Complement system Flashcards
Function of the complement system
Cytolytic destruction of cellular antigens by specific antibody
Chemotaxins in complement system
C5a, C5b, C6, C7
Immune adherence mediator
C3b
Kinin activator in complement system
C2b
Anaphylotoxins in complement system
C3a, C4a, C5a (most potent)
Opsonins in complement system
C3b, C4b, C5b
Complement fragments involved in virus neutralization
C4b, C1
Number of IgM molecules required to activate complement
1 molecule
Number of IgG molecules required to activate complement
2 molecules
Complement fragment exception where ‘a’ is larger than ‘b’
C2a is larger than C2b
Most abundant complement protein
C3
Most commonly measured complement protein fragment
C3b
Three major pathways of complement activation
Classical, Alternative, Lectin
Pathway activated by antibody bound to antigen
Classical pathway
Pathway activated on microbial cell surfaces without antibody
Alternative pathway
Pathway activated by plasma lectin binding mannose on microbes
Lectin pathway
Recognition unit in the classical pathway
C1
Activation unit in the classical pathway
C4, C2, C3
Membrane attack complex (MAC) components
C5, C6, C7, C8, C9
Classical pathway C3 convertase
C4bC2a
Classical pathway C5 convertase
C4b2a3b
Subunit of C1 that binds to antibody Fc region
C1q
Role of C1r and C1s subunits
Generate enzyme activity to start cascade
Enzyme classification of C1r and C1s
Serine protease proenzymes (zymogens)
First complement component to bind in the classical pathway
C1
Subunits of C1
C1q, C1r, C1s
Stabilizing factor for C1
Calcium
Components of the alternative pathway
C3, Factor B, Factor D, Properdin
Triggering substances for the alternative pathway
Pathogens (bacteria, fungi, viruses, parasites), non-pathogens (immune complexes, RBCs, polymers)
First step in the alternative pathway
Conversion of C3
Factor B role in the alternative pathway
Analogous to C2 in the classical pathway
Plasma protein that cleaves Factor B after binding to C3b in the alternative pathway
Factor D
Cleavage products of Factor B by Factor D
Ba and Bb
Alternative pathway C3 convertase
C3bBb
Alternative pathway C5 convertase
C3bBb3bP
Similarity between alternative and classical pathways
Identical after C5 cleavage
Key Protein in MBL Pathway
Lectin (binds to carbohydrates)
MBL Function
Binds to mannose on bacterial cells
MBL Production Site
Liver (during acute-phase inflammatory reactions)
Enzymes Binding MBL
MASP-1, MASP-2
Function of MASP-1 and MASP-2
Similar to C1 activity in the classical pathway
Recognition Unit
MBP, MASP-1
Role of MASP-1
Cleaves C3
Role of MASP-2
Cleaves C4 and C2
C3 Convertase in MBL Pathway
C4b2a
C5 Convertase in MBL Pathway
C4b2a3b
Importance of MBL Pathway
Innate host defense in early childhood
Structural Similarity of MBL Complex
Resembles C1q of the classical pathway
Activation Triggers of MBL Pathway
D-mannose, GlcNAc, microbial polysaccharides
Prevents activation of complement proteins
Plasma complement regulation
Dissociates C1r and C1s from C1q
C1 Inhibitor (C1 Inh)
Cleaves C3b and C4b
Factor I
Inactivates C3b and prevents Factor B binding in the alternative pathway
Factor H
Cofactor with Factor I to inactivate C4b
C4 Binding Protein
Prevents attachment of the C5b67 complex to cell membranes
S Protein (Vitronectin)
Accelerates dissociation of C3 convertase
Decay Accelerating Factor (DAF)
Inhibitors of Membrane Attack Complex (MAC)
HRF (Homologous Restriction Factor) and MIRL (CD59)
Mediates immune complex transport and is a cofactor for Factor I
CR1 (CD35)
B-cell co-receptor for antigens with CD19
CR2 (CD21)
Enhances adhesion and activity of phagocytic cells
CR3 (CD11b/CD18) and CR4 (CD11c/CD18)
Prevents insertion of C9 into the cell membrane
MIRL (CD59)
Cofactor for Factor I cleavage of C3b and C4b
MCP (CD46)
Assay for hemolysis via classical pathway
CH50 assay (using sensitized sheep RBC)
Assay for hemolysis via alternative pathway
AH50 assay (using magnesium chloride, ethylene glycol tetraacetic acid)
Test for concentration of complement component C3
Radial Immunodiffusion (RID)
Decreased complement components is due to
Consumption (autoimmune) or genetic defect
Detects complement-fixing antibodies after antigen-antibody binding
“Complement fixation test”
Systems involved in complement fixation
Test/Bacteriolytic System & Indicator/Hemolytic System
Complement fixation test reporting
Positive: no hemolysis; Negative: hemolysis