### Re: How many different clones of antibodies does a human being possess ?

Date: Fri Apr 9 13:58:38 2004
Posted By: Peter Burrows, Faculty, Microbiology
Area of science: Immunology
ID: 1080226999.Im
Message:
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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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