Final answer:
An allergen is any substance that can provoke hypersensitivity reactions, such as pollen, food, and animal hair. Allergens can lead to symptoms ranging from mild sneezing and itchy eyes to severe reactions like anaphylactic shock.
Step-by-step explanation:
An allergen is any antigen that causes an allergic response, which is an immune reaction resulting from immediate hypersensitivities where an antibody-mediated immune response occurs within minutes of exposure to a usually harmless substance. Common allergens include pollen, dust mites, mold, specific foods such as peanuts and shellfish, insect stings, and certain medications such as aspirin. These allergens may be inhaled, ingested, or come into contact with the skin, leading to various symptoms ranging from mild issues like sneezing and itchy eyes to severe reactions like anaphylaxis.