AssessmentsIsolated primary immunoglobulin M deficiency
Isolated primary immunoglobulin M deficiency
Let’s take a look at how B cells end up secreting different types of antibodies.
An antigen is any substance recognized by that particular antibody.
Each antibody has two identical light chains and two identical heavy chains that combine into a Y shape.
So this Y-shaped antibody’s got two arms with identical tips, which is called the variable region.
This variable region contains an antigen binding domain that’s unique to that antibody.
Below the variable region, or toward the point where the arms meet, is the constant region where every member of an antibody class is identical – so all IgM antibodies have the same constant regions, but IgM and IgA constant regions are different.
For example, IgMs are part of B cell receptors, and are the first free-floating antibodies produced in an immune response.
They’re secreted as a pentamer, meaning there are five antibodies connected together, which provides many binding sites for grabbing antigens and taking them out of the blood.
IgG antibodies stick to the surface of bacteria and viruses – and that prevents them from adhering to and infecting cells.
IgA antibodies line mucosal tissues like the gastrointestinal and respiratory tracts and stop microbes from invading in the first place.
IgE antibodies work with eosinophils to destroy parasites, and as for IgD antibodies, they’re also used in some B cell receptors, just like IgMs are, but their function as free-floating antibodies is still actually unclear.
Each B cell has over 100,000 B cell receptors spread across its surface, all of which bind the same unique antigen.
When a B cell comes in contact with an antigen it recognizes, the B cell internalizes that antigen and then presents a piece of it on a major histocompatibility complex class II molecule, or MHC-class II for short.
Then, at some point, along comes a CD4+ helper T cell that binds to the presented antigen and when that happens, it expresses a protein called CD40 ligand on its surface.