Re: Why are people allergic to cats and not dogs?

I'm not sure of the answer to your question.  I did some searching of the 
medical literature at http://www.ncbi.nlm.nih.gov/entrez/query.fcgi

I found some studies which looked at the percent of people allergic to cat 
and/or dog dander (as well as many other allergens).  These studies said 
that upon testing many people were allergic to both types of dander, likely 
due to the fact that both types of dander contain very similar proteins, to 
which the people are allergic.  If this is true, and you think more people 
show symptoms to cats vs. dogs, then maybe there is something to what you 
suggest.  In other words, maybe cats shed more, or their hair stays more 
airborne, etc.

Another possibility that I thought of, for which I have no proof, is the 
following.  There are some studies which suggest that exposure to a pet 
during the newborn period lowers the incidence of later allergy to this type 
of animal.  This is called immunological tolerance.  I know that many 
pregnant women get rid of their cats, as cats can harbor toxoplasmosis, 
which is very dangerous for the developing fetus.  If fewer babies are 
exposed to cats during their first few months of life, maybe they are more 
likely to later develop allergies specifically to cats?  (One problem is 
that probably some expecting mothers also get rid of their dogs, just 
because they don't want a big dog around with a newborn, although I don't 
know of any health danger to babies that is carried by dogs.)  It's just an 
idea, but I couldn't find anything more specific to your question.  Hope 
this helps.