Final answer:
MHC II molecules are composed of two similar-length subunits, an alpha (a) and a beta (B) chain, which together present antigen fragments to T cells. The exact number of amino acid residues for each subunit is not specified.
Step-by-step explanation:
The major histocompatibility complex (MHC) II molecules are integral membrane glycoproteins involved in the immune system's ability to recognize foreign particles. Specifically, MHC II molecules consist of two subunits, an alpha (a) and a beta (B) chain, each with similar lengths and with both chains possessing transmembrane portions. The number of amino acid residues for each of these chains is not specified in the provided text, but generally, the two chains of MHC II would each have a sequence long enough to span the cellular membrane and to fold into the necessary structure to form the antigen-binding cleft that characterizes these molecules. These two subunits work together to present antigen fragments to T cells, playing a crucial role in the adaptive immune response.