Reactivity and sensitivity are distinct measures of basophil response
Basophil reactivity refers to the proportion of basophils that express CD63 compared to the negative control and can be expressed as %CD63+ basophils at a given allergen concentration or as the ratio of %CD63+ to allergen and the IgE-mediated positive control (anti-IgE or anti-FcεRI). It serves to document the presence of biologically relevant sensitisation to allergen through IgE. Basophil reactivity needs to be measured before sensitivity can be assessed and should hence be reported first. Two recent studies of peanut allergy found a relationship of reactivity and symptom severity25,33; however, in a study of wasp venom allergy, basophil reactivity to wasp allergen extract could not predict patients symptom severity34. The latter study set the scene for developing methods to assess basophil sensitivity35, that has been shown to be useful in the diagnosis of allergic asthma36, rhinitis37, food allergy33,38-40, allergen immunotherapy41-45 and anti-IgE therapy46-48.
Basophil sensitivity refers to the allergen concentration eliciting half-maximal basophil activation and can be expressed as EC50 that decreases with increasing severity42,45, or CD-sens which is the inverse of EC50 multiplied by 100 and can be calculated based on the slope of the dose-response curve25,35. CD-sens increases with the severity of allergic reactions35. Determination of sensitivity of basophils to allergen by flow cytometry was preceded by studies determining basophil sensitivity to allergen by measuring the release of histamine, PGD2 or Cys-Leukotrienes3. Activation of blood basophils should be assessed at each of 5-12 log dilutions of allergen. The degree of reactivity at each allergen concentration is plotted against allergen concentration, and both maximal reactivity and half-maximal reactivity are determined by fitting a non-linear curve to the dose-response. Basophil sensitivity correlates with the patient´s sensitivity to allergen at the clinical level, both in respiratory36,49 and in food allergies25,38-40,50 and changes in sensitivity reflect the clinical improvement in allergic rhinitis41,42,44,45,51,52. Basophil reactivity and basophil sensitivity appeared to be distinct parameters of activation53,54; however, systematic analyses of signalling molecules in the pathway leading from IgE crosslinking to degranulation show that are interdependent and regulated by syk55,56.