questions 3

Antibodies

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Topic updated on 01/11/17 7:40pm

Overview
 

 
General antibody properties
  • Isotype defines function 
    • e.g. IgE, A, G, M, etc
  • Idiotype
    • defines antigenic specificity
      • unique to a clonal population of cells
  • Fragments of the antibody
    • Fab
      • antibody binding
      • defines idiotype
        • has a unique peptide-binding pocket
        • 1 idiotype per B cell
      • made up of heavy and light chains
        • antigen binding region is in the variable regions
    • Fc
      • constant
      • carboxy terminal of protein
      • complement fixation
      • carbohydrate side chains
      • has a heavy chain contribution
        • light chain only contributes to Fab
      • determines isotype
        • IgM, IgD, etc.
  • Antibody diversity generation is similar to T-cell receptor      
    • VJ (light-chain) or V(D)J (heavy-chain) gene recombination
      • mediated by rag
      • during process terminal deoxynucleotidyl transferase (TdT) adds nucleotides
    • heavy chains combined randomly with light chains
    • somatic hypermutation
      • after B-cell activation with antigen
Specific antibodies properties

  • IgM
    • μ constant chain
    • bound to surface of B-cell
    • secretion of IgM pentamer
      • monomers held together by J chain
      • can bind 10 regions of the same antigen
        • IgM has a valence of 10
    • essential for early immune response
      • highest avidity of all immunoglobins
        • multipoint binding
      • potentially low affinity
        • strength of binding
    • produced in primary humoral response
  • IgD
    • δ constant chain
    • expressed with IgM on B-cell surface
    • purpose unknown
  • IgG
    • γ constant chain
    • secreted following class switching by plasma cells
    • exists if 4 sub-isotypes
    • circulates as a monomer
    • essential for late immune response and memory
    • only IgG that can cross placenta
    • IgG has a valence of 2
    • bind to phagocytic cells (e.g. macrophages) via Fc region
  • IgA
    • α constant chain
    • common in submucosa as a dimer                        
      • transported across mucosal epithelium via specfic Poly Ig Receptor
      • dimer held together by J chain
        • may be cleaved by Neisseria species virulence factors to enable mucosal colonization 
      • dimer form has a valence of 4
      • found in high concentrations in colostrum to protect infant mucosal tracts 
    • circulates as a monomer
  • IgE
    • ε constant chain
    • "homocytoropic antibody"
    • binds to mast cells and basophils
    • crosslinks to mediate degranulation

Reference



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Qbank (3 Questions)

TAG
(M1.IM.8) A 19-year-old male is found to have Neisseria gonorrhoeae bacteremia. This bacterium produces an IgA protease capable of cleaving the hinge region of IgA antibodies. What is the most likely physiological consequence of such a protease? Topic Review Topic

1. Membrane attack complex formation is impaired
2. Opsonization and phagocytosis of pathogen cannot occur
3. Impaired adaptive immune system memory
4. Impaired antibody binding to mast cells
5. Impaired mucosal immune protection

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TAG
(M1.IM.20) In which location is dimeric IgA most likely to be found in highest concentrations? Topic Review Topic

1. Colostrum
2. Blood from the fetus
3. Blood from the mother
4. Fetal bone marrow
5. Fetal thymus

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TAG
(M1.IM.22) A 15-year-old Canadian male reinfected with a bacterium that his immune system successfully cleared previously, has generated a profound secondary immune response. Serum levels of IgG are found to be elevated. IgG is capable of binding to cells such as neutrophils, eosinophils, and macrophages. Which site on the IgG molecule is the site of this binding? Topic Review Topic
FIGURES: A          

1. A
2. B
3. C
4. D
5. E

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