A8: Effector Mechanisms of Humoral Immunity Flashcards
What is an Fc receptor?
A transmembrane protein that has the capability of binding to a specific portion of a dimeric heavy chain Ig molecule at a site located in Fc region of the Ig
Which cell type primarily produces IgA?
B-1 cells in a T cell independent fashio
What two main processes mediate neonatal immunity?
IgG cross the placenta so maternal IgG mediate immunity. Gut immunity is via IgA transfered through breastmilk
What is the mechanism of classical activation of complement?
IgM or enough IgG binds to an antigen such that two C1q heads can bind to antibody C region, thus making C1 enzymatically active (via serine protease activation) which cleaves C4
What is the pathway of classical activation of complement leading to C3 cleaving?
C1 becomes enzymatically active and cleaves C4, C4b binds to C2 generating C4b2a which is a C3 convertase
What is the key intermediate in the complement that mediates the step from C3 cleavage to C5 cleavage?
C4b2a is a C3 convertase, and C3b binds to the C4b2a generating the C4b2a3b complex which can cleave C5.
What is the main bacteria type and classic example that is attacked by the MAC?
Gram negative bacteria with thin cell walls like Neisseria
What are the steps of the lectin binding pathway that lead to C4 cleaving?
MBL binds mannose on bacteria or virus which allows serine proteases related to C1 to associate and activate C4
What is the mechanism of alternate complement activation?
Normal hdyrolysis of C3 leads to C3b that binds to microbes and is stabilized. Bound C3b binds to factor B and factor D breaks it down to make Bb fragment. C3bBb complex can break down more C3, resulting in more C3b which results in C3bBb3b which is a C5 convertase
What are the two key C5 convertases made by the alternative pathway and by the classical/lectin pathway?
Alternative: C3bBbC3b complex
Classical/lectin: C4bC2a3b complex
Which complements are key chemotaxic products?
C3a and C5a
What is the complement receptor present on B lymphocytes?
CR2 receptor (recognizes C3d)
How, via complement, are antigens presented to B cells in germinal centers?
A complement protein bound to antigen-antibody can be recognized by follicular dendritic cells in germinal centers and display it to B cells
What results if there is a deficiency of C3? What results if there is a deficiency of C2 or C4?
C3 deficiency is vulnerable to bacterial infection. C2 or C4 has little clinical significance.
What are key inhibitors of complement activation?
C1 inhibitory (C1 INH)
Decay accelerating protein (DAF) stops more C3b cleaving
Factor I cleaves C3b or C4b to inactive fragments
Factor H is required for factor I
MCP also regulates by working with factor I
