Immunoglobulins (Igs), commonly known as antibodies, are Y-shaped proteins that are key players in the adaptive immune system. These proteins are made to bind to specific antigens, such as those found on bacteria and other foreign substances. Immunoglobulins generally consist of two heavy chains and two light chains that are joined by disulfide bonds. Both types of chains (heavy and light) are comprised of two regions: a constant region and a variable region. The variable region is made up of variable domains from both the heavy and light chains which determines the specificity of the immunoglobulin to bind an antigen. Immunoglobulins can either be membrane-bound (B-cell receptor) or secreted (antibody). All jawed vertebrates possess immunoglobulin M (IgM), but other immunoglobulin types vary among species. For example, humans have five total immunoglobulin isotypes: IgM, IgD, IgG, IgE, and IgA.
IgM can trace its roots all the way back to cartilaginous fishes making it the immunoglobulin with the longest evolutionary history. In addition to its presence in myriad animal taxa, it is the first immunoglobulin that responds when an antigen is present. Another primordial immunoglobulin is IgD. This immunoglobulin is still poorly understood, but the secreted form is known to bind to basophil surfaces and, overall, IgD is thought to enhance mucosal immunity. In humans, the most abundant immunoglobulin is IgG (accounting for ~75% of our serum antibodies) which is the only human antibody that can cross the placental barrier to confer immunity to newborn babies–an essential role. While IgA plays a crucial role in mucosal immunity, IgE is critical for defense against parasites, and is also involved in the dreaded allergic responses. IgE is specific to mammals and is the least abundant immunoglobulin in humans.
IgY is an important immunoglobulin found in reptiles, birds, and amphibians, and although mammals lack this particular immunoglobulin, both IgG and IgE are believed to have been derived from an IgY ancestor. In fact, IgG and IgY are believed to have the same functionality: high-affinity memory responses.
There are a few examples in Kingdom Animalia where immunoglobulins get weird and interesting: in some reptiles, birds, and amphibians, another form of IgY exists that is smaller: some animals have one or the other form of IgY, but some animals have both! It’s still unclear exactly how the smaller form of IgY works, or what evolutionary advantages it presents to those animals which make it. The humble camelids (which include camels, alpacas, llamas, etc.) also have a unique immunoglobulin which is a modified form of the mammalian IgG. The camelid Ig lacks light chains (an exception to the “two heavy chains, two light chains” rule)! A similar form is found in cartilaginous fishes, called immunoglobulin new antigen receptor (IgNAR).
The immune system is complex and full of variability between animal groups (and sometimes even within them!), but this complexity is essential for life to compete in the constant arms race against the mostly harmless, but sometimes harmful, world of microbes.
Featured Image: BiotechMichael, Wikimedia Commons Public Domain