An allergy is when the body’s immune system sees a substance as a harmful “invader” and overreacts to it. Allergens are the substances that trigger an immune response, called an allergic reaction.
Some symptoms of an allergy can be similar to those of the common cold or flu, but there are some ways you may be able to tell what is causing your symptoms. Allergy symptoms usually last while exposed to the allergen, which can be around six weeks during pollen seasons in the spring, summer, or fall. Colds and flu rarely last beyond two weeks.
Symptoms of allergies that may be similar to or different from colds and the flu include:
- Fever
- Never with allergies
- Common with colds, rare with the flu
- Itchy, watery eyes/itchy nose
- Common with allergies
- Not normal with colds or the flu
- Headache
- Uncommon with an allergy or a cold
- Common with the flu
- General aches and pains
- Never with allergies
- Mild with a cold, common and sometimes severe with the flu
- Fatigue, weakness
- Sometimes with allergies or colds
- Usual with the flu, and can last up to 3 weeks
- Extreme exhaustion
- Never with allergies or colds
- Usual at the start of the flu
- Runny, stuffy nose
- Common with allergies or colds
- Only sometimes with the flu
- Sneezing
- Usual with allergies or colds
- Only sometimes with the flu
- Sore throat
- Sometimes with allergies or the flu
- Common with colds
- Cough
- Sometimes with allergies
- Common with colds and the flu (may be severe with the flu)
- Chest discomfort
- Rare with allergies, except for those with allergic asthma
- Mild to moderate with colds, common with the flu
Other common allergy symptoms include:
- Rash
- Hives
- Swelling
- Skin redness
- Stomach cramps
- Vomiting
- Diarrhea
- Bloating
- Pain
- Tongue swelling
- Swelling in the mouth and throat/throat closing
- Wheezing
- Chest tightness
- Shortness of breath
- Lightheadedness or fainting
- A sense of “impending doom”
- Low blood pressure (hypotension)
- Rapid and weak pulse
Some of the above symptoms may be severe and can be signs of a serious and potentially life-threatening allergic reaction (anaphylaxis). Sudden, severe, widespread reactions require emergency care. Call 911 if you suspect someone is having a severe allergic reaction.
See a doctor for a diagnosis if you are unsure whether your symptoms are caused by allergies, a cold, or the flu. Treatment is different for each condition.
What Causes Allergies?
Allergic reactions result when the body’s immune system sees a substance as a harmful “invader” and overreacts to it.
Allergens can enter or come into contact with the body in several ways:
- Inhaled, such as pollen, dust, mold, and dander
- Ingested, such as foods and medications
- Injected, such as from injected medications or insect bites and stings
- Absorbed through the skin, such as from plants like poison ivy, latex, metals, and ingredients in household products
How Are Allergies Diagnosed?
Allergies are diagnosed starting with a medical history and physical exam.
Tests used to diagnose allergies may include:
- Skin prick test
- Intradermal skin test
- Blood tests (specific IgE)
- Physician-supervised challenge tests
- Patch Test
What Is the Treatment for Allergies?
The first line of treatment for allergies is to avoid known allergens if possible. When this is not possible, medications used to treat allergies include:
- Nasal corticosteroids (nose sprays) for nasal allergies
- Antihistamines to help with sneezing, itching, runny nose, and hives often caused by seasonal and indoor allergies
- Mast cell stabilizers to help with itchy, watery eyes, or an itchy, runny nose
- Decongestants to reduce nasal congestion
- Corticosteroid creams or ointments to relieve itching and rash
- Oral corticosteroids to reduce swelling and stop severe allergic reactions
- Immunotherapy
- Allergy shots
- Sublingual immunotherapy (SLIT)
- Epinephrine is administered during a life-threatening anaphylaxis (severe allergic reaction)
From 
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https://www.aafa.org/allergies.aspx
https://newsinhealth.nih.gov/2014/10/cold-flu-or-allergy