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Figure 2. Patients with follicular lymphoma at the time of diagnosis were CD20 positive (A). Neoplastic follicles replaces nodal architecture with back-to-back pattern (HES X 1) (A1). Low grade (1-2) with a predominance of centrocytes (HES X20) (A2). CD20 positive B cells are seen in both follicular and interfollicular areas (HES X 20) (A3). Staining CD10 (A4) and BCL2 (A5) were found positive within the follicles and also infiltrate interfollicular areas (HES X20). Follicular lymphoma (X10) of the same patient at relapse and after anti-CD20 therapy (rituximab) with loss of CD20 (B). Neoplastic follicles in low-grade follicular lymphoma, low grade (grade 1-2) with predominance of centrocytes (B1). Neoplastic follicles replaces nodal architecture with back-to-back pattern (HES X 10). Immunohistochemistry with immunoperoxidase stains in paraffin sections shows CD20 negative B cells (B2). The other B cell markers CD79A (B3), PAX5 (B4), and BCL6 (B5) remain positively expressed by follicular lymphoma cells.