A patient receives rituximab for rheumatoid arthritis and experiences fever and muscle pain shortly after infusion. What is the most likely cause of her symptoms?

• A. Basophil degranulation
• B. B-lymphocyte release of cytokines
• C. Natural killer cell activation
• D. Neutrophil activation

Answer: (B)

Rituximab is a monoclonal antibody that targets CD20 on B-lymphocytes, leading to their depletion. When a patient receives rituximab, the infusion can trigger a response from the immune system. The symptoms of fever and muscle pain that the patient experiences shortly after the infusion are most consistent with a cytokine release syndrome (CRS), which occurs when there is a rapid release of pro-inflammatory cytokines by activated immune cells, particularly B-lymphocytes.

In this scenario, when rituximab binds to CD20 on B-cells, it can stimulate the release of various cytokines, including interleukins and tumor necrosis factor (TNF). This release can lead to systemic symptoms such as fever, chills, and muscle pain. The timing of these symptoms, occurring shortly after the infusion, further supports that they are due to this immediate immune response rather than a delayed allergic reaction or other mechanisms.

Thus, the most likely cause of the patient's symptoms is the release of cytokines from activated B-lymphocytes as a result of the interaction with rituximab.