Allergen that is ubiquitous or occurs in numerous allergen sources. This means that the same IgE molecule can bind to different allergens (antigens) that carry the same epitope.
Often panallergens are evolutionarily highly conserved allergens (their structure has hardly changed over evolution). This "structural fidelity" explains many unexpected cross-reactions.
Examples of panallergens are profilins or tropomyosins.