During the immune response T cell function is influenced by extrinsic factors, some of which lead to increased protein and DNA damage and are thought to play a role in age-related immune dysfunction. Damage is in part due to reactive oxygen species produced as a result of aerobic metabolism during a vigorous immune response, but in the in vitro models commonly used to study human immunity may also be due to culturing cells under hyperoxic conditions, i.e., in air. Reactive oxygen species (ROS) are ubiquitously generated but an imbalance between ROS production and protection against ROS may severely affect T cell activation. Controlling and modulating oxidative stress in the extracellular milieu may influence T cell signalling and activation. Here, we discuss the relevance of oxidative stress modulation to prevent T cell dysfunction. We draw attention to some technical, but critical, aspects of T cell culture under hyperoxic conditions.