Where does loading of antigens onto MHC class II occur?

The loading of antigens onto MHC class II molecules occurs in the phagolysosome. This process is essential for the presentation of extracellular antigens to CD4+ T helper cells. Once a professional antigen-presenting cell (APC) such as a dendritic cell, macrophage, or B cell engulfs an extracellular pathogen, the pathogen is internalized into a compartment that ultimately fuses with lysosomes, forming the phagolysosome.

Within this specialized environment, proteolytic enzymes break down the antigen into peptides. Simultaneously, MHC class II molecules, which are synthesized in the endoplasmic reticulum, are transported to the phagolysosomal compartment. There, the processed peptides bind to the MHC class II molecules. Once the MHC class II-antigen complex is formed, it is transported to the cell surface, where it can be recognized by CD4+ T cells, triggering an immune response.

Other options are not the correct sites for MHC class II loading. The endoplasmic reticulum is involved in the initial assembly and transport of MHC class II molecules but does not participate directly in loading peptides. The cell nucleus does not play a role in antigen presentation, and