MHC-II molecules, such as HLA-DR, are primarily found in which cellular compartment?

MHC-II molecules, including HLA-DR, are primarily found in the endosomal and lysosomal compartments of antigen-presenting cells. These molecules play a crucial role in the immune response by presenting processed antigens derived from extracellular proteins to CD4+ T helper cells. The synthesis of MHC-II molecules begins in the endoplasmic reticulum, where they are initially assembled. Following assembly, they are transported through the Golgi apparatus and then sent to specialized endosomal compartments.

In these endosomal compartments, MHC-II molecules encounter and bind to antigenic peptides that have been internalized and processed. This loading of peptides onto MHC-II takes place within the endosomes, allowing for the presentation of these peptides on the cell surface for recognition by T cells.

Though MHC-II molecules ultimately reach the cell membrane to interact with T cells, their critical processing occurs primarily in the endosomes, which is why this compartment is the correct answer. The other options refer to locations where MHC-II molecules may not be functionally active in the context of antigen presentation.