Omalizumab and the World Health Organization. It is also used to treat people 12 years and above with chronic spontaneous urticaria (also called chronic idiopathic urticaria), which cannot be treated with elevated doses of H1-antihistamines. The main adverse effect is anaphylaxis (a life-threatening systemic allergic reaction), with a rate of occurrence of 1 to 2 patients per 1,000. Like other protein and antibody drugs, omalizumab also causes anaphylaxis, although at a relatively low frequency among antibody drugs. The allergic reaction is probably not due to the binding characteristics of the antibody drug, but to the protein nature of the antibody. The
Allergic inflammation hours and is mediated by mast cells) should not be confused with delayed hypersensitivity Type IV allergic reaction (which takes 48–72 hours to develop and is mediated by T cells). The products of the early phase reaction include chemokines and molecules that act on endothelial cells and cause them to express Intercellular adhesion molecule (such as vascular cell adhesion molecule and selectins), which together result in the recruitment and activation of leukocytes from the blood into the site of the allergic reaction. Typically, the infiltrating cells observed in allergic reactions contain a high proportion of lymphocytes, and especially, of eosinophils.