Immunoglobulin isotypes

Question 1

IgG

Answer 1

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.

Question 2

IgA

Answer 2

Prevents attachment of bacteria and viruses to *mucous membranes

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,
Guardia)

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.

Question 3

IgM

Answer 3

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.

Question 4

IgD

Answer 4

Unclear function

Found on surface of many *B cells and in *serum.

Question 5

IgE

Answer 5

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.