Your immune system is designed to protect your body against harmful substances, such as bacteria, viruses, and foreign substances (allergens). In that sense, allergic responses are not inherently bad. But in a person with allergies, the immune response is exaggerated, and you react to substances that are not generally harmful.
The word “allergies” is kind of a catch-all term, since it can refer to allergic conjunctivitis, atopic dermatitis (eczema), contact dermatitis, hay fever (seasonal allergies), or food or drug allergies, which tend to manifest symptoms all over the body.
Common allergens include drugs, dust, food (these can be to the protein, starch, additive, or pesticide on the food), insect bites, mold, pet dander, pollen, hot or cold temperatures, sunlight, or other environmental triggers. Sometimes even friction can cause symptoms in highly reactive individuals.