|S/no.||Diagnostic points and pitfalls|
|1.||Immunohistochemistry for B cell markers/transcription factors including CD20, CD79a, PAX5, Oct-2 and BOB.1 may be variably or weakly positive in the HRS-like cells in AITL, and together with expression of CD30 and sometimes CD15 can produce appearances closely resembling CHL.|
|2.||Failure to evaluate for or recognise atypical T cells and related architectural features (eg, irregular tentacular expansion of FDC meshworks) in the presence of HRS-like cells can lead to misdiagnosis of AITL as CHL.|
|3.||Pattern I of AITL can easily be dismissed as reactive lymphoid hyperplasia if attention is not paid to the interfollicular infiltrate and immunohistochemistry for TFH cells and FDC meshworks is not performed. The age and clinical presentation of the patient should be taken into account.|
|4.||Extrafollicular PD-1-positive lymphocytes should be at least of a similar immunostaining intensity to normal follicular PD-1-positive lymphocytes to suggest AITL. Although extrafollicular lymphocyte PD-1 positivity is an important feature of AITL, this finding may be present in other neoplastic and reactive conditions. Furthermore, PD-1 positivity is not restricted to TFH cells. Overdependence on this feature may lead to an incorrect diagnosis of AITL.|
|5.||Thorough immunoarchitectural assessment, including the use of multiple FDC markers, TFH markers and B cell and T cell markers, will often resolve diagnostic dilemmas, but the pathologist should be familiar with the distribution and intensity of these markers in normal/reactive tissues to avoid misinterpretation.|
|6.||Clonality assessment for immunoglobulin gene rearrangement may be positive in AITL, which may result in an erroneous diagnosis of B cell lymphoma, especially if no clonal T cell receptor gene rearrangement is identified (ie, false negative). Interpretation of molecular clonality results should, as always, be done in conjunction with the morphological appearances.|
AITL, angioimmunoblastic T cell lymphoma; CHL, classical Hodgkin lymphoma; FDC, follicular dendritic cell; HRS, Hodgkin/Reed-Sternberg; TFH, follicular helper T cell.