Antigen presentation describes a vital immune process. Immune cells cannot penetrate other cells, which may be infected with viruses or bacteria, and thus rely on information conveyed by fragments of intracellular components being presented on major histocompatibility complex (MHC) molecules on the cell surface. Likewise, many pathogens are too large to be recognised directly by immune cells, and must first be digested into smaller fragments that can be presented by specialised antigen-presenting cells (APCs), such as dendritic cells and macrophages.