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The basophil activation test (BAT) is a flow cytometric assay that evaluates the percentage of activation or degranulation of peripheral blood basophils, after “in vitro” exposure to specific allergens. In sensitized patients, the stimulation of peripheral blood basophils with a specific allergen induces or up-regulates the expression of molecules, such as CD63 and CD203c, which represent, markers of degranulation and activation of basophils, respectively. The validity of the BAT requires a negative control (sterile saline) and a positive control (anti- IgE molecules). Several studies have demonstrated the role of the BAT in supporting the diagnosis of drug, food and hymenoptera venom allergy. The BAT has shown a low sensitivity but good specificity in diagnosing allergy to drugs such as NSAIDs, beta-lactam antibiotics, quinolones and muscle relaxants. In food allergy, the sensitivity and specificity of the BAT depends on the food; in the case of peanut allergy the sensitivity reaches 96% while the specificity the 100%. In addition, the BAT is an useful tool to monitor the natural resolution of allergies and the clinical effects induced by either immunotherapy or anti-IgE treatment. Finally, the BAT has been utilized to study the pathogenetic mechanisms underlying several IgE-mediated diseases. For example, in patients affected by severe bronchial asthma, the BAT has demonstrated the ability of Staphylococcus aureus enterotoxins to induce the activation of basophils supporting the role of these enterotoxins as “triggers” of the inflammatory cascade in bronchial asthma. In patients with cystic fibrosis the BAT can be used to discriminate allergic bronchopulmonary aspergillosis from Aspergillus colonization. More recently, the BAT has been demonstrated as a potential diagnostic tool to evaluate allergy to the polyethylene glycol (PEG) present in the anti-SARS-CoV-2 BNT162b2 mRNA vaccine. [ABSTRACT FROM AUTHOR]