Table 1.

Distinguishing features and proposed classification of different immediate type hypersensitivity reactions.

	Pseudo-Allergic (Mas-Related G-Protein-Coupled Receptor Member X2 (MRGPRX2) Activation)	Non-IgE Mediated, Immunologic Activation (IgG, rarely Described to be Involved in Immediate Type Reaction)	Non-IgE Mediated, Non Immunologic Activation (i.e., Opioid, Complement)	IgE Mediated	Non-Allergic (Immune System Not Primary Involved)
Mast cell involvement	Yes	Yes	Yes	Yes	No
Skin test (immediate reading)	Positive	Negative	Positive	Positive	Negative
Specific IgE	Can be positive without clinical relevance	Can be positive without clinical relevance	Can be positive without clinical relevance	Presumably positive	Can be positive without clinical relevance
Basophil activation test (BAT)	Can be positive without clinical relevance	Presumably negative	For most negative	Presumably positive	Negative
Could explain reaction after first time exposure	Yes	No, except if previous sensitization by cross-reactivity	Yes	No, except if previous sensitization by cross-reactivity	Yes
Dose dependency	Yes	Probably	Yes	Classically no, marginally significant	Yes
Adverse drug reactions (ADR) Classification	New proposal: type A	Type B	Type B	Type B	Type A
Re-administration possible	Theoretically possible, with reduced speed or lower doses. No data available yet	Theoretically not recommended	Theoretically not recommended	Not recommended (consider desensitization protocol)	Yes, with reduced speed or lower doses if not pharmacologically contraindicated