Antigens are macromolecules that elicit an immune response in the body.
The most common antigens are proteins and polysaccharides . The two categories of antigens are processed and presented to T cells by quite different mechanisms. Now for more details. I histocompatibility molecules are transmembrane proteins expressed at the cell surface. All of these must be present within the lumen of the RER if they are to assemble correctly and move through the Golgi apparatus to the cell surface.