Antibodies are groups of proteins composed of between 82 to 96% polypeptide and 4 to 18% carbohydrate. They are present in the serum and tissue fluids of vertebrates. Some are curried on the cell surface where they act as receptors while others are free in the blood or lymph, referred to as the antibodies. Plasma cells which are formed from precursor B-cells are responsible for producing immunoglobulins, and it does so in large quantities.
There are five immunoglobulin (antibody) heavy glycoprotein chains namely;
Immunoglobulin M (μ)
This constitute approximately 10% of the total immunoglobulins in the human adult; mainly occurring as a pentamer. These immunoglobulins are not flexible because they lack a hinge region in their structure. They are prominent in the primary immune response and predominates in certain antibody responses such as the natural blood group antibodies. Together with IgD, they are co expressed on the surface of B cells thus acting as antigenic receptors.
This immunoglobulin is the most efficient in compliment fixation. It is sometimes referred to as a primitive immunoglobulin as it is found also in lower animals and is the first immunoglobulin to be synthesized by the fetus.
Immunoglobulin D (δ)
This occurs in trace amounts (about 0.2%) as a monomer with slightly higher molecular weight than IgG. It is relatively labile to degradation by heat and proteolytic enzymes. IgD is co-expressed with IgM in the B-cell surface membranes. There has been isolated report of IgD activity against certain antigens such as insulin, penicillin, milk protein nuclear antigens, and diphtheria toxoids. However, its main function is still unclear.
Immunoglobulin A (α) (A antibodies)
This is the most predominant immunoglobulin in the body secretions existing in two forms namely the serum IgA and the secretory IgA, both of which account for about 15% of the total immunoglobulins in the body. IgA appears in the intestines at four weeks after birth. The secretory immunoglobulin A is a D-dimer and contains a secretory component which is coiled and wrapped around the fc fragments.
The secretory component is synthesized by the epithelial cells and it is responsible for transporting the IgA from the epithelial cells into the secretion and stabilizing as well as protecting the IgA from the proteolytic enzymes produced by the pathogens such as streptococci and Neisseria.
IgA is responsible for protection of the epithelia mucosal areas such as the upper respiratory, urogenital and the gastrointestinal tract. They are present in secretions such as the saliva, sweat, mucus and the colostrum where they provide protection against allergies, autoimmune disorders and respiratory tract infections.
Immunoglobulin G (γ)
This class of antibodies normally occur as trisomers and are the most abundant immunoglobulins in a normal adult. IgG constitute approximately 75% of the total serum immunoglobulins. It is the only immunoglobulin class that can cross the placental barrier and other vascular barriers due to its flexible molecules, therefore providing protection to the unborn child during the first few months of life. It is also capable of fixing or stimulating compliment reaction. They are the antibodies of secondary response.
IgG is further divided into γ1, γ2, γ3, and γ4 whose distinction is based on the number and arrangement of the inter heavy chain disulfide bonds in that the subclasses 1 and 4 have 2 disulphide chain bonds, subclass 2 has 4 disulphide chain bonds and subclass 3 has 15 disulphide chain bonds. Their normal serum concentrations are 60-70% for subclass 1, 14-20% for subclass 2, 4-8% for subclass 3 and 2-6% for subclass 4. Some of the exception in the subclasses of IgG are that IgG2 does not cross the placenta and IgG2 and IgG4 cannot stimulate a compliment reaction. However, they are all potent antitoxin and opsonic antibodies.
Immunoglobulin E (ε) (E antibodies)
IgE is the least common of the antibodies in a human adult constituting of about 0.004% of the total serum immunoglobulins. It has no hinge regions and sometimes occurring as pentamers. Its distinctive characteristic is that it binds with very high affinity to the mast cells or basophils through a site in the fc region in the presence of allergens.
Notably, high levels of IgE in the serum is associated with allergic hypersensitivity disease, such as atopic dermatitis, and extrinsic asthma conditions. Therefore, it is considered to mediate hypersensitive reactions and involved in protective immunity against helminthic infections with a very short half life of between 2 and 3 days.