Oral Lichen Planus: This is a chronic inflammatory condition that can cause an increase in lymphocytes in the lamina propria and epithelium of oral mucosa. The clinical presentation and histological findings are consistent with this diagnosis, especially if the patient has a history of oral lichen planus or other autoimmune conditions.

Chronic Graft-Versus-Host Disease (GVHD): If the patient has a history of hematopoietic stem cell transplantation, chronic GVHD could be a possible cause of increased lymphocytes in the oral mucosa.
Contact Mucositis: Irritation or allergic reactions to dental materials, oral care products, or food can lead to an increase in lymphocytes in the oral mucosa.
Lymphocytic Epithelial Lesion (LEL): This condition is characterized by an infiltration of lymphocytes into the epithelium and can be associated with various conditions, including autoimmune disorders.

T-Cell Lymphoma: Although less likely, it is crucial to rule out lymphoma, as it can present with similar histological findings. A thorough examination of the biopsy specimen for atypical lymphocytes and further testing (e.g., immunohistochemistry, flow cytometry) may be necessary.
Sjögren's Syndrome: This autoimmune disorder can cause lymphocytic infiltration in the oral mucosa and should be considered, especially if the patient has symptoms of dry mouth or dry eyes.

Crohn's Disease or Ulcerative Colitis: Although these conditions primarily affect the gastrointestinal tract, they can rarely cause oral manifestations, including an increase in lymphocytes in the oral mucosa.
Sarcoidosis: This systemic granulomatous disease can affect various organs, including the oral mucosa, and may cause an increase in lymphocytes, although this is a rare presentation.
Immunodeficiency-Associated Lymphoproliferative Disorders: In patients with immunodeficiency (e.g., HIV/AIDS), certain lymphoproliferative disorders can occur, which may present with increased lymphocytes in the oral mucosa.