There are many allergic diseases manifesting in different organs and on the basis of different pathomechanisms (see Sect. 1.3). The most common allergies develop via IgE antibodies and manifest within minutes to hours after allergen contact ("immediate-type reactions"). Not infrequently, there are biphasic (dual) reaction patterns when after a strong immediate reaction in the course of 6-12 h a renewed hypersensitivity reaction (late-phase reaction, LPR) occurs which is triggered by IgE, but amplified by recruitment of additional cells and mediators. These LPRs have to be distinguished from classic delayed-type hypersensitivity (DTH) reactions (type IV reactions) (see Sect. 5.5).What may be confusing for the inexperienced physician is familiar to the allergist: The same symptoms of immediate-type reactions are observed without immune phenomena (skin tests or IgE antibodies) being detectable. These reactions are called "pseudo-allergic reactions" (PARs), the term "pseudo" only reflecting the not detectable participation of the immune system and not implying "psychological" phenomena. People can die from pseudoallergic reactions! The term is negatively defined; with better techniques allowing the detection of antibodies or sensitized cells, PAR may turn into true allergy. IgE-mediated drug allergies will be covered together with other adverse drug reactions (see Sect. 5.7).In atopic eczema, IgE antibodies play a role in many patients in the eliciting phase, albeit in close connection with T-cell-mediated reac-tions (combination of type I and type IV b reactions). Atopic eczema will be discussed in a separate section (see Sect. 5.5.3).The maximal manifestation of IgE-mediated immediate-type allergic reaction is anaphylaxis. In the development of clinical symptoms, different organs may be involved and symptoms of well-known allergic diseases of skin and mucous membranes [also called "shock fragments" (Karl Hansen)] may occur according to the severity (see Sect. 5.1.4).