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.