|MadSci Network: Immunology|
This is a very interesting question and to come straight to the point there is no good way to measure the total number of different antibodies present at a particular point in time. We could gather a very large panel of common antigens and determine antibody titers to each of them, but there would still be a lot of antibody of unknown specificity that we could not account for. Moreover, even fairly simple antigens can have multiple antigenic determinants or epitopes, which can bind different antibodies. Determining the number of epitopes on an antigen is very difficult. (see http://www.clu net.edu/BioDev/omm/epitope/molmast.htm for nice 3D structures of antibodies in complex with protein antigens to get a visual idea of an epitope). However, by making several assumptions we can calculate the theoretical maximum number of different antibodies in our bodies at one time and by making even more assumptions we can come up with a number that may give a very approximate answer to your question. IgG is the major antibody in blood (10mg/ml). It has a molecular weight of ~ 160,000 and can also be found in the interstitial fluids (fluids that exist outside of the blood circulation). For a 70 kg person the estimates are 5 L blood (but 2.1L of that is cells) and 10 L interstitial fluid giving a total of about 13 L of fluid that we could squeeze out of you. If the IgG levels are uniformly 10 mg/ml = 10 g/L = 130 g of antibody total. One mole of antibody (6.02 X 10^23 molecules) = 160,000 g. If you do the calculations you will come up with ~ 5 X 10^20 IgG molecules. (This is probably an underestimate since there is a very large amount of IgA antibody that is present in your intestine and other mucosal surfaces that we haven’t included.) This is a lot of antibodies but in reality each IgG antibody is not different and the number of identical molecules is probably quite large. For example, two weeks after a booster injection with tetanus toxoid, the specific antibody concentration in serum is about 33 ug/ml according to a recent publication: (http://www.sciencemag.org/cgi/data/304/5667/104/DC1/1) This would be about 0.3% of your total IgG or 1.6 X 10^18 molecules. But tetanus is a fairly complex protein antigen with different antibodies recognizing different epitopes. If we guess 100-1000 epitopes, that would mean 10^15 – 10^16 identical anybody molecules. If tetanus is a representative antigen then you could have in the range of 50,000 – 500,000 different antibodies at one time. As you probably realize, we have made a lot of assumptions here, so I hope you will take this number with a grain of salt.
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