Acute Urticaria: This is the most common cause of hives, often triggered by allergic reactions to foods, medications, or infections. It presents with itchy, raised, and transient wheals.

Chronic Urticaria: Similar to acute urticaria but lasts more than 6 weeks. The cause is often autoimmune or idiopathic.
Insect Bites or Stings: Can cause localized or generalized hives, depending on the severity of the reaction.
Allergic Contact Dermatitis: A type IV hypersensitivity reaction to substances like latex, metals, or certain chemicals, leading to hives and skin inflammation.
Drug-Induced Urticaria: Certain medications, such as antibiotics or NSAIDs, can trigger hives as an allergic reaction.

Anaphylaxis: A severe, life-threatening allergic reaction that can present with hives, along with other symptoms like difficulty breathing, rapid heartbeat, and a drop in blood pressure. Immediate medical attention is required.
Mastocytosis: A condition characterized by the accumulation of mast cells in the skin, which can cause hives, flushing, and other systemic symptoms.
Autoimmune Disorders: Such as lupus or rheumatoid arthritis, which can have hives as part of their presentation.

Urticarial Vasculitis: An inflammation of the blood vessels presenting with hives that can last longer than 24 hours and may leave bruises.
Erythema Multiforme: A skin condition characterized by target lesions, which can sometimes resemble hives, often triggered by infections or medications.
Hereditary Angioedema: A genetic disorder that causes recurrent episodes of severe swelling, which can sometimes be confused with hives.