When an antigen is presented by the Major Histocompatibility Complex (MHC) on the surface of an antigen-presenting cell, it binds to the TCR. This binding event triggers a conformational change that activates the CD3 complex. The CD3ΞΆ subunit is especially important in this process because it contains immunoreceptor tyrosine-based activation motifs (ITAMs) that propagate the signal through a cascade of intracellular kinases, ultimately leading to T-cell activation.
