Which accessory molecule primarily signals intracellular activation via the T cell receptor?

The accessory molecule that primarily signals intracellular activation via the T cell receptor is CD3. CD3 is an integral part of the T cell receptor (TCR) complex and is essential for T cell activation. When the TCR recognizes an antigen presented by Major Histocompatibility Complex (MHC) on an antigen-presenting cell, the CD3 complex transmits a signal into the T cell. This signaling is crucial for the subsequent activation of intracellular pathways that lead to T cell proliferation, differentiation, and effector functions.

CD3 consists of several protein chains that, upon engagement with the TCR, initiate a cascade of intracellular signaling events through the phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs). This phosphorylation is a critical step in T cell activation and is what ultimately allows the T cell to respond to the antigen.

Other molecules such as CD2, CD4, and CD8 also play important roles in T cell function and help in stabilizing the interaction between T cells and antigen-presenting cells. CD2 is involved in adhesion, while CD4 and CD8 serve as co-receptors that help define the T cell subset and assist in the TCR's recognition of antigens presented by MHC class II and