Which of the following statements is true about classical class II MHC molecules?
1) They are able to directly bind bacteria and viruses on the cell surface
2) They are able to bind and transport peptides derived from extracellular proteins and degraded intracellularly, to the cell surface
3) They are able to bind polysaccharide and lipid molecules
4) They mainly serve antigen presentation from virally infected and tumor cells
5) They are expressed on the surface of all nucleated cells.