An allergen is a type of antigen that produces an abnormally vigorous immune response in which the immune system fights off a perceived threat that would otherwise be harmless to the body. Such reactions are called allergies.
Antibodies against Phl p 5 can be used as a marker of allergy to grasses in adult patients. Children reacted exclusively to minor allergens more frequently than adults. Prolonged allergen exposure is evidently necessary to induce sensitization to Phl p 5.