By generating peptides from intracellular antigens, which are then presented to T cells, the ubiquitin/26S proteasome system plays a central role in the cellular immune response. Under the control of interferon-γ the proteolytic properties of the proteasome are adapted to the requirements of the immune system. Interferon-γ  induces the formation of immunoproteasomes and the synthesis of the proteasome activator PA28. Both alter the proteolytic properties of the proteasome complex and enhance proteasomal function in antigen presentation. Thus, a combination of several of regulatory events tunes the proteasome system for maximal efficiency in the generation of MHC class I antigens.