of healthy dogs over an age range of 0 6 to 14 2 years of age the cd3 t cell marker which defines total t cells encompassing both the cd4 and cd8 t cell subgroups demonstrated a significant increase in relative percentage with age fig1 a this may partly be a result from a relative increase in cd8 t cells that co express the cd3 marker being greater than a relative decrease in cd4 t cells that express the same marker studies in humans have also demonstrated an increase in levels of cd8 t cells with age suggesting that these cell types correspond to immature t cells that are unable to attain their full mature functional status due to the age associated processes highlighted above also when combined with interpretation of data from other canine and human studies which have examined absolute and relative values of total t cell subsets it was shown that an increase in percentage of total t cells is apparent because the absolute numbers of the t cell subsets are declining at a lower rate while the cd3 marker is commonly shared by the cd4 and cd8 t cells their functions are very distinct from each other the function of cd8 t cells cytotoxic t cells is to identify and destroy host cells that contain intracellular pathogens e g viruses cd4 t cells helper t cells are specialized to activate other cells to destroy extracellular pathogens e g bacteria and parasites and fall into two functional classes t helper 1 th1 cells which activate monocyte macrophages to kill bacteria they harbor and th2 cells which activate b cells to produce antibody the significant increase in relative percentage of canine cd8 t cells fig1 c observed could.