Digestive 
Blood
Cardiovascular
Dermatology
Genitourinary,
Hormones
Infections
Oncology and
Musculo-skeletal
Mental health and
Parasites
Respiratory
Senses
Various
Immunological
They are usually caused by allergies to a medication or food:
- Drug allergies, frequently to antibiotics in the penicillin group such as amoxicillin and analgesics from the pyrazolone group such as dipyrone.
- Food allergy, some of the most common are eggs, shellfish, nuts, fruits and vegetables.
- Autoimmune urticaria of non-allergic mechanism, such as in serum sickness and in so-called idiopathic urticaria (of unknown cause).
Non-immunologic
Includes physical hives, provoked by various circumstances such as cold, sweating, exercise, or rubbing or skin contact.
