What change is observed in leukocytes during an allergic disorder (type I hypersensitivity) often caused by asthma, hay fever, and drug reactions?

Increase in eosinophils.

Explanation:

The leukocytes are the white blood cell that plays an important role in the immune system. The white blood cell include eosinophil, basophil, neutrophil and monocytes.

The allergic reactions that are caused by hay fever or asthma is marked by the excess increase in the number of eosinophils in the body. The eosinophils are the first to reach at the site of parasitic infections and protect the body during the allergic reactions.

Thus, the answer is eosinophils.