You head back to the gym after a layoff and you are definitely sore the next day.  But then you head back to gym and you are less sore after the 2nd workout.  People often wonder why they are less sore after successive workouts. A Study in January 2016 at BYU may now shed some light on this subject.This article suggests that T-cells enter the muscle after the first workout in an inflammatory response and then after repeated workouts it suggests that muscles become more effective at recruiting immune cells following a second bout of exercise and that these cells may facilitate accelerated repair. In other words, the T-cells seem to remember the damaging insult to the muscles and reacts similarly to when the immune system responds to antigens like bacteria or viruses.

The article surmised that inflammation is not necessarily bad and actually increased after the second set of exercise and suggests inflammation itself probably does not worsen exercise-induced muscle damage.

Skeletal muscle responds to exercise-induced damage by orchestrating an adaptive process that protects the muscle from damage by subsequent bouts of exercise, a phenomenon called the repeated bout effect (RBE). The mechanisms underlying the RBE are not understood. We hypothesized that an attenuated inflammation response following a repeated bout of lengthening contractions (LC) would be coincidental with a RBE, suggesting a potential relationship. A 29-plex cytokine array identified alterations in inflammatory cytokines. Immunohistochemistry quantified inflammatory cell infiltration and major histocompatibility complex class 1 (MHC-1). Muscle soreness was attenuated in the days following B2 relative to B1, indicating a RBE. , CD8+ T-cells were observed infiltrating apparently necrotic muscle fibers. No changes in MHC-1 were found. We conclude that inflammation is not attenuated following a repeated bout of LC and that CD8+ T-cells may play a role in muscle adaptation following LC. Moreover, it appears that the muscle or the immune system becomes sensitized to an initial bout of damaging exercise such that inflammatory cell infiltration into the muscle is enhanced upon a repeated bout of damaging exercise.

Source: Frontiers | Skeletal muscle inflammation following repeated bouts of lengthening contractions in humans | Striated Muscle Physiology