This study plans to learn more about why some people with Chronic Idiopathic Urticaria (CIU) respond to treatment with omalizumab (Xolair). It will test people before they receive treatment with omalizumab as part of standard of care, to see if there are differences in their blood and skin that can predict who responds to treatment.

Inclusion Criteria:

• CIU as defined as frequent urticarial lesions for ≥ 6 weeks
• Symptoms not controlled with standard dose antihistamines (loratadine 10mg daily, desloratadine 5mg daily, fexofenadine 180mg daily, cetirizine 10mg daily, or levocetirizine 5mg daily)
• Planned initiation of treatment with omalizumab (Xolair) as part of standard of care for antihistamine-refractory urticaria

Exclusion Criteria:

• Use of immunomodulatory drugs in the past 1 month prior to beginning the study
• Use of systemic steroids in the past 1 month prior to beginning the study
• Use of omalizumab in the past 3 months prior to beginning the study
• Use of any investigational agent in the past 30 days
• Untreated intercurrent illness
• Severe Asthma
• Primary diagnosis of flushing