Antibodies, also known as immunoglobulins, are like the ammunition of the body’s humoral defense against microorganisms. Plasma cells are like the turret guns of the immune system that secrete thousands of antibodies per second. You should know the general structure of the 5 types of antibodies, as well as the specific structure of an individual antibody.
First, I’m going to draw a 2 by 2 grid to demonstrate the difference between the light and heavy chains, as well as the variable and constant regions. Each antibody has 2 light chains with a variable region known as V-L and a constant region known as C-L. Each heavy has a variable region known as V-H and three constant regions known as C-H-1, C-H-2, and C-H-3. Disulfide bonds join the light chain to the heavy chain here, and the heavy chains of IgG to each other here. These disulfide bonds bend here at the Hinge region so that the overall antibody structure forms a Y shape.
Four regions, V-L, C-L, V-H, and C-H-1 make up the fragment of antigen binding, or Fab. The variable regions of the Fab fragment determine the specific idiotype: the unique structure of which binds to a corresponding specific antigen. Each B cell expresses only one idiotype of an antibody. As an example, a lab-made immune Fab specific to digoxin is used as an antidote to overdoses of the medication.
C-H-2 and C-H-3 together make up the Fc region of the antibody, which corresponds to the stalk of the Y. C-H-2 has the special property of binding to complement, something I will cover in a future video.
Now let’s discuss the different isotypes of immunoglobulins. There are 5 types: IgG, IgE, IgD, IgM, and IgA. They are differentiated biochemically by their Fc regions and their different functions. All have monomer forms, but only IgA and IgM have different forms when secreted. IgM can exist as a pentamer linked together by disulfide bonds and a J chain, or joining chain. IgA can exist as a dimer with a J chain as well as a Secretory protein. If you were to know just very simple associations to each type then know these. IgM – Immediate response. IgG – delayed response. IgA – mucous membranes. IgE – allergies and parasites. IgD – unclear function.
Question: how many binding sites exist on secreted IgM? Think carefully! Most people’s initial reaction is to say five because secretory IgM exists as a pentamer. However, it is important to remember that each antibody monomer has two binding sites, and thus, secretory IgM has 2 times 5 = 10 binding sites.
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