DCs or macrophages engulf antigens into phagosomes. The phagosomes may fuse with endoplasmic reticulum, and antigens from the particle are directed to the proteasome where they are digested; cognate peptides with the appropriate linear amino sequence then bind to MHC I. This complex moves to the surface of an antigen-presenting cell and is subsequently engaged by a CD8⁺ T cell. An alternative pathway is that antigens “leak” or escape from the phagosome and enter the cytosol in the absence of fusion of phagosome/endoplasmic reticulum; the antigens in the cytosol are digested by the proteasome, bind to MHC I, and traffic to the cell surface.