Most people grow into allergy, not out of it. Allergies can have a big impact on daily activities to many people like feeling constantly fatigued, irritable, or just not feeling well. People with allergies often have difficulty in concentrating, which can affect performance of work or school. When people are allergic, their immune system releases a chemical substance inside their body called histamine. Once histamine is released, it attaches to histamine receptors, and it is this interaction that triggers allergy symptoms. The antihistamine blocks histamine from attaching to the histamine receptors. This is how antihistamines work to relieve allergy symptoms.