BackgroundIn Japan, pollinosis caused by Japanese cypress Chamaecyparis obtusa (JCy) is one of the very common seasonal allergies. In JCy pollinosis, Cha o 1 is the first major allergen and Cha o 2 is the second major allergen. Recently, Cha o 3 was identified as a new JCy pollen allergen in JCy pollinosis. However, the relative contribution of Cha o 3 to JCy pollinosis compared to Cha o 1 and Cha o 2 has not been fully elucidated. ObjectiveThis study clarified the allergenicity of Cha o 3 compared to Cha o 1 and Cha o 2 in JCy pollinosis. MethodsWe recruited 27 patients with JCy pollinosis and performed basophil activation test (BAT) with native (n) Cha o 1, 2, and 3 purified from JCy pollen. In addition, we performed JCy-specific IgE suppression test. ResultsIn the BAT, 26 out of 27 patients (96%) or 18 out of 27 patients (67%) showed positive basophil activation in response to n Cha o 1 or n Cha o 2, respectively. Little CD203c expression in response to n Cha o 3 was seen. The presence of n Cha o 3 marginally reduced the titer levels of JCy-specific IgE. ConclusionCha o 3 showed little ability to activate basophils and suppress JCy-specific IgE titer levels compared to that of Cha o 1 or Cha o 2 in patients with JCy pollinosis. Thus, Cha o 3 may not be a major allergen in JCy pollinosis.