Basophils are circulating blood cells, recruited to allergic tissue sites in a similar way like eosinophils (via CCR3). They are, like mast-cells, armed by high affinity IgE receptors, which bind IgE with a very high affinity and react enormously quick to antigen/allergen. Their activation via the surface bound IgE (due to binding and cross-linking of bound IgE molecules) leads to secretion of mediators (histamine, leukotriens, cytokines) and change of the cell membrane (upregulation of activation markers like CD203c and CD63). This basophile system is very sensitive and can be used like a IgE-assay (e.g. CAP assay of Phadia, Uppsala Sweden). However, the main advantage is that the basophil assay offers new perspectives as they combine specificity (IgE) with a very sensitive biological read out system, namely basophil activation.
We use at present the basophil assay in different ways:
whole blood assays (direct or conventional BAT):
The conventional BAT relies on basophils derived directly from the patient and stimulated with a battery of potential allergens. CD63 & CD203c upregulation is used to detect activation and degranulation upon stimulation.
in vitro sensitization assay (indirect BAT)
The indirect BAT is based on well-defined human donor basophils. Endogenous IgE can be stripped from Fc-IgE receptors by low pH treatment (lactic acid) and incubation of the IgE stripped basophils with IgE containing serum allows passive sensitization. The passively sensitized basophils are tested with the respective allergens. At the moment this procedure is still experimental and needs further improvement.