Acute Allergic Urticaria: This is the most common cause of urticaria, often resulting from an allergic reaction to foods, medications, or insect bites. It presents with sudden onset of itchy hives and can be accompanied by angioedema.

Chronic Idiopathic Urticaria: This condition is characterized by the recurrence of urticaria for more than 6 weeks without an identifiable cause. It's a diagnosis of exclusion after ruling out other potential causes.
Autoimmune Urticaria: In this condition, the body's immune system produces antibodies against the mast cells, leading to their degranulation and the release of histamine, causing urticaria.
Physical Urticaria: This includes conditions like dermatographism, cold urticaria, heat urticaria, and solar urticaria, where urticaria is triggered by physical stimuli.

Anaphylaxis: Although not a form of urticaria itself, anaphylaxis can present with urticaria as one of its symptoms. It's a life-threatening condition that requires immediate medical attention.
Mastocytosis: A condition characterized by the accumulation of mast cells in one or more organs, which can lead to episodes of urticaria, among other symptoms.
Urticarial Vasculitis: An inflammation of the blood vessels that can present similarly to urticaria but is associated with more severe symptoms and potential organ damage.

Erythema Multiforme: A skin condition characterized by target lesions, which can sometimes be confused with urticaria. It's often associated with infections or medication use.
Stevens-Johnson Syndrome/Toxic Epidermal Necrolysis: Severe skin conditions usually triggered by medications, which can present with urticaria-like lesions but are much more severe and potentially life-threatening.
Hereditary Angioedema: A rare genetic disorder that leads to recurrent episodes of severe angioedema, which can sometimes be confused with urticaria but lacks the urticarial rash and is not responsive to antihistamines or corticosteroids.