CD3 is a protein found on the surface of T-cells, making it essential for activating these cells and intracellular signaling . The protein consists of four subunits (CD3γ, CD3δ, CD3ε, and CD3ζ) that are associated with the TCR. When it interacts with its antigen on the major histocompatibility complex (MHC), CD3 activates signaling pathways to activate the T-cells .