Complement system Flashcards
(24 cards)
What is the complement system?
A part of the innate immune system made up of plasma proteins that enhance immune responses like inflammation, opsonization, and cell lysis.
What are the 3 main complement activation pathways?
Classical (antibody-dependent)
Lectin (via mannose-binding lectin)
Alternative (pathogen surface directly)
What do all three pathways of complement activation lead to?
What do all three pathways of complement activation lead to?
What is the function of C3a
Acts as an anaphylatoxin, promoting inflammation and recruiting immune cells.
What is the function of C3b?
Acts as an opsonin – it coats pathogens and enhances their uptake by phagocytes.
What initiates the formation of the Membrane Attack Complex (MAC)
C5b, after C5 is cleaved by C3b-mediated convertase
What is the function of C5a?
Potent anaphylatoxin
Powerful chemoattractant for neutrophils and macrophages
Promotes inflammation
What proteins form the Membrane Attack Complex (MAC)?
C5b + C6 + C7 + C8 + multiple C9 molecules.
How does the MAC kill pathogens?
By forming pores in the pathogen’s membrane, leading to osmotic lysis.
What are the main roles of complement proteins in immunity?
Opsonization (C3b)
Inflammation (C3a, C5a)
Cell lysis (MAC via C5b–C9)
What are anaphylatoxins, and which complement components act as them?
Molecules that induce inflammation; C3a and C5a are the primary anaphylatoxins.
Which immunoglobulins can activate the classical complement pathway?
IgM and IgG (especially IgG1 and IgG3)
What is the function of complement regulators (e.g., CD59)?
Prevent complement activation on host cells and protect against accidental self-damage.
What is the role of mannose-binding lectin (MBL) in complement activation?
MBL binds to sugars on pathogens and activates the lectin pathway.
What process creates diversity in antibodies before antigen exposure?
V(D)J recombination in the bone marrow (antigen-independent)
What is junctional diversity?
Variation created when nucleotides are added or deleted at V-D-J joining sites.
What is combinatorial diversity?
Any heavy chain can pair with any light chain, multiplying the possible combinations.
What does class switching involve?
Changing the constant region of the heavy chain to switch antibody isotype (e.g., IgM → IgG).
Does class switching affect antigen binding?
No — the VDJ region remains unchanged, so specificity stays the same.
What enzyme is key to initiating class switching?
AID (Activation-Induced Cytidine Deaminase)
What are switch (S) regions?
DNA sequences upstream of constant region genes where recombination happens during class switching.
Name one cytokine that influences antibody isotype switching and what it promotes.
IL-4 → IgE and IgG1
Where does somatic hypermutation occur?
n germinal centers after B-cell activation (antigen-dependent).
What does the ER do during antibody production?
Translates mRNA into polypeptides, folds them, and assembles light + heavy chains.
