What are Antibodies?
January 7th, 2009 by The Doc
We’ve all heard about them: Antibodies are those “things” that protect us from infection. But not many of us have a really good grip on what they look like, where they come from, or how they work.
Antibodies are large, protein molecules composed of four polypeptide chains (two identical heavy chains and two identical light chains). Each heavy chain is shaped like a boomerang; the two boomerangs are joined along one of their arms by disulfide bonds to form a “Y” shape. One light chain is linked to the outside of each of the spreading arms of the “Y”.
The Y-shaped antibody molecule is divided into a variable (V) region and a constant (C) region. The V region is located at the ends of the Y’s spread arms, while the C region is located along the Y’s upright base.
Five different types of heavy chains determine the five different classes of antibodies present in human serum and tissues. These antibody classes are: IgM, IgA, IgG, IgE, and IgD. To make things more complicated (of course!), there are four subclasses of IgG.
IgM is the first antibody formed after exposure to a new antigen. IgG, the most prevalent antibody in serum, is the primary circulating antibody after re-exposure to an antigen. IgA is mainly found on the surface of mucous membranes and in secretions (tears, saliva, GI and respiratory tract, etc.). IgE is present in low levels in serum; it is involved in generating allergic responses. IgD is found on the surface of immature (“naïve”) B cells, where it plays a role in B cell maturation.
Antibodies are produced by B cells, which are specialized white blood cells. When a foreign invader (antigen) first enters the body, other specialized white blood cells (natural killer cells, for example) capture the invader and present it to an immature B cell. The B cell then “learns” to produce an antibody that is directed specifically against the new antigen.
Once a B cell has learned to produce an antibody, it always produces antibodies that are designed to deal specifically with one antigen (or antigens that are very similar to the original invader). A B cell can switch back and forth between antibody classes (e.g., it can convert from IgM to IgG), but the different classes of antibody produced by that particular B cell are only designed to deal with antigens that look alike.
In general, antibodies are immune system “snitches.” Once they encounter their assigned antigen, they grab onto it, making it more visible to the remainder of the immune system. Antibodies also create a tasty coating on the antigen, making it more appetizing to the white cells that will eventually consume and destroy it.
Great article! A complex topic explained in simple terms. I learned more with this than I did in my college biology class…
Hey there, Muted Mapmaker!
Thanks for the input. Biology teachers have to complicate matters by trying to explain things like opsonization, receptors, Bence-Jones proteins, kappa chains, highly-variable regions, epitopes, etc.
I can dispense with all of that, because Simple is my middle name…
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