Re: T-cell and B-cell immunology

The short answer is no.  It is difficult to be sure if you still have 
immunity to something you were immunized with 20 years ago.  The strength 
and length of memory depends on the nature of the antigen that you 
received, either as an immunization or as the natural disease.  However, 
based on experience, it appears that immunity to most childhood diseases is 
life long, either because of the persistence of memory cells, or because 
you are no longer susceptible for some non-immunologic reason.  Of the 
immunizations you have probably received, the only one routinely 
re-administered is tetanus, an inactivated toxin from the bacterium 
Clostridium tetani that apparently doesn't elicit good memory cell 
responses.  Smallpox vaccination is no longer practiced since the disease 
has been eradicated from the world due to a combined effort of the World 
Health Organization and local governments.  The virus only exists 
(hopefully) in two high-containment research facilities, and there has been 
a vigorous debate about whether or not these stocks should be destroyed.