Enzyme extracted from papaya. Topically, papaya latex can cause severe irritation and blisters. Severe allergic reactions have been reported in sensitive individuals. Other research has confirmed that papain is a strong allergen on skin. 
- Stremnitzer C, Manzano-Szalai K, Willensdorfer A, Starkl P, Pieper M, König P, Mildner M, Tschachler E, Reichart U, Jensen-Jarolim E. Papain Degrades Tight Junction Proteins of Human Keratinocytes In Vitro and Sensitizes C57BL/6 Mice via the Skin Independent of its Enzymatic Activity or TLR4 Activation. J Invest Dermatol. 2015;135(7):1790-800.