IgG
IgG is a divalent Ig. This is the most common immunoglobulin in the CSF,
blood, lymph, and peritoneal fluid. It has a half-life of 23 days and is
good for passive immunity (serum transfer). It causes agglutination of
insoluble antigen. IgG is the only immunoglobulin which crosses the
placenta. Placental crossing of IgG: +++ rodents, ++ primates, + dog/cat,
0 ruminants, pig, horses. IgG is a good opsonin as the phagocytes have an
IgG Fc receptor. This immunoglobulin stimulates Antigen Dependent
Cell-mediated Cytotoxicity - IgG Fab binds target cell, NK Fc receptor
binds IgG Fc, and NK cell releases cytotoxic contents on target cell. IgG
activates complement, neutralizes toxins, immobilizes bacteria, and
inhibits viral attachment.
IgA
IgA is the secretory Ab. It's found in saliva, sweat, mucous, tears, milk,
gastric fluid, etc. Only the dimer is active - monomers are not. The
secretory portion of the dimer is actually the epithelial cell IgA
receptor, which remains with the IgA as it is transported into the lumen
by the epi cell. IgA prevents pathogens from attaching to the mucosal
surface. IgA is NOT effective at fixing complement. IgA is bactericidal in
the presence of lysozyme, which is present in secretions which contain
IgA. IgA is antiviral and an effective agglutinin.
IgD
IgD is not found in high serum concentrations. IgD is a surface marker for
mature B cells. IgD is made by all mature, normal B cells, along with
IgM. B cells make both IgD and IgM due to differential splicing of RNA
IgE
IgE is found in low serum concentrations, mainly because it binds so
effectively to mast cells and basophils, with a lesser affinity for
eosinophils. It binds to the Fce receptor on these cells. In the presence
of Ag for which IgE is specific, the IgE becomes crosslinked, triggering
degranulation and release of histamine and other compounds - this can
cause anaphylaxis. IgE is useful in fighting parasites.
IgM
IgM is found on the surfaces of mature B cells. IgM has a half life of 5
days and is a pentameric molecule with a valence of five. This is the only
Ig made by the fetus. An increase in IgM signifies a recent infection or
exposure to antigen (immunization). IgM is an efficient agglutinin and
natural isohemagglutinins are IgM. IgM is efficient at activating
complement. IgM is made after exposure to T independent Ag, and after
immunization with T dependent Ag.