Background: Grass pollen allergy is one of the most common allergies worldwide, and patients often show sensitization to an array of phylogenetically related species.

Objective: 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.

Methods: 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.

Results: 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).

Conclusions: Patients with rhinoconjunctivitis diagnosed using skin prick testing with a grass mix allergen extract and treated with a short course of SIT based on a single species P pratense allergen extract are able to develop an immune response that targets not only the immunizing species, but also the grass mix allergen extract.

Key words: Allergen. Grass pollen. Cross-reactivity. Specific immunotherapy (SIT).