Concept:
Histamine is an important mediator released during allergic reactions.
Step 1: Histamine causes sneezing, itching, watery eyes, and nasal discharge.
Step 2: Antihistamines block histamine receptors.
Step 3: By blocking histamine action, they reduce allergic symptoms.
Step 4: They are commonly used in allergic rhinitis, urticaria, and itching.
Step 5: Therefore, antihistamines are mainly used for allergic conditions.
\[
\boxed{\text{Allergic conditions}}
\]