Immunoglobins

An **immunoglobin** is a specific protein called an antibody synthesized in response to the presence of a foreign substance (antigen). The antibody has a specific molecular structure capable of recognizing a complementary molecular structure on the antigen which might be some proteins, polysaccharides, and nucleic acids. Small organic foreign molecules do not by themselves elicit antibody formation unless they become bonded to one of the larger biomolecules listed above. Studies have shown that immunoglobin G (IgG), the most common human immunoglobin, consists of two long "heavy" chains (A and B) and two short "light" chains (C and D). These are in the shape of a Y bonded together with disulfide bonds. There are several major areas of globular tertiary structure on the chains. The globular structure on the ends of the chains may be variable and accounts for some of the differences in specificity for different antigens. There are also two carbohydrate chains in between the A and B chains. There are several types of immunoglobins where the composition and structure is slightly different, but still maintains the Y shape. IgG - major serum immunoglobin (antibody).

Ig M
- first antibody produced in response to an infection.

IgA
- protects body surfaces, mucous membranes, oral cavity.

IgD
- thought to regulate antibody synthesis.

IgE
- found in larger amounts in people with allergies

Among allergen-specific IgG antibodies in allergic individuals, there is a preponderance of IgG1 and IgG4, while IgG2 and IgG3 responses are small. Other findings in allergic patients include the following: -Elevated IgG4 concentrations often occur in sera of patients with atopic eczema and dermatitis, probably as the result of prolonged antigenic stimulation (88). -In allergy to many different allergens, allergen-specific IgG antibodies are predominantly of the IgG4 subclass and their levels increase during desensitisation therapy. In the antibody response to desensitization/immunotherapy, initially mainly IgG1 is formed, whereas IgG4 becomes more prominent after 1-2 years. Allergen-specific IgG4 has often been regarded as a two-headed phenomenon: potentially harmful as well as potentially protective However, when more is found out about IgG4 antibodies, the harmful effects are hard to substantiate. The protective effects are still debated, but particularly from the field of parasitology the evidence is accumulating that IgG4 does, under certain conditions, effectively interfere with allergen-induced, IgE-medical effector cell triggering, i.e. IgG4 acts as a blocking antibody. Recent data indicate a striking similarity with respect to the type of antigen that triggers the IgG4 and IgE immune responses. Since a marked difference in epitope specificity exists between the IgG4 and IgE antibodies, only a fraction of the allergen-specific IgG4 can interfere effectively with IgE binding. The use of IgG4 antibody assays to monitor immunotherapy is justifiable, but its value should not be overrated. However, if no IgG4 antibody is induced by conventional immunotherapy, the therapy is likely to have been ineffective. An immunotherapy may be considered to be immunologically effective if a substantial increase (10 to 100 fold) in allergen-specific IgG4 is induced (89).