Phototoxicity and photoallergy

Area of Science:

• Dermatology
• Photobiology
• Toxicology

Background:

• Photosensitivity encompasses phototoxic and photoallergic reactions.
• Phototoxicity is significantly more prevalent than photoallergy.
• Phototoxicity presents in two forms: photodynamic (oxygen-requiring) and nonphotodynamic.

Purpose of the Study:

• To differentiate between phototoxic and photoallergic reactions.
• To describe the mechanisms and clinical manifestations of photosensitivity.
• To highlight the role of ultraviolet A (UVA) radiation in photoreactions.

Main Methods:

• Review of existing literature on photosensitivity.
• Classification of photoreactions based on mechanism (photodynamic vs. nonphotodynamic) and clinical presentation.
• Analysis of causative agents and their associated reaction types.

Main Results:

• Phototoxic reactions, common and varied, can manifest acutely (erythema, edema, hyperpigmentation) or chronically (sun damage, skin cancer).
• Specific agents like porphyrins, coal tar, and certain drugs induce photodynamic reactions, while psoralens primarily cause nonphotodynamic reactions.
• Photoallergic reactions are uncommon, involving immune responses, with solar urticaria and exogenous chemical allergies as examples. UVA is implicated in most photoreactions.

Conclusions:

• Understanding the distinct mechanisms of phototoxicity and photoallergy is crucial for diagnosis and management.
• Certain substances and drugs can induce diverse and sometimes severe phototoxic reactions, including skin cancer.
• Photoallergy, though rare, involves immune system alterations, and UVA plays a key role in both phototoxicity and photoallergy.