Type A thymomas (spindle cell thymomas) vs Type AB thymoma: Type A thymomas have no or very few (easily countable) lymphocytes. If there are lymphocyte-dense areas (lymphocytes too numerous to count) or >10% of tumor has moderate infiltrate of lymphocytes, the tumor is classified as a type AB thymoma. The lymphocytes in type AB thymomas are predominantly TdT+ immature T-cells. Micronodular thymoma with lymphoid stroma vs Type AB thymoma: Micronodular thymoma with lymphoid stroma contains multiple small cytokeratin-positive epithelial islands surrounded by lymphoid stroma containing CD20+ mature B-cells and CD3+/TdT negative T-cells. Type AB thymoma contains admixture of cytokeratin-positive epithelial cells and TdT+ immature T-cells. Type B1 thymoma vs Type AB thymoma: Type B1 thymoma resembles normal thymus and contains TdT+ cortical areas and medullary structures with Hassall corpuscles. This image of a type AB thymoma shows spindle cell component in the lower right which merges imperceptibly with the lymphocyte-rich B component on the upper left.