[Overview of lymphoid neoplasms in the fourth edition of the WHO classification].
The fourth edition of the "WHO Classification of Tumours of Haematopoietic and Lymphoid tissues" was published in 2008 as an updated version of the third edition published in 2001. In this review, the revised points in the lymphoid neoplasms in the fourth edition were summarized from the viewpoint of doctors and medical technologists in clinical laboratories in hospitals. The diseases are classified based on information about morphology, immunophenotype, genetic features, and clinical features. B lymphoblastic leukemia/lymphoma with 7 recurrent genetic abnormalities is individually classified as a provisional entity. Anaplastic large cell lymphoma is divided into two entities, ALK positive and ALK-negative. The pathogenesis of the former is involved with ALK gene rearrangement with several partner genes. Two borderline categories between diffuse large B-cell lymphoma (DLBCL) and Burkitt lymphoma, and between DLBCL and classical Hodgkin lymphoma are newly recognized as a distinct disease entity based on the overlapping morphological and genetic features. In the diagnosis of lymphoid neoplasms, understanding the morphological features is fundamental. On the other hand, the importance of immunohistochemistry and flow cytometry to clarify the immunophenotype, and chromosomal analysis and genetic examination to clarify the genetic features has been raised.