CD4 is a glycoprotein found on the surface of immune cells such as T-helper cells, monocytes, macrophages and dendritic cells. It is a co-receptor that assists the T-cell receptor (TCR) with an antigen-presenting cell and also interacts directly with MHC class II molecules on the surface of the antigen-presenting cells using its extracellular domain. In lymphatic tissues, the CD4+ T-cells are seen in large numbers in the parafollicular zone, while scattered cells are found in the germinal centers and mantle zone. CD4 is demonstrated in hepatic sinusoidal cells, monocytes, and monocytes-derived cells, but is not expressed on B-cells and immature thymocytes. Precursor T-lymphoblastic lymphomas are therefore variable in their expression of CD4. CD4 plays an important role in classification of lymphocytes in inflammatory lesion and malignant lymphoma.