Antibodies Flashcards
Five classes of antibodies:
IgG, IgA, IgM, IgE, IgD
Basic structure of antibody
- Four chain protein
- 2 heavy chains, 2 lights chains
- linked by disulfide bonds
What is responsible for differentiation into the five classes of antibody?
H chain isotypes: gamma, alpha, mu, epsilon, delta
The amino acid sequences of the variable H and L regions varies with:
The epitope toward which the particular antibody is directed
What is an idiotype?
The two epitope binding regions formed by the combination of the hypervariable regions of the H and L chains
IgG structural properties:
- composed of two L chains and two H chains
- MW 150,000 Da
- Four subclasses: Lambda 1, 2, 3, 4
- Papain cleaves it into two Fab and one Fc region
Function of IgG Fc region
- activates complement
- controls catabolism of IgG
- fixes IgG to tissues or cells via Fc receptor
- mediates placental transfer of antibodies
IgG functional properties
- highest serum level of all immunglobulins
- half life of 18-25 days
- adheres to cells that possess a receptor for the Fc fragment from IgG
- fixes complement leading to cell lysis
- mediates placental passage of maternal antibody to the fetus
IgM structural properties
- on B cell membrane as a monomer but secreted as a pentamer
- Pentamer has five high affinity and five low affinity binding sites
- Has one more constant region than A, G and D
IgM functional properties
- earliest immunoglobulin to appear after antigenic stimulus
- fixes complement avidly
IgA structural properties
- exists in three forms: monomer, dimer (two monomers joined by J chain), dimer plus a secretory piece
- secretory piece (receptor of IgA Fc region) transports dimer across respiratory and intestinal mucosal barriers
- two subclasses a1 and a2
IgA functional properties
- found in high concentrations in secretions
- in serum exists mostly as a dimer with 5 day half life
- located in and protects mucosal tissues, saliva, tears, colostrum
- blocks bacteria, viruses, toxins from binding to host cells
IgE structural properties
- four constant domains like IgM
- IL-4 mediates the B-cell switch to IgE production
IgE functional properties
- extremely low serum concentration and half life because Fc region binds with very high affinity to mast cells and basophils
- adheres to tissue bound mast cells and circulating basophils via Fc receptors
- triggers release of vasoactive amines (histamine)
- atopic disease and anaphylaxis associated with IgE
- binding of IgE to IL-5 activated eosinophils eliminates Helminths
- both total and allergen specific IgE can be quantified
IgD functional properties
- low serum levels
- found on 15% of newborns’ B-cell membranes
- found on adult blood lymphocytes in conjunction with IgM
- receptor on B-cell membranes for antigen
