Immunoglobulin isotypes Flashcards
IgG
Main antibody in 2° (delayed) response to an antigen. Most abundant isotype in serum. Fixes
complement, crosses the placenta (provides infants with passive immunity), opsonizes bacteria,
neutralizes bacterial toxins and viruses.
IgA
Prevents attachment of bacteria and viruses to mucous membranes; does not fix complement.
Monomer (in circulation) or dimer (with J chain when secreted). Crosses epithelial cells by
transcytosis. Produced in GI tract (eg, by Peyer patches) and protects against gut infections (eg,
Giardia). Most produced antibody overall, but has lower serum concentrations. Released into
secretions (tears, saliva, mucus) and breast milk. Picks up secretory component from epithelial cells,
which protects the Fc portion from luminal proteases.
IgM
Produced in the 1° (immediate) response to an antigen. Fixes complement but does not cross the
placenta. Antigen receptor on the surface of B cells. Monomer on B cell, pentamer with J chain
when secreted. Pentamer enables avid binding to antigen while humoral response evolves.
IgD
Unclear function. Found on surface of many B cells and in serum.
IgE
Binds mast cells and basophils; cross-links when exposed to allergen, mediating immediate (type I)
hypersensitivity through release of inflammatory mediators such as histamine. Mediates immunity
to worms by activating eosinophils. Lowest concentration in serum.
Thymus-independent
antigens
Antigens lacking a peptide component (eg, lipopolysaccharides from gram ⊝ bacteria); cannot
be presented by MHC to T cells. Weakly immunogenic; vaccines often require boosters and
adjuvants (eg, pneumococcal polysaccharide vaccine)
Thymus-dependent
antigens
Antigens containing a protein component (eg, diphtheria vaccine). Class switching and
immunologic memory occur as a result of direct contact of B cells with Th cells (CD40–CD40L
interaction).
