A friend has an allergic reaction to touching some but not all of my short-haired cats. (He does not have any issues with just being in the same room; petting or holding the cat is what sets it off. He also did not react to the one long-haired cat I've had.) In one case he reacted to one of a pair of littermates but not the other. All of the cats eat the same food and live in the same indoor environment. What characteristics of the cats affect this? Short of bringing my friend with me to the animal shelter, is there anything I can do to minimize this consideration when adopting a new cat?
Oddly enough, it could be the color of their fur.
While the reason is unclear, researchers at Long Island College Hospital in New York speculate that darker cats may produce more of an allergen called fel d 1 in their skin and saliva than do lighter colored felines.
A study of 321 patients found that those who owned dark-colored cats were up to four times more likely than those with light-colored cats or no feline friends to suffer moderate or severe allergy symptoms, according to a report to be published in the Annals of Allergy, Asthma and Immunology.
Then again, as is often the case with science, another study has reportedly found the exact opposite result. There is a study titled Fel d 1 levels in domestic living rooms are not related to cat color or hair length, but I cannot find details of the actual study (possibly due to the current government shutdown). It is possible that the previous study may have simply been too small, and insufficiently randomized, as the cohort was only 321 patients.
According to yet another study, it is also possible that male cats may produce more fel d 1 than female cats, and intact males may produce more than neutered males.
In any case, fel d 1 levels do seem to be the most likely culprit. Since the hormone is secreted in saliva and from the sebaceous glands, it is most abundantly produced during grooming.
To mitigate your friend's exposure, you can try to keep him away from areas where the cats are most likely to groom themselves.