Four Assays for the Testing of Skin Sensitizing Materials

April 19, 2024

A skin sensitizer is defined by the United Nations Globally Harmonized System of Classifications and Labelling of Chemicals as a substance that induces an allergic response after coming in contact with the skin. Skin sensitization testing is often necessary to meet regulation guidelines and, more generally, is key to ensuring the safety of your product for consumer use.

Skin sensitisation testing has historically been done in animal studies involving mice and guinea pigs. However, as research techniques have improved, animal studies have become less ubiquitous. In vitro assays, experiments where biological systems are studied outside of their natural environment, offer an alternative for those who are put off by the ethical considerations as well as the cost in time and money associated with animal testing. Additionally, in vitro assays may provide a mechanistic understanding of the sensitization process that would otherwise be difficult to obtain in animal studies.

Skin sensitisation happens as a result of biochemical processes that ultimately lead to an inflammatory response, namely allergic contact dermatitis. This process is known as the Adverse Outcome Pathway (AOP), which is made of 4 key events. One of these events is the activation of dendritic cells, which are immune cells that play a key part in immune system signalling. The tests to be discussed all target this specific event.

It is important to keep in mind that all four events in the AOP must be thoroughly investigated to gain a complete understanding of the skin sensitizer in question. Nevertheless, the four assays described here can be incredibly useful in better understanding the potential skin sensitizer you are working with.

The Organisation for Economic Co-operation and Development (OECD) recommends four different assays for in vitro assaying of dendritic cell activation:

  1. Human Cell Line Activation Test (H-CLAT)
  2. U937 Cell Line Activation Test (U-SENS™)
  3. Interleukin-8 Reporter Gene Assay (IL-8 LUCAssay)
  4. Genomic Allergen Rapid Detection (GARD™) for Assessment of Skin Sensitizers (GARD™Skin)

Assays 1, 2 and 3 are protein-based assays that quantify a change in expression of specific protein markers whose increased expression is associated with the dendritic cell activation. On the other hand, assay 4 is nucleic acid-based, designed to detect the elevated transcription levels of a set of genes associated with dendritic cell activation.

The full guidelines to these tests are listed in the OECD Test Guideline No. 442E, In Vitro Skin Sensitisation assays addressing the Key Event on activation of dendritic cells on the Adverse Outcome Pathway for Skin Sensitisation. Biohubx is capable of performing these tests to these standards. Please contact Dr. Steven Jones ( for further information.

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