This multicenter, randomized, double-blind, placebo-controlled, parallel-group study will assess the efficacy and safety of additional quilizumab therapy in adult patients with Chronic Spontaneous Urticaria resistant to antihistamine treatment.

Inclusion Criteria:

• Diagnosis of CSU refractory to H1 antihistamines with/without LTRAs, as defined by the following:
• The presence of itch and hives for > 6 consecutive weeks at any time prior to enrollment despite use of H1 antihistamines during this time period
• UAS7 score greater than 16 and itch component of UAS7 greater than 8 during 7 days prior to Day 1. Patients must have been on daily stable doses of H1 antihistamine for at least 3 consecutive days immediately prior to the screening visit and must document current use at screening
• CSU diagnosed for more than 6 months
• Patients must have a minimum diary compliance (6 out of 7 consecutive days) prior to Day 1

Exclusion Criteria:

• Treatment with an investigational agent within 30 days of screening, and previous treatment with monoclonal antibody therapies used to treat CSU 9 months prior to screening
• Chronic urticarias other than CSU, including the following: Acute, solar, cholinergic, heat, cold, aquagenic, delayed pressure or contact.
• Other diseases and conditions with symptoms of urticaria.
• Routine doses of the following medications within 30 days prior to screening: systemic or topical corticosteroids (prescription or over the counter), hydroxychloroquine, methotrexate, cyclosporine, or cyclophosphamide
• IV immunoglobulin G (IVIG), or plasmapheresis within 30 days prior to screening
• Patients with cancer, history of cancer considered uncured or in complete remission for < 10 years, or currently under work-up for suspected cancer except non-melanoma skin cancer that has been treated or excised and is considered resolved
• History of anaphylactic shock without clearly identifiable avoidable antigen
• Presence of clinically significant cardiovascular, neurological, psychiatric, metabolic or other pathological conditions.
• Medical examination or laboratory findings that suggest the possibility of decompensation of co-existing conditions for the duration of the study.
• Evidence of current drug or alcohol abuse
• Nursing women and women of childbearing potential, unless using effective contraception as defined by protocol