Immunoglobulin(Ig); IgM, IgG, IgA, IgD, IgE
Structure and Function
Immunoglobulins(Ig) have a basic four-chain monomeric structure consisting of two
identical heavy chains and two identical light chains with interchain disulfide bonds. There
are five heavy chain classes (M, D, G, E and A), four G subclasses(G1-4) , and two A
subclasses(A1, 2). There are two light chain isotypes, K(kappa) and L(lamda).
IgG
: Antibody molecules have biological properties such as transport
across the maternal-fetal mebranes, interaction with the classical complement system and
fixation to heterologous tissues resided in the Fc fragment of IgG. IgG molecules have a molecular
weight of 150,000 and a sedimentation coefficient of 7S. Four subclasses of IgG have been
identified on the basis of antigenic and structural difference residing in the heavy chains
designated r1, r2, r3 and r4.
IgM
: IgM has a molecular weight of about 900,000 daltons. Each molecula
consists of five identical subunits of about 180,000 daltons. Five monomeric units are formed in
a circle via disulfide bonds between heavy chains of the subunits to form a pentamer. In
addition, the IgM molecule contains an additional chain, the J or joining chain. The J chain
is a component only of polymeric Igs of many species such as IgM, and IgA. The molecular
weight of J chain is about 15,000 daltons. J chain consists of about 118-125 amino acid and
7-8 carbohydrate residues. The presence of six cystein residues is unique, and J chain is
synthesized by plasma cells engaged in the production of IgM(and IgA).
IgE
: The molecular weight is 190,000 and the sedimentation coefficient is 8S.
IgE is present in normal human serum in very low concentration, and it has the shortest survival
(the half-life in serum is about 2-3 days), highest fractional catabolic rate and lowest synthetic rate.
IgE is susceptible to denaturation by the treatment with heat and reducing agents under
conditions that have little effect on other Ig.
IgA
: Monomeric IgA molecules have a molecular weight of 150,000 daltons;
polymers range inweight up to 350,000 daltons. Polymer forms of IgA contain the J or joining chain.
Two subclasses of IgA are identified on the basis of differences in structure of the heavy alpha
chain; IgA1 and IgA2. IgA2 molecules are further classified into two categories based on the
presence of the genetic markers; A2m(1) and A2m(2).
Secretory IgA: The Ig patterns of external secretions is characterized by a secretory IgA in
saliva, tears, nasal, tracheobronchial, intestinal and cervical fluids as well as bile and urine. The
majority of IgA molecules in external secretions exist as dimers composed of two 7S IgA monomers
plus two other non-Ig proteins, J chain and secretory component(SC)(M.W.71,000). Schematic
representation of the scretory IgA molecule is shown as follows; 
Plasma cells containing IgA predominate in the lamina propria of the respiratory and gastrointestinal
tract. Secretory component is localized in mucosal epithelium, in intracellular spaces of the mucosa
and on the surface of the epithelial cells lining the lumen of secretory glands; SC is not found in
plsama cells. J chain is localized to plasma cells of the lamina propria. SC provides a receptor for
dimeric IgA, and covalent and noncovalent interaction between SC and dimeric IgA are completed
during the passage of the dimer between or through the epithelial cells. SC functions to protect
dimeric Iga from proteolysis by gastrointesinal enzymes.
IgA protease: Several bacterial species, including some normally residing in the intestinal
tracts, secrete a proteolytic enzyme with substrate specificity for IgA. This enzyme cleaves a bond
in the hinge region of IgA1. This bond is absent in IgA2 and the enzyme does not appear to cleave
IgA2 molecules.
IgD
: Normal human IgD is difficult to investigate because of the low concentration
of this protein inserum, average of 30 ug/ml, and because of the tendency of IgD to aggregate and
fragment during isolation. IgD is susceptible to proteolysis by the enzyme plasmin, formed during the
clotting of blood. IgD, as well as IgE, is susceptible to denaturation by heat and acid treatment under
conditions which do not affect IgS, IgA or IgM. The rate of synthesis of IgD is 100 times lower than that
of IgG, and has a fast catabolic rate(serum half life; about 3 days).
Secretory IgA: The Ig patterns of external secretions is characterized by a secretory IgA in saliva, tears, nasal, tracheobronchial, intestinal and cervical fluids as well as bile and urine. The majority of IgA molecules in external secretions exist as dimers composed of two 7S IgA monomers plus two other non-Ig proteins, J chain and secretory component(SC)(M.W.71,000). Schematic representation of the scretory IgA molecule is shown as follows;
Plasma cells containing IgA predominate in the lamina propria of the respiratory and gastrointestinal tract. Secretory component is localized in mucosal epithelium, in intracellular spaces of the mucosa and on the surface of the epithelial cells lining the lumen of secretory glands; SC is not found in plsama cells. J chain is localized to plasma cells of the lamina propria. SC provides a receptor for dimeric IgA, and covalent and noncovalent interaction between SC and dimeric IgA are completed during the passage of the dimer between or through the epithelial cells. SC functions to protect dimeric Iga from proteolysis by gastrointesinal enzymes.
IgA protease: Several bacterial species, including some normally residing in the intestinal tracts, secrete a proteolytic enzyme with substrate specificity for IgA. This enzyme cleaves a bond in the hinge region of IgA1. This bond is absent in IgA2 and the enzyme does not appear to cleave IgA2 molecules.
