How Structures of the Different Classes of Immunoglobulin Molecules Affect Their Biological Activity - Essay Example

The physical, chemical, as well as antigenic variations in the immunoglobulins, have led to their classification into five classes which are known as IgG, IgM, IgA, IgD, and IgE (Ananthanarayan and Paniker 2005 p84,85; Levinson 2008 p426).

The IgG molecule is made up of two light chains and two heavy chains which are both connected with the help of disulfide bonds. It is the most abundant immunoglobulin found in the serum. IgG is the most significant immunoglobulin for combatting bacteria and viruses. Also, it possesses the property of easily crossing the placental barrier. This is because of its Fc portion which can attach to the receptors that are present on the outer surface of the cells of the placenta and thus allow its entry. This is also the reason why this immunoglobulin is found in significant quantities in a newborn child. Opsonisation is also a characteristic of IgG. This function is possible owing to the gamma heavy chain. Receptors for these gamma heavy chains are located on the outer portion of the phagocytes and this binding leads to opsonization. The IgG immunoglobulin also has different sugars bound to its heavy chain and in particular to the CH2 domain. The attachment of different sugars results in differing roles of this immunoglobulin. If the terminal sugar is N-acetyl glucosamine, the action of IgG is to promote the inflammatory process. This is caused by the binding of N-acetyl glucosamine to the mannose-binding ligand and thus resulting in the activation of the complement system. On the other hand, if the IgG has sialic acid on its terminal, there is downregulation of the inflammatory process (Levinson 2008 p. 427,429; Schroder and Cavacini 2010 p. 48; Kayser and Bienz 2005 p.56).

IgA is another immunoglobulin that is marked to be the significant antibody of the mucosa. It is produced by the plasma cells of the intestine, respiratory tract as well as mammary glands. The IgA prevents the invasion of the epithelial layer by toxins as well as pathogens and prevents their attachment to the epithelial layers. When it is produced at the mucosal surfaces, it is found as a dimer possessing two heavy chains and two light chains which are joined together with the assistance of a J chain. On the other hand, in serum, it exists as a monomer and it is believed to be a probable activator of the complement mechanism by the alternate pathway. Its role in opsonization is also suspected but not confirmed due to the presence of FcRs for IgA on the macrophages (Macpherson and Austyn 2012 p.227).

IgE is an immunoglobulin that does not have a very high quantity in the human body. In normal physiological conditions, IgE exists in very low quantities (Approximately 0.04) in the serum. IgE has an important function in allergic reactions. This is mediated by the Fc portion of the IgE immunoglobulin which attaches to the outer surface of the mast cells and basophils. This complex of IgE with mast cells and basophils functions as a receptor molecule for allergens. Cross-linking of the attachment point of the IgEs which lie together is done by allergens and this leads to the production of allergic reaction mediators which include histamine (Levinson 2008 p.430,431). Also, IgE acts on the eosinophil for the secretion of enzymes that destroy worms. IgE can also work towards preventing the entry of infectious agents that have escaped the defense mechanisms of IgA in the epithelial layer. This function is achieved by the action of IgE on the mast cells which leads to the release of chemotactic factors for granulocytes and hence promotes an inflammatory reaction (Shetty 2005 p.31).

IgM is also an important immunoglobulin as it is the first immunoglobulin to be secreted as part of the primary immune response of the body.  It exists as a monomer on the outer surface of the B cells where it performs the task of the receptor of antigens. This function of the IgM is due to the presence of a hydrophobic sequence on the heavy mu chain of IgM which results in the binding of the IgM to the B cells. This hydrophobic sequence lacks the heavy mu chain of IgM in the serum which results in it being present freely. Another characteristic of this immunoglobulin is that it exists as a pentamer in the serum and possesses ten sites for the binding of antigens. This immunoglobulin can utilize all its binding sites simultaneously in combatting against antigens and hence is beneficial in the immune response. It has a significant role in agglutination as well as the initiation of the complement system (Levinson 2008 p.430; Kayser and Bienz 2005 p.56).