Second-generation antihistamines such as loratadine (claritin) are prescribed for seasonal allergies because they:

Antihistamines are drugs that are used to treat allergic rhinitis (both seasonal and perennial) and other form of allergies that cause hives or urticaria. They can give relief when a patient has nasal congestion, sneezing or hives because of pollen, dust, or animal allergy (allergens). Second generation antihistamines such as loratidine are prescribed for seasonal allergies because they are less sedating than the first generation antihistamines.  

The second generation antihistamines like loratadine are prescribed for these allergies because the sedation caused by them is less than what is caused by first generation antihistamines They cure all kind of allergies caused by pollen, dust and animals. This sedation may lead to drowsiness. We can find these antihistamines naturally in onions and apples.