Data in this chapter may be updated by PubMed and Ovidsp Medline searches using the keywords: allergy basic mechanisms, allergic rhinitis, anaphylaxis, skin prick tests, specific IgE, allergen component, basophil activation test, immunotherapy, mast cell signalling. The references of relevant articles were also checked. In addition, major immunology and allergy journals, and immunology textbooks were scanned. The evidence in this chapter is mainly levels 2/3 with some level 1 evidence. The clinical and laboratory recommendations are based on guidance from the UK and international professional associations, grade A and B.