Acute Allergic Reaction: This is often the most common cause of urticaria, resulting from an allergic reaction to foods, medications, insect bites, or other allergens. It presents with sudden onset of itchy hives and can be associated with other systemic symptoms like angioedema.

Chronic Idiopathic Urticaria: This condition is characterized by the recurrence of hives without an identifiable cause, lasting more than 6 weeks. It's a diagnosis of exclusion after ruling out other causes.
Autoimmune Urticaria: In this condition, the body's immune system attacks its own tissues, leading to the release of histamine and the development of hives.
Infections: Certain infections, such as viral upper respiratory infections, can trigger urticaria.
Drug-Induced Urticaria: Many medications can cause urticaria as a side effect, including antibiotics, NSAIDs, and blood pressure medications.

Anaphylaxis: A severe, life-threatening allergic reaction that can present with urticaria, along with other symptoms like difficulty breathing, rapid heartbeat, and a drop in blood pressure. Prompt recognition and treatment are crucial.
Hereditary Angioedema: A rare genetic disorder characterized by recurrent episodes of severe swelling, which can be life-threatening if not properly managed.
Mastocytosis: A condition where there's an accumulation of mast cells in the body, leading to the release of histamine and other mediators, causing symptoms including urticaria.

Urticarial Vasculitis: An inflammation of the blood vessels that can cause urticaria, along with other symptoms like skin purpura and joint pain.
Erythema Multiforme: A skin condition characterized by target-like lesions, which can sometimes present with urticaria-like symptoms.
Schnitzler Syndrome: A rare disorder that includes chronic urticaria, bone pain, and a monoclonal gammopathy.
Cold Urticaria: A condition where exposure to cold triggers the onset of hives.
Solar Urticaria: Hives that develop in response to sunlight exposure.