Intravenous high-dose immunotherapy is one of the highly effective proven treatments for a number of autoimmune diseases of the nervous system. Nowadays there is enough knowledge about the spectrum of side effects and approaches to their prevention and monitoring. Most of them are leveled by improving the technology of the production process, but anaphylactic reactions remain one of the uncontrolled, albeit rare, undesirable reactions. The development of allergic reactions is associated with the presence of antibodies to immunoglobulin class A (IgA) in the patient, which is manifested by a decrease in the level of IgA during routine immunological examination. The article provides a review of the literature on the prevalence of IgA deficiency, its causes. A modern view of the need for routine testing of IgA level before the course of intravenous immunotherapy, approaches to reduce the risk of developing serious adverse reactions in such cases is described. A clinical case of treatment of a patient with chronic inflammatory demyelinating polyneuropathy and a reduced level of IgA is presented.