CD3d, or the CD3 delta subunit of the T-cell receptor complex, is a crucial component of the T-cell receptor (TCR) complex. The TCR/CD3 complex, including CD3d, is essential for signal transduction in T-cells. When the TCR recognizes and binds to an antigen presented by MHC molecules, the CD3 complex, including CD3d, transduces the activation signal into the T-cell. Defects in the CD3d can lead to severe combined immunodeficiency autosomal recessive T-cell-negative/B-cell-positive/NK-cell-positive (SCIDBNK), highlighting the importance of CD3d in the immune system.