In normal lymphoid tissue CD19 is observed in germinal centers (on both B cells and follicular dendritic cells), in mantle zone cells and in scattered cells in the interfollicular areas, with an overall immunoreactivity pattern similar to that of CD20 and CD22. However, in contrast to CD20, CD19 is also expressed in pre-B cells. CD19 positivity is seen in the vast majority of B-cell neoplasms (B-lymphoblastic lymphoma, small lymphocytic lymphoma/CLL, mantle cell lymphoma, follicular lymphoma, Burkitt lymphoma, marginal zone lymphoma, diffuse large B-cell lymphoma, T-cell-rich B-cell lymphoma, lymphoblastic lymphoma, hairy cell leukemia) and commonly at a lower intensity than normal B-cell elements. Plasma cell neoplasms are consistently negative as are T-cell neoplasms. CD19 was undetectable in 14% of diffuse large B-cell lymphomas, 30% of T-cell-rich B-cell lymphomas and 75% of post-transplant B-lymphoproliferative disease. CD19 expression is not seen in Reed Sternberg cells of classic Hodgkin lymphoma.