CD8b is the 32 kD β chain of CD8, also known as Lyt-3.2 or Ly-3.2. It is a member of the Ig superfamily expressed as a heterodimer with the CD8α chain on a subset of MHC class I-restricted T cells and most thymocytes. CD8 is a co-receptor for the TCR complex involved in T cell activation. The antibody 53-5.8 is specific for Ly-3.2 and has low reactivity with Ly-3.1.