a = generated from C3
a = small soluble product
b = big and bound product

Question 7

Complement activation

Answer 7

-triggered enzyme system
-three activation pathways
-each activation pathway has a separate trigger
-activation pathways generates enzymes
-enzymes generate biologically active products
-generation of C3 convertases is the key event in complement activation

Question 8

Complement trigger

Answer 8

Molecular binding - pathogens, immune complexes

Question 9

Complement activation

Answer 9

Formation of active enzymes, cleavage of components - release of biologically active products

Question 10

Complement consequence

Answer 10

Products perform various immune functions

Question 11

Classical pathway

Answer 11

-C1 complex binds to antibody (IgG/IgM) as part of Ag:Ab complex - C1 crosslinks C1 binding sites on the antibody
-Binding activates C1 which can then bind and activate C4 (C4 –> C4b (large, active) + C4a (small soluble))
-C4b binds to microbe surface - opsonisation
-C2 attaches to bound C4b
-C1s splits bound C2 giving C4b2a
-C4b2a is a C3 convertase

Question 12

C3 tickover

Answer 12

-spontaneous hydrolysis of thioester to C3(H2O) which is rapidly inactivated [C3(OH)]
-C3(H2O) binds to FB forming C3(H2O)B
-FD splits bound FB giving C3(H2O)Bb
-C3(H2O)Bb is a C3 convertase
starts alternative pathway -> generates C3b
-if microbe present, C3b binds to surface

Question 13

Alternative Pathway

Answer 13

Microbe present:
-C3b binds to microbial surface - opsonisation
-FB binds to membrane-bound C3b -> C3bB
-FD splits bound FB forming C3bBb
Alternative pathway C3 convertase
-C3bBb forms more C3a and C3b
-C3b binds to microbe and cycle repeats (((amplification loop)))

Question 14

Lectin pathway

Answer 14

-Pathway activated by mannose-binding lectin (MBL) binding to pathogens surface via carbs
-MBL is made in the liver in response to cytokines released by macrophages activated by pathogen binding
-MBL and two serum proteases, mannan associated protease (MASP-1,MASP-2) form a complex similar to C1

Question 15

C5 convertases

Answer 15

C3b combines with C3 convertases to form C5 convertases

Question 16

Lytic pathway

Answer 16

-C5b-6 - soluble complex
-C5b-7 - inserts into membrane via hydrophobic complex
-C5b-8 small transmembrane channels formed
-(C5b-8)^9n - pore formed

Question 17

Opsonisation

Answer 17

-C3b molecules bind to the microbial surface (react with proteins & carbs)
-Phagocytes have receptors for C3b so recognise the C3b (opsonin)
-Initiates phagocytic cell function
-Increases rate of phagocytosis -> increased rate of microbe clearance

Question 18

MAC-mediated cytolysis

Answer 18

-C5b6789(n) assemble to create the membrane attack complex (MAC)
-ring structure (pore) of MAC disrupts the membrane of a microbe (hydrophobic external face, hydrophilic internal face)
-Uncontrolled influx of ions & water -> cytolysis

Question 19

Inflammation 1 - action on mast cells

Answer 19

-complement components involved include (C3a,C4a,C5a)
-Activate mast cells & basophils (C5a>C3a>C4a (anaphylatoxins))
-Trigger release of inflammatory mediators (histamine –> vascular changes in infected tissue to facilitate movement)
-Increase adhesion of cells to endothelium

Question 20

Inflammation 2 - chemotaxis

Answer 20

• C5a & C3a are released at the site of infection
• These diffuse through the tissues forming a concentration gradient
  -Neutrophils enter the tissues from the blood
• Migrate along gradient of C5a/C3a
  -Encounter opsonised bacteria
  -Phagocytose bacteria -> clear infection

Question 21

Receptors overview

Answer 21

-Mediate opsonic & inflammatory activity
-Range of receptors - CR1, CR2, CR3, CR4, C1qR, C3aR, C5aR
-Range of ligands - C3a, C3b, C4a, C4b, C5a, C5b, C1q, degraded C3b (iC3b/C3bi)
Range of effects - Phagocytosis, cell adhesion, immune complex clearance, antigen persistence, smooth muscle contraction

Question 22

CR1 (CD35)

Answer 22

Activity:
-Receptor for C3b (primary opsonin) & C4b bound to immune complexes (Ag:Ab complex)
Function:
-leukocytes & tissues: adherence of C3b/C4b particles in preparation for phagocytosis
-Erythrocytes: C3b/C4b immune complexes transported to clearance systems (liver/spleen)

Question 23

C5a effects via C5aR

Answer 23

Mast cells - chemotaxis & degranulation (histamine, serotonin)
Monocytes/macrophages - cytokine production (IL-1, IL-6, IL-8)
Neutrophils:
-chemotaxis, adhesive properties (via P-selectin expression on vascular endothelium)
-activates cells (secretory granule release)
-NADPH-oxidase pathway activator (respiratory burst)