The substantial differences between photoallergic and phototoxic reactions.

With great interest, yet growing concern, I read the article published by Jakubowicz et al. in a recent issue of the journal AAEM [1], in which the authors addressed phototoxic reactions caused by Heracleum sosnowskyi. Without any doubt, this is a relevant challenge to public health in Poland, for the plant is spreading across the country in an uncontrolled way and people exposed to this plant and sunlight may su er severe adverse cutaneous reactions. However, the scienti cally incorrect terminology used by the authors throughout this article cannot be le without comment, as it fosters misunderstandings about this very important topic. Phototoxic reactions resulting from exposure to plants were rst described in France by Maurice Oppenheim in 1932, and then reinforced by his further observations in the USA [2]. e disease was then referred to as dermatitis striata pratensis bullosa (grass or meadow dermatitis), which typically presented as bullous eruptions on the areas of the skin which have been in contact with certain plants and subsequently exposed to sunlight. is historical name so accurately depicts the clinical picture and circumstances of the emergence of this disease that it is still preferred by some authors even today [3], although ‘phototoxic dermatitis’ seems much more sound scienti cally. e road to understanding of the underlying mechanisms was paved in 1938, when Hans Kuske isolated from various plants with known sun-sensitizing properties compounds responsible for phototoxicity, including psoralens [4]. We now know that there are 2 types of photoxicity: photodynamic, which requires oxygen, and non-photodynamic, which is not oxygendependent. e reactions induced by psoralens, for the most part, are non-photodynamic [5]. By 1959, the relationship between the structures of furocoumarins and their biological e ect were quite well-known [6], and the therapeutic use of their phototoxic properties had begun. ‘Phototoxic’ indeed, and certainly not ‘photoallergic’ as Jakubowicz et al. have put it. e di erences between photoallergic and phototoxic reactions (Table 1) are substantial – as substantial as are those between allergic contact dermatitis and irritant contact dermatitis in the skin, or allergy and toxicity in general [7]. us, calling the archetypically phototoxic psoralens in Heracleum as ‘photoallergic’ agents is as incorrect scienti cally as would be referring to arsenic poisoning as ‘arsenic allergy’. Photoallergic reactions depend on individual predispositions and involve speci c immunologic reactions, including antigen recognition by speci c T-cells. ese reactions, as a rule, are unpredictable, but very rare. erefore, photoallergies seem not well suited to be addressed with the means of public health, health policy or consumer protection. Simply put, a great e ort would bring bene t to a very small group of a ected people. It would be like e.g. imposing a ban on terbina ne – a safe and e ective antifungal drug that has already been on the market for several decades, just because of a single recently diagnosed case of photoallergic reaction to terbina ne [15]. us, photoallergies remain a domain of specialized clinicians keen on tracking down rare and unusual cases. Having said that, ‘endemics’ of photoallergic reactions may sometimes occur Table 1. The most important di erences between phototoxic and photoallergic reactions [8-14].