Help! Immune function decrease with time (not age)??

[Dr. Hans J. Kugler]

 IF YOU HAVE ANY REFERENCES OR OTHER DOCUMENTATION SHOWING THAT THERE IS
A DECREASE (or any kind of change) IN IMMUNE FUNCTIONS OVER TIME
(20, 40 years ago versus today) IN HUMANS (or animals) PLEASE RESPOND!

With all the recent publications regarding decreases in human
reproductive capacity over the past 50 years (blamed on toxic 
chemicals in the environment), we wondered if the immune system
of the - generally accepted as "healthy" - average population has 
also suffered. 
I vaguely remember a statement by a professor of pathology saying
that "modern" men's thymus atrophies more quickly. 
I did a lit search via NERAC but nothing came up!
With most areas of cellular and molecular immunology defined only
recently, how could one show if there really was a decrease of
immune functions over time? Possibly a greater onset of immune-
related diseases in specific age-groups 20 or 50 years ago versus
today?
Any suggestions?              Hans J. Kugler, PhD
                              Editor, PREVENTIVE MEDICINE UP-DATE
       E-mail or              218 Ave. B
                              Redondo Beach, CA 90277