Cytotoxic immunity is critical for protection against intracellular bacteria and virus infections. Young foals are uniquely vulnerable to Rhodococcus equi and rotavirus infections, which induce serious diseases that require expensive treatment and clinical care, and negatively impact the equine industry. Age-related immune competence or lack of may explain exclusive susceptibility to these type of microorganisms by the young equine. Cytotoxic immune response has not been characterized in the foal. Our study design explores potential limitations of the equine neonate immune responses associated with the effector cytotoxic cell, the suppressive regulatory cell, and the activator antigen presenting cell. The overall hypothesis of our investigation is that the equine neonate immune system generates limited cytotoxic immune response due to suppressive effect of Tregs and/or limited stimulation by antigen presenting cells. Our in vitro experiments will measure several molecules that individually characterize the function of these 3 cell types through age, starting with the equine neonate and the 1-month-old foal, and comparisons with adult horse cells. In addition, this study will inform how cytotoxic immune function and antigen presenting cells gain competence in the young foal, in response to different stimuli, and how suppression from Tregs impacts these responses. The knowledge gained from this study is critical to understand the limitations in the immune response to certain pathogens, and consequent development of vaccines adapted to the equine neonate.