Histological presentation of primary nodal T- and NK-cell lymphomas. Anaplastic large cell lymphomas (ALCLs), ALK⁺ or ALK⁻, tend to invade the peripheral sinuses and produce cohesive sheet-like infiltrates in the lymph node. Peripheral T-cell lymphoma, NOS (PTCL, NOS) is not associated with a characteristic pattern of growth, and can to a variable extent replace the normal lymphoid tissue. Subtypes of PTCL, NOS are composed of mostly CD4⁺ or CD8⁺ lymphoid cells with a phenotype resembling that of Th1 or Th2 cells, or of cells expressing cytotoxic molecules. The tumor microenvironment tends to be more abundant in Th1-type PTCL, NOS, and can sometimes comprise abundant epithelioid histiocytes. Primary nodal EBV⁺ lymphomas derived of cytotoxic T cells, or less commonly NK cells, represent a rare aggressive entity, which is mostly reported in Asians. Lymphomas derived from CD4⁺ TFH cells (TFHL) represent the most prevalent nodal PTCL with a variety of histological patterns. The most common form of TFHL is the angioimmunoblastic type (TFHL-AI), which in its usual form is a diffuse microenvironment-rich tumor, comprising a proliferation of arborizing vessels and follicular dendritic cells, and an infiltrate of many reactive large (blastic) and small B cells, plasma cells, histiocytes, and non-neoplastic T cells. Less commonly, the neoplastic cells of TFHL-AI concentrate around reactive or regressive germinal centers (patterns 1 and 2), and these may be more difficult to diagnose because of the association with reactive follicles. In the uncommon follicular type of TFHL (TFHL-F), the neoplastic TFH cells grow in follicles, resembling folliicular lymphoma (FL-like), or in clusters within large B-cell nodules resembling progressively transformed germinal centers, which is an uncommon form of reactive follicular hyperplasia (PTGC-like). The NOS subtype of TFHL, defined by the TFH phenotype of the neoplastic cells, does not contain the complete microenvironment of TFHL-AI, grows diffusely, and may preferentially distribute in the paracortex and between preserved follicles. TFHL, in particular the AI and follicular subtypes, may contain large atypical cells resembling Reed-Sternberg cells, a source of frequent diagnostic difficulties in distinguishing them from classic or nodular lymphocyte predominant Hodgkin lymphoma.