It is assumed that, in immunocompetent patients, the immune response will allow a normal inflammatory response to tissue trauma. However, alterations in immune system function may be seen in the surgical patient, which can have an adverse effect on recovery in the postoperative period. The main causes of alteration of the immune response are tissue injury, either intentional (e.g. surgery) or accidental (e.g. trauma), and haemorrhage. The main points covered in this section are The immune response; Factors affecting the postoperative immune response; Effect of perioperative immune dysfunction on the surgical patient; and Minimizing immune dysfunction following anaesthesia and surgery.