Immunoglobulin isotypes

Question 1

IgG

Answer 1

Main antibody in secondary (delayed) response to an antigen. Most abundant isotype. Fixes complement, crosses the placenta, opsonizes bacteria, neutralizes bacterial toxins and viruses

Question 2

IgA

Answer 2

Prevents attachment of bacteria and viruses to mucous membranes; does not fix complement. Monomer (in circulation) or dimer (when secreted). Crosses epithelial cells by transcytosis. Found in secretions and early breast milk. Picks up secretory component from epithelial cells before secretion

Question 3

IgM

Answer 3

Produced in the primary (immediate) response to an antigen. Fixes complement but does not cross the placenta. Antigen receptor on surface of B cells. Monomer on B cells or pentamer. Pentamer shape allows for easy trapping of free antigens out of tissue while humoral response evolves

Question 4

IgD

Answer 4

Unclear function, found on surface of B cells and in serum

Question 5

IgE

Answer 5

Binds to mast cells and basophils; cross links when exposed to allergen, mediating immediate (type I) hypersensitivity through release of inflammatory mediators like histamine. Mediates immunity to worms by activating eosinophils. Lowest concentration in serum