Skin sensitization is one of the key adverse effects to be addressed in the human hazard characterization and risk assessment of chemicals and cosmetic ingredients. Traditionally, the skin sensitization potential of substances has been addressed by using animal models, but an increasing political and ethical demand to replace animal experiments resulted in:
the regulatory requirement in Europe to assess the safety of cosmetic ingredients without testing on animals 
a recent amendment of the industrial chemical regulation explicitly requests information on skin sensitization from non-animal test methods and prescribes that animal testing can only be conducted if the non-animal methods are not applicable or the obtained results are inadequate for classification and risk assessment purposes. 
The OECD has summarized the detailed understanding of the biological mechanism of skin sensitization by structuring the various biological steps in the process into an Adverse Outcome Pathway (AOP) [3,4].