They can develop a food allergy from a food they have been eating for years. Their systems change and what was fine one day can give them an allergic reaction the next day.
The signs of a food allergy are head and neck scratching as well as possibly hair loss from the scratching. Ear infections are also a sign of a food allergy. Diarrhea and vomiting are 
associated with food allergies as well as wind. These are responsible for around 10% of all cat allergies.
It is caused by the cat reacting to a particular agent in food causing an antibody response that occurs in the intestinal tract and is not fully comprehended.
Food allergies in cats can be caused from the cat eating beef, lamb, seafood, corn, soy, dairy products and wheat gluten. The vet may try an elimination process by placing the cat on a special diet over 12 weeks.
If the symptoms go away and then return after going back onto the diet it was eating before then a food allergy may be diagnosed. (Read more)
Inhalant allergies are common in cats. They often start in young cats and worsen as the cat gets older. They can be affected by weed pollens, grass, tree pollens, house dust mite, molds and mildew. The symptoms are itching severely.
Red and crusty rashes on head, neck, and back with the itching being very intense. Facial crust and scabby lesions as well as hair loss. Cats with inhalant allergies tend to have problems with other allergens also.
If they have many allergens then this itching and scratching can be severe and all year round whereas those with less allergies may only have this problem for a few weeks maybe twice in the year. Your vet is likely to perform a interdermal skin test or blood test.
Flea allergies are the most common allergy in cats. They tend to be affected by the seasons much more severe in the summer months and fall. It’s the saliva of the flea that causes the problem and watch for them biting frequently on their tail as this is a sign of flea allergy.
They will scratch and bite themselves and even cause scabs and sores. They have a wide range of symptoms including military dermatitis meaning they develop small red raised skin lesions.
To determine whether your cat is suffering from a flea allergy, you vet may well perform an intradermal or vitro allergy test. A positive test confirms a flea allergy. (Click here to read more)
Contact allergies are more local to the area where the cat has come into contact with something that they are allergic to. Not as common as the other allergies, they can get these from flea collars rugs and blankets. Your vet is likely to perform a skin biopsy or a patch test.
These are just generalized symptoms but if you believe your cat is experiencing a problem with allergies then take them to your vet.
It is hard to diagnose the problem yourself as there can be similar reactions from all the allergies and other illness can produce similar symptoms, so leave it to the vet to decide.