The effectiveness of the immune system decreases with increasing age. This process is known as immunosenescence. Recent studies showed the influence of aging on neutrophil granulocytes (PMNs) and T-cells, with the extent of the influence appearing to depend on various co-factors (such as the primary diseases of a patient). In this study, the PMNs and T-cells of younger and older adult patients were tested for their immunoreactivity before and after an operation in order to examine the consequences of the aging process on the moderately triggered immune system. Whole blood was taken from young patients (aged 18–65 years) and old patients (> 65 years) before and one day after an operation. Previous illnesses and medication intake were taken from the patient’s file. PMNs and T-cells were isolated. Immunoassays, live cell imaging (LCI) and flow cytometric examinations (FACS) were performed in order to assess certain properties of the PMNs (chemotactic migration, ROS production, NET formation, change of surface epitopes), their expression of adhesion molecules as well as their cell viability. In addition, the blood samples were subjected to a laboratory chemical examination. Above all during the initial LCI observation period (< 40 min), the PMNs of old patients covered longer distances than those of young patients. NETosis, ROS production and surface antigen expression were influenced neither by age nor by the surgical procedure. Regardless of age, PMNs´ ROS production started earlier 24 h after the operation compared to the pre OP values. By labeling the translocator protein (TSPO), it was demonstrated that mitochondrial release occurs only during suicidal NETosis. Old patients showed significantly more TSPO-labeled mitrochondria per PMN. The neutrophil-to-lymphocyte ratio (NLR) and the CD4/CD8 ratio were significantly increased in older patients. The share of CD28- and CD8-positive cells was increased in younger patients. All patients showed postoperative leukocytosis caused by an increase in monocytes and PMNs, which was independent of the extent of the trauma. Only young patients showed a postoperative increase in lymphocytes. Old patients had higher IL-6 levels than young patients. The operation did not lead to any increases in the IL-6 and CRP levels. Age influences the function of PMNs and T-cells more strongly than a moderate surgical trauma in combination with anaesthesia. The results advance our understanding of the decreasing effectiveness of the immune system in old age.