Follicular lymphoma (FL) is a subclass of mature B-Cell lymphoma, which can involve the skin primarily or secondarily, making up approximately 40% of all cutaneous B-cell lymphomas.

Clinically, patients usually present with a red indurated plaque or nodule with slowly progressive enlargement. Microscopically, FL in the skin usually reveals marked histopathologic changes in the dermis. A narrow zone (Grenz zone) separates the epidermis from a marked, intense lymphocyte dermal infiltration. The infiltrate may show a nodular or diffuse pattern. Lesional cells are a combination of centrocytes and centroblasts.

Centrocytes, the majority of cells found in FL, are small lymphoid cells with condensed irregular, cleaved nucleus, whereas centroblasts, the minority of cells found in FL, are larger lymphoid cells with round nuclei and peripheral nucleoli.

Differential diagnosis includes atypical follicular lymphoid hyperplasia which may be idiopathic or related to infection, and other forms of mature B cell lymphoma.

Immunophenotyping:
- BCL2 is often strongly positive in systemic FL which involves the skin secondarily, and negative or faint staining in most cases of primary skin FL

- CD10 and BCL6, are often positive in systemic/nodal FL, while in other mature B-cell lymphomas these are often negative. Strong staining of both BCL2 and CD10 should always raise suspicion of systemic follicular lymphoma involving the skin secondarily (WHO classification of tumors of hematopoietic and lymphoid tissues, 4th edition)

- t(14,18) is positive in systemic FL and negative in primary cutaneous FL

In this case, CD10, BCL2, BCL6 and t(14;18) are positive. Thus, the diagnosis is systemic/nodal follicular lymphoma involving the skin secondarily.