Which subunits compose MHC-I molecules?

MHC-I molecules are primarily composed of an alpha chain and a beta-2 microglobulin (often referred to as the beta subunit). The alpha chain consists of three domains (alpha 1, alpha 2, and alpha 3) and is responsible for presenting endogenous antigens to cytotoxic T cells. The beta-2 microglobulin does not penetrate the membrane but stabilizes the structure of the MHC-I molecule, making it necessary for its proper function.

The correct answer highlights the inclusion of the beta-2 microglobulin, which is indeed an essential component of MHC-I structure alongside the alpha chain. MHC-I molecules do not contain beta 1 and beta 2 subunits, nor are they composed of HLA-D subunits, which are associated with MHC-II molecules, rather than MHC-I. The inclusion of both the alpha chain and beta-2 microglobulin is critical for the molecule's ability to display peptides effectively on the cell surface, which is crucial for immune recognition and response.