Urticaria, also known as welts, hives, or wheals, is characterised by the appearance of intensely pruritic erythematous plaques. It appears clinically as pruritic, pale, blanching swellings of the superficial dermis that last for up to 24 hours. Lesions may be small, large, giant, oval, or annular.
Urticaria affects 10% to 25% of the population and can occur in any age group.  It is a mast cell driven disease. The triggering mechanisms for mast-cell activation in urticaria are not well defined but seem to be multiple and diverse.  Urticaria is classified according to duration of symptoms:
Chronic urticaria: urticaria that has been continuously or intermittently present for at least 6 weeks. 
Chronic urticaria has been classified into the following 2 subtypes: 
Chronic spontaneous urticaria: spontaneous appearance of wheals, angio-oedema, or both ≥6 weeks due to known or unknown causes.
Inducible urticaria, for example delayed pressure urticaria, heat urticaria, solar urticaria, symptomatic dermatographism, vibratory angio-oedema, aquagenic urticaria, cholinergic urticaria, and contact urticaria.
Some people have urticaria that is complicated by angio-oedema, a swelling of the deeper dermis and tissues (e.g., mucosal surfaces), with laryngeal oedema potentially causing respiratory distress and death.  Angio-oedema is commonly associated with urticaria and may occur at any age.  It may not necessarily be seen at the first urticarial episode, and its risk increases with greater exposure to the allergenic substance.  Symptoms are typically numbness, pain, or paraesthesias.  Angio-oedema without urticaria should prompt further evaluation.
Assistant Professor of Clinical Pediatrics, Clinician Educator
Director, Quality Improvement and Educational Activities For The Breathmobile Program
Keck School of Medicine, USC
ML was a speaker for MEDA Pharmaceuticals in 2016.
Clinical Associate Professor of Pediatrics
Division of Allergy-Immunology
Los Angeles County+USC Medical Center
KYCK has received grant support from and acts as a consultant and speaker for Thermo-Fisher Scientific.
Dr Marilyn Li and Dr Kenny Y.C. Kwong would like to gratefully acknowledge Dr Nanette Silverberg, and Dr Mary Lee-Wong, previous contributors to this topic. NS and MLW declare that they have no competing interests.
Pediatrics and Internal Medicine
Division of Allergy and Immunology
CP declares that he has no competing interests.
New York University School of Medicine
AK declares that he has no competing interests.
Head of Allergy Division
Department of Dermatology and Allergy
Hannover Medical School
BW declares that she has no competing interests.
Use of this content is subject to our disclaimer