Immunoglobulins (IgG) or antibodies are serum proteins, produced by B cells, which
a) inactivate, agglutinate antigens for phagocytosis,
b) activate complement system for cytolysis.
These immunoglobulins may be present on the surface of B- cells (surface immuno globulins) or secreted in extracellular medium (secretory immuno globulins).
Depending on their properties and role in immune system, immunoglobulins may be divided into 5 types —
• It constitutes —10% of serum and mainly remains within the circulatory compartment. 1gM antibodies are the first class of immunoglobulins secreted after antigenic challenge, though levels decline soon after the infection is controlled. Thus, raised levels indicate acute infection. 1gM does not cross the placenta and high 1gM levels in newborn indicate true fetal infection.
• It is the most abundant serum Immunoglobulin (—70%), mainly produced during second antigenic challenge.
After primary antigenic exposure, IgG levels slowly than 1gM levels and decline gradually after 6- 12 weeks (persisting longer than 1gM). However, on subsequent challenge with same antigen, IgG response is anamnestic, profound and persistent. Thus, IgG levels may be high due to past infection and not necessarily indicate acute infection.
Although predominantly present in serum, Immunoglobulins diffuses well in tissues as well as crosses the placenta to transfer the maternal immunity to baby. Hence, raised lgG titers in fetus or newborn not necessarily indicate fetal infection.
Four major subclasses are – IgG1, IgG2, IgG3 and IgG4C. Of these, IgG2 is unique, as it also responds to polysaccharide antigens.
• IgA contributes —15-20% of serum immunoglobulins (serum IgA), as well as also present in various body secretions (secretory IgA) e.g. nasobronchial or gut secretions, saliva, lacrimal fluid and colostrum (protective antibody in human milk).
Secretory IgA is predominantly responsible for local immunity i.e. clearance of antigen from mucosal sites. IgA does not activate complement system or provoke inflammatory response.
• IgE is present iii miniscule amounts in serum but acts as principal mediator of immediate hypersensitivity (allergy) as well as also has some role in immunity against helminthes.
Two types of IgE antibodies are — non specific IgE, present in sera/tissues of normal individuals which appear to defend against parasitic invasion; and specific or reaginic antibodies, synthesized on exposure to a specific allergen.
• IgD accounts for <1% of serum immunoglobulins with indefinite role, probably as an antigen receptor site on circulating B-cells.