What accessory molecule is used by the T cell receptor to detect an MHC Class II molecule?

The accessory molecule that is utilized by the T cell receptor to recognize an MHC Class II molecule is CD4. This molecule is a co-receptor specifically found on the surface of helper T cells (CD4+ T cells). CD4 interacts with the MHC Class II molecules present on professional antigen-presenting cells, such as dendritic cells, macrophages, and B cells.

This interaction between CD4 and MHC Class II is crucial for the stabilization of the T cell receptor-MHC interaction, which enhances the signal transduction necessary for T cell activation. As a result, this interaction plays a vital role in the immune response by helping to initiate and regulate the activation of T cells following antigen recognition.

The other molecules mentioned do not perform this specific function. CD3 is involved in signaling within all T cells regardless of their co-receptor status, while CD2 interacts with other accessory proteins during T cell activation but does not specifically bind MHC molecules. CD8 is associated with cytotoxic T cells (CD8+ T cells) and recognizes MHC Class I molecules, making it unsuitable for the interaction with MHC Class II.