T cells express cell surface proteins, described by cluster determination (CD) numbers
TH cells express CD4 molecules on their cell surface

• Enables the lymphocyte to bind to a MHC class II molecule
• T cell receptor is unique in that it is only able to identify antigen when it is associated with a MHC molecule on the surface

Cytotoxic cells possess CD8 cell surface markers, which bind to antigenic peptides expressed on MHC class I molecules