An immunoglobulin heavy chain is one of the four main components of an antibody and is composed of 450 to 550 amino acids. All immunoglobulin is produced by plasma cells when an immune response is initiated within the body. The name immunoglobulin comes from the discovery that they bind with globular proteins in serums that contain an antibody. The term "heavy chain” refers to the polypeptide sequence length, which can be compared to an immunoglobulin light chain that has only around 200 amino acids. Antigen binding is the most prominent function of an immunoglobulin heavy chain, but in some cases, this initial binding does not provide immune-mediated protection or other function until other “effector functions,” like the fixation of antigen-specific complements, occur.
When the mere binding of an antibody does not initiate an immune response on its own, it may bind to other cells to increase biological activity. For example, lymphocytes, the body’s main “fighter” cells, phagocytes, which are cells that absorb and dispose of foreign substances, and platelets in the blood all have receptor sites for immunoglobulin. Another function of an immunoglobulin heavy chain is to help bind the immunoglobulin to receptors on cells called trophoblasts found on the placenta during pregnancy. This binding allows the immunoglobulin to transfer across the placental barrier, resulting in transferred antibodies and inherited immunity from mother to newborn. Fixation of other chemical complements by immunoglobulin is responsible for activities like the lysis of unwanted cells and the initiated release of secondary chemicals.
The specific sections of an immunoglobulin heavy chain that contain the majority of antigen binding sites are called Fab fragments. An immunoglobulin is often times broken down into its basic pieces before the Fab fragments can be efficiently utilized. An enzyme, papain, breaks the immunoglobulin at its hinged areas, which produces two identical immunoglobulin heavy chains and two identical immunoglobulin light chains. One of the Fab fragments that is only found on an immunoglobulin heavy chain is the Fab Fc fragment. The Fab Fc fragment contains two regions, specifically known as H2 and H3, and any immune-mediated activity that uses these sections depends on the breakdown of the immunoglobulin molecule because it leaves H2 and H3 exposed on the heavy chains.
Immunoglobulin is often organized by researchers into five classes based on the readily apparent differences in the amino acid sequence on its heavy chain. The differences can be detected by directing antibodies at heavy immunoglobulin chains and noting the biological reaction or lack thereof. Some common immunoglobulin classes include the Gamma heavy chain (IgG), Mu heavy chain (IgM), and the Alpha heavy chain (IgA).