An 88-year-old man underwent a routine full blood count (FBC) for monitoring of essential thrombocythaemia. He was noted to have a mild lymphocytosis, the FBC showing haemoglobin concentration 122 g/l, white cell count 13·6 × 109/l, neutrophils 6·66 × 109/l, lymphocytes 6·24 × 109/l and platelets 597 × 109/l.
A peripheral blood film examined by light microscopy showed abnormal lymphocytes with intracytoplasmic crystalline inclusions (top). Flow cytometry demonstrated a population of lymphocytes (5·2 × 109/l) positive for CD5, CD19, CD20 and CD23 with weak lambda expression, consistent with chronic lymphocytic leukaemia. Examination by electron microscopy showed rhomboid crystals within the cytoplasm (bottom left). Under higher power the crystals appeared granular with microfibrils of 5–7 nm in length (bottom right).
Crystals within the cytoplasm of lymphocytes are a recognized finding in lymphoproliferative disorders. The crystals are thought to be composed of immunoglobulin. The formation of crystalline inclusions may be related to a dysfunction of immunoglobulin synthesis.