To date, eleven groups of grass pollen allergens eliciting a specific IgE response in atopic individuals have been identified. Groups 1 and 5 allergens are the most critical (major) pollen allergens leading to the sensitization of 90% and 65–85% allergic patients, respectively. Other allergens frequently involved in the IgE response include groups 2/3, 4, 6, 7, 10–13 allergens. Allergens found in various Pooideae exhibit high homology in terms of their amino acid sequence composition, which translates into significant cross-reactivity in terms of antibody (IgE and IgG) as well as T cell responses. Nevertheless, for a given allergen group, there is evidence of both interspecies (i.e. differences in amino acid sequences) and intraspecies (multigenes, post-translational modification, mRNA splicing or editing) molecular variability.