Grass pollen immunotherapy decreases the number of mast cells in the skin

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Abstract
Allergen injection immunotherapy is effective for summer hay fever and reduces cutaneous sensitivity to grass pollen. We have addressed whether this effect of immunotherapy may be due to a decrease in mast cell numbers in the skin. Total mast cells and mast cell subtypes in the dermis were measured by dual immunocytochemistry in 40 adult patients who had received either ‘active’ grass pollen immunotherapy or placebo injections for 9 months in a double-blind clinical trial. Clinical improvement in hay fever was accompanied by a greater than 10-fold reduction in the immediate cutaneous response to grass pollen (P = 0.0002) and a sevenfold decrease in mast cell numbers in the skin (P = 0.0001). The number of mast cells after immunotherapy correlated with the clinical response in terms of seasonal symptoms (r = 0.61, P = 0.001) and rescue medication use (r = 0.75, P = 0.0001). Specific double immunostaining showed that the majority of mast cells (greater than 60%) were tryptase/chymase-positive (MCTC) and the remainder tryptase-only (MCT) cells. Following immunotherapy both subtypes were equally reduced. One mechanism by which immunotherapy may act is to reduce mast cell numbers with a consequent reduction in immediate allergic sensitivity.