Cats are the domestic animals that are most likely to provoke an allergic reaction. This reaction occurs as a result of exposure to dried animal saliva known as dander which is prevalent on felines as a result of their constant preening. This dander can even be spread onto peoples clothes through contact and this in turn can independently cause a reaction from individuals without them being directly exposed to the animal itself.
Dogs are also a cause of allergic reaction among many people for the same reason as cats but not to such an extent. It is often thought that the length of an animal's coat might influence the likelihood of adverse reaction. The truth is there is no direct correlation that would conclude that individuals are more likely to be allergic to a long haired dog and visa versa.
Although rare, some people feel that they are allergic to pet birds. The most likely explanation however is that they are reacting to the tiny mites that live on them.
Even if the animal has been removed there remains a chance that individuals might continue to suffer attacks for 1-3 months after the animal has gone as its allergens could still linger on the furniture, carpets and curtains, etc. This can however be eradicated by thorough cleaning.