Used in immunological research to study T-cell activation and immune responses. Facilitates the development of assays for HIV entry inhibition, as CD4 is a primary receptor for HIV. The His tag allows for easy purification and detection in protein expression systems. Employed in protein-protein interaction studies, including binding assays with MHC class II molecules or therapeutic antibodies. Commonly used in ELISA, surface plasmon resonance, and pull-down experiments to analyze ligand binding and inhibitor efficacy. Supports vaccine development and monoclonal antibody testing by providing a purified, functional target protein.