The Clinical Meaning of Histamine Skin Reactivity

The definition of the “atopic state”, i.e. subjects presenting at least one skin wheal with a minimum diameter of 3\udmm induced by an allergen skin-prick test (ASPT), is based on the assumption that wheal size depends entirely on\udthe amount of histamine produced in the antigen-antibody reaction. Several epidemiological studies have, however,\uddemonstrated that an ASPT-elicited wheal is heavily modulated by “histamine skin reactivity” (HSR), i.e. the size of\udthe wheal induced by a prick test performed with a given solution of histamine. HSR not only varies widely\uddepending on the individual characteristics and geographical setting, but also changes over time; these differences\udin HSR markedly influence the amount of specific IgE required to produce a wheal of at least 3 mm in an ASPT. We\udshould therefore ideally conceive the existence of two types of” atopic patients”: one type in whom “atopy” is mainly\udthe result of an increased level of specific IgE antibodies, and another type in whom positive ASPTs are mainly the\udresult of marked skin reactivity to even small amounts of histamine. If hyper-reactivity to histamine occurs not only in\udthe skin but in parallel also in other parts of the organism, especially at the mucosal level, “normal” histamine\udproduction may cause chronic or recurrent clinical symptom