CD8, also known as Ly-2 or Leu-2, is a heterodimeric glycoprotein (alpha and beta subunits) that functions in conjunction with the T cell receptor in the recognition of MHC class I/peptide complexes. CD8 alpha is expressed on double positive (CD4+8+) thymocytes and mature CD8+ cytolytic T cells, intraepithelial lymphocytes, some gamma-delta T cells, subsets of thymic and splenic dendritic cells, and peritoneal mast cells. It can form disulfide linked homodimers or heterodimers with CD8 beta. Thymic CD8+ DC express primarily alpha-beta heterodimers, while splenic CD8+ DC primarily express alpha-alpha homodimers. CD8 alpha+ DC can present viral antigenic peptides in complex with MHC I and prime CTL responses.