Immunoglobulins (Ig), also called antibodies (Ab), are large Y-shape proteins produced by plasma cells that are used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, called an antigen
They are grouped into isotypical subclasses called IgG, IgD, IgE, IgA, IgM, all of which presents a different heavy chain (H-chain).
The configuration of the heavy chains in IgM, for example, is such that IgM forms into pentamers (five molecules joined together), and forms a five-pointed star shape. IgA, however, ordinarily exists as dimers (i.e. two molecules together).
Immunoglobulins are related to other molecules: the "immunoglobulin superfamily" contains CD4, CD8, the T Cell Receptor (TCR), and HLA.
While the rule is that the genome is the same in every cell in the body, some white blood cells that function as part of the immune system provide exceptions to that rule. The plasma cell is able to synthesize proteins called immunoglobulins that can function as antibodies. For decades, immunologists puzzled over how the immune system could possibly generate so many different types of antibodies. Could all the 10 million different types of antibody proteins be encoded in the genome? This would take up an enormous amount of chromosomal space. Moreover, how could the immune system “know” how to make an antibody to some foreign molecule (antigen) that isn’t even found outside the laboratory? Researchers eventually discovered that the genome of the B cell does not contain DNA encoding any of the antibody proteins. Rather, plasma cell DNA is rearranged during the cell’s development from a B cell to create the antibody-encoding genes. Moreover, while the mammalian organism has the ability to synthesize over 10 million different types of antibody proteins, each plasma cell can synthesize only one.
All immunoglobulin proteins have a similar structure. Each consists of two pairs of polypeptide subunits. There are two identical heavy chains and two identical light chains; the chains are linked together by disulfide bonds (Figure 1). The specificity of the immunoglobulin molecule (i.e., whether it will bind to a poliovirus, an E. coli cell, or some other antigen) is determined by the amino acid sequence of the variable regions at the amino-terminal ends of the heavy and light chains. The variable regions of the immunoglobulin molecule are attached to constant regions that give the antibody the effector properties needed for inactivating the antigen.
Ig as growth factors receptors