We say "have an allergy" (not "have allergy"). Most names of symptoms are countable, including the word "allergy".

We say "have an allergy" (not "have allergy").

My sister has an allergy to pollen.

My sister has allergy to pollen.

Most names of symptoms are countable, including the word "allergy".

My mother has a food allergy.

And we use a/an with singular countable nouns.

Do you have an allergy to peanuts?

Follow the same convention with the expressions "cause an allergy", "treat an allergy", "develop an allergy", "diagnose an allergy", etc.

Any type of food can potentially cause an allergy.

Your doctor can diagnose an allergy by using a skin test.

In many situations, we use a possessive pronoun (my, your, etc.) or another determiner (this, that, the, etc.) instead of "an".

What is causing your allergy?

You can also use another determiner (these, some, any, a number of, etc.) to identify or quantify a group of allergies.

Do you have any allergies?

But omit the article if you're using the plural form (allergies) in a general sense.

Allergies occur when your immune system reacts to a normally harmless substance.

Drug allergies can cause itchy skin and facial swelling.

A partial list of common symptoms that take the indefinite article (a/an) can be found below:

• Backache (The article can be omitted in British English.)
• Cold
• Cough
• Cramp/Muscle Cramp (The article can be omitted in British English.)
• Earache (The article can be omitted in British English.)
• Fever (The article can be omitted in British English.)
• Headache
• Heart attack
• Runny nose
• Sore throat
• Stomachache (The article can be omitted in British English.)
• Stroke
• Toothache (The article can be omitted in British English.)