Antigens presented by major histocompatibility complex molecules have been classified into those presented by ‘endogenous’ and ‘exogenous’ pathways. Some microorganisms reside within host-cell vacuoles that appear to avoid both pathways. Novel presentation mechanisms are being unraveled for these microorganisms, and their antigens, rather than being just peptides, can also consist of lipids or DNA fragments.
Read more at:
https://www.cell.com/trends/microbiology/fulltext/0966-842X(96)81511-3
