Can One Side of the Body Be More Allergic Than the Other? A. There are several possible explanations for one-sided sensitivity to a substance. One hypothesis is that the more sensitive side was the one where the substance first provoked a reaction, "and there are immune cells that remain in this area and can respond more rapidly," said Dr. Donald Belsito, a dermatologist at NewYork-Presbyterian Hospital/Columbia University Medical Center. This is called a recall reaction. Another suggestion, he said, is that the site that reacts most intensely is the one that experienced the highest level of the chemical causing the initial reaction. It has been shown that the higher the concentration of the chemical that induces the allergy, the lower the concentration needed to bring it back out. In a one-sided reaction involving the eyes, right-handed people tend to rub the right eye more than the left, making that side react more, said Dr. Marjorie L. Slankard, director of the allergy clinic at NewYork-Presbyterian. The many allergy cells around the eyes, called mast cells, can be physically disrupted so they release chemicals that cause swelling, redness and more itching. Mast cells in the skin may be more common in one area than another, Dr. Slankard said. In rare cases, an arm that shows greater sensitivity to one kind of allergy shot can show a more extreme reaction even to a different kind of shot.