['Grass pollen allergy is one of the most common allergies worldwide, and patients often show sensitization to an array of phylogenetically related species.', 'To determine the effect of specific immunotherapy (SIT) with Phleum pratense extract on induction of the immune response to a mixture composed of 5 grass pollen extracts.', 'Forty-six adult patients suffering from rhinitis and sensitized to a mix of grass pollen allergen extracts were randomized 3:1 to receive a short course of SIT with P. pratense or to an open control group without SIT. At baseline and after 3-4 months, we evaluated levels of specific immunoglobulin (Ig) E and IgG4, as well as the immediate and delayed cutaneous responses to the grass mix and P. pratense. IgG4 to Lolium perenne was also determined.', 'Levels of IgE and IgG4 to grass mix and P. pratense increased significantly during treatment (P<.001). However, this increase was only significantly higher in the SIT group than in the control group for IgG4 (P<.001). The levels of IgG4 to Phl p 5 and Lol p 5 were highly correlated (r=0.99, P<.001). The immediate and delayed cutaneous responses were significantly diminished to both extracts after SIT (P<.001).'] Does immunotherapy with a Phleum pratense allergen extract induce an immune response to a grass-mix allergen extract?