Final answer:
Several molecules are needed for the expression of the TCR at the cell surface, including the T-cell receptor (TCR) and CD4 or CD8 molecules. The TCR binds to presented antigens, while CD4 or CD8 molecules bind to MHC molecules on antigen-presenting cells.
Step-by-step explanation:
For the expression of the TCR at the cell surface, several molecules are needed. The T-cell receptor (TCR) is a protein dimer embedded in the plasma membrane of a T cell. The TCR is responsible for binding to a presented antigen and initiating an immune response. In addition to the TCR, CD4 or CD8 molecules, which are cell adhesion molecules, are also required. These molecules bind to the MHC molecules on antigen-presenting cells, helping to keep the T cell in close contact with the antigen-presenting cell.