Bone marrow is normo- or hypercellular with normal or reactively changed hematopoiesis depending on the inciting agent. The marrow also contains increased numbers of polyclonal small round lymphocytes, usually with the majority of T cells. Sometimes lymphocytes form aggregates, which are well circumscribed and may even contain germinal centres. These benign aggregates are localized in the centre of the intertrabecular space, they do not occur in peritrabecular locations.