Immunoglobulins - Ig

Immunoglobulins are Y shaped glycoproteins that bond with antigens. We referred to these as antibodies in previous essays. The difference in terminology is this: immunogloblins that are on the surface of lymphocytes are called immunoglobulins, those that are secreted into the blood by plasma cells are called antibodies. It’s all about location. They are made of four polypeptide chains – two ‘heavy’ chains and two ‘light’ chains with a hinge in the neck region to allow easier coupling with antibodies. The binding sites on these molecules are at the tips of the Y arms. Have a look at this picture I pinched off the internet: 

There are five types of heavy chain peptides used to construct these molecules, resulting in five different classes of immunoglobulins or antibodies. We abbreviate immunoglobulin to Ig and the types of immunoglobulins are called IgA, IgD, IgE, IgG, and IgM. Due to their different structures they have different properties when it comes to battling antigens.

In alphabetical order the general roles of the different classes of immunoglobulins are:

• Ig A – these occur on surface tissues that are likely to be infected such as the lung and the gut lining. They are also found in tears and sweat. These work in conjunction with barriers to prevent infection.
• Ig D – Discovered in 1965 but what it does is still a mystery – potential PhD thesis for some budding young scientist?
• Ig E – These activate mast cells and get them to release histamine
• Ig G – These kick start macrophages and the complementary protein system into action. They primarily attack bacteria and viruses
• Ig M – These are the most numerous on the immunoglobulins and Ig M's are the first class of immunoglobulins to be produced during an antibody response. The role of these is very similar to that of Ig G and at this level, that’s all you need to know.

Here's a 3D model showing protein structure:

Remember: that these are broad classes of immunoglobulins and specific antibodies within these groups target specific, individual pathogens/antigens.