Rituximab (RTX) is currently used in the treatment of lymphoproliferative diseases and of several rheumatologic disorders and is a frequent cause of acute infusion reactions, usually classified as cytokine release syndrome (CRS). Some infusion reactions to RTX raise concern for immediate type I hypersensitivity, even if to date RTX-specific IgE antibodies have not been reported. To improve knowledge of the mechanisms of reactions to RTX, we investigated humoral and cellular immune responses to this drug in a patient suffering from rheumatoid arthritis who displayed two immediate infusion-related reactions. RTX-exposed tolerant patients and healthy untreated subjects were used as controls. Non-isotype-specific and IgE anti-RTX antibodies were positive in the serum samples collected from the reactive patient but not in those from the control groups. Only the reactive patient also displayed skin testing positivity with RTX. More importantly, RTX-stimulated peripheral blood mononuclear cells from the reactive patient, but not from the controls, displayed a dose-dependent proliferative response associated with a Th2 cytokine production profile. Our results show the presence of RTX-specific Th2-type cells and IgE antibodies, thus suggesting that type I hypersensitivity may be an additional mechanism to CRS in the development of RTX reactions.