Experts define an allergy as an extreme response to the introduction of a foreign body(called an allergen) into the system of an individual. Generally, our immune systems are set up to fight against substances they deem harmful to our bodies. However, specific allergens tend to drive our defenses over the top. Sometimes the allergens(such as perfumes and pollen) are completely harmless but our bodies simply read them all wrong. Common allergens include pet dander, dust, pollen and specific types of perfumes. Cases of food allergy have also been reported across the board but these are not very common. Mostly, food poisoning involves legumes, shellfish and sea food.
It would be important to note that our bodies do not react the same way to the same allergens. Pollen may affect you in extreme ways but be totally harmless to the next person. Symptoms of allergy conditions include fatigue, irritability, stomach ache, coughing, wheezing, sneezing, hives, itches, nose congestion, running nose and a host of others, depending on the individual's immune responses.