The 116-13.1 monoclonal antibody reacts with mouse CD8 also known as Lyt-2. The CD8 antigen is a transmembrane glycoprotein that acts as a co-receptor for the T cell receptor (TCR). Like the TCR CD8 binds to class I MHC molecules displayed by antigen presenting cells (APC). CD8 is primarily expressed on the surface of cytotoxic T cells but can also be found on thymocytes natural killer cells and some dendritic cell subsets. CD8 most commonly exists as a heterodimer composed of one CD8αand one CD8βchain however it can also exist as a homodimer composed of two CD8αchains. Both the CD8αand CD8βchains share significant homology to immunoglobulin variable light chains. The molecular weight of each CD8 chain is approximately 34 kDa. The 116-13.1 antibody exhibits depleting activity when used in vivo .