Severe Late-Onset Neutropenia in a Patient With Multiple Sclerosis Treated With Ocrelizumab.

Ocrelizumab, a humanised monoclonal antibody targeting cluster of differentiation 20 (CD20), is widely used to treat multiple sclerosis (MS). While it is effective in reducing relapse rates, adverse effects such as late-onset neutropenia can pose significant clinical challenges. We report the case of a 36-year-old female with a history of relapsing-remitting MS (RRMS) who developed severe neutropenia following ocrelizumab administration. The patient presented with fever, fatigue, and oral aphthous ulcers, with laboratory tests confirming an absolute neutrophil count (ANC) of 0.11 × 10⁹/L. Prompt initiation of broad-spectrum antibiotics and granulocyte colony-stimulating factor (G-CSF) resulted in significant clinical improvement. This case highlights the importance of monitoring blood counts in patients receiving ocrelizumab and the need for timely intervention for late-onset neutropenia (LON).