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

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.