One of the most common major surgeries that older patients undergo is coronary artery bypass grafting surgery (CABG), which is performed in approximately 400,000 patients in the United States each year. CABG invokes a massive surgical stress response, with systemic epinephrine increasing 33-fold and norepinephrine increasing 3-fold. Initially, local tissue injury results in a sterile inflammation, releasing damage-associated molecular patterns (DAMPS). DAMPS activate neutrophils, bringing a cascade of cytokines, complement, and coagulation changes. Activation of nociceptors results in a neurometabolic response involving the sympathetic nervous system and hypothalamus-pituitary axis. This brings about systemic effects including changes in basal metabolic rate, hyperglycemia, lipolysis, negative nitrogen balance, and release of cytokines and complement. Although the surgical stress response is essential for wound healing and is usually self-limiting, an exaggerated response may occur resulting in multiple organ dysfunction. The acute phase of the surgical stress response is often followed by secondary insults that may be either sterile or pathogen-induced (such as postoperative infection).In the two-hit model of surgical stress response, there is an exaggerated response even to minor insults in vulnerable individuals who were primed by the initial stress response. Changes in the microbiome may also occur, developing a pathobiome that may enter the systemic circulation. If left unchecked, this second hit may result in the development of systemic inflammatory response syndrome (SIRS) and multi-organ failure. Chronological ageing changes the innate and adaptive immunity of patients. Biological hallmarks of aging such as genomic instability, mitochondrial damage, glycation of proteins, and cellular senescence all result in increased oxidative stress and systemic inflammation. Aging brings about a pro-inflammatory innate immune responsiveness that often occurs even in the absence of an inflammatory threat. This is termed inflammaging. Paradoxically, inflammaging is associated with an increased risk of infection and poor response to stressful events. At the same time, there is an age-associated loss of T-cell function, particularly in naïve CD8 T-cells. This deficit is termed immunosenescence and is characterised by reduced pathogen recognition, chemotaxis, and phagocytosis.