Mild allergy to cats... will I build up an immunity?

I have had cats for DECADES and not had a major problem... however, I went with out any cats for two months, and I now how two new cats and am having mild allergy symptoms. Is it from a "NEW" cat my body simply has to get used to? Could all the cats in a closed off space at the shelter have given me the symptoms, and not necessarily just the two cats? I also have seasonal allergies, it is also possible that it came simply from mowing the lawn or high pollen counts.