The GK1.5 monoclonal antibody specifically binds with the mouse CD4 molecule, also known as L3T4, a 55 kDa differentiation antigen which binds to the MHC class II. CD4 is expressed on most thymocytes, a subpopulation of mature T lymphocytes, dendritic cells, pluripotent hematopoietic stem cells, B cell precursors, and lymphoid precursors inside the thymus. It is also expressed on the mouse egg cell membrane, enhancing adhesion to MHC class II bearing sperm. By interaction with MHC class II on the surface of APC, CD4 initiates the development of T lymphocytes and helps the optimum functioning of mature T lymphocytes.

The binding of the GK1.5 antibody blocks the binding of the Anti-Mouse CD4 RM4-5 antibody.