The main protein thought to be responsible is Chicken serum albumin (which is identical to alpha livetin found in egg yolk).
This is a partially heat-labile allergen (partially damaged by heat); Small studies have shown that IgE reactivity to Chicken albumin was reduced by 88% after heating at 90 degrees Celsius for 30 minutes.
There is some anecdotal evidence that people may be unable to eat cooked fresh chicken without having allergic symptoms but are able to eat chicken which has been fully frozen before being thawed, cooked, and eaten. This can be explained by a protein shape change caused by freezing or by cooking for an extended period, essentially damaging the protein so it cannot cause a reaction.
The alpha livetin protein is partially heat labile, so some sufferers will only get reactions from raw chicken and others may get reactions depending on how high a temperature the meat was cooked at and how long for.
An allergy to poultry can sometimes be associated with an allergy to egg, this is called Bird-Egg Syndrome.
In these cases sensitivity to egg is to the proteins in egg yolk, alpha livetin (gal d 5), which is found in both chickens and eggs.
IgE allergy to egg is usually from the egg white (proteins gal d 1 to 4), so whilst people who are allergic to chickens can become intolerant or allergic to egg, it doesn’t usually work the other way around due to the different proteins involved.