The immune system is designed to protect the individual from foreign substances or organisms. It is expressed as cellular and humoral immunity. The former is dependent upon T lymphocytes and the latter on B lymphocytes, which become plasma cells and secrete antibodies. The immune system can be influenced by protein-energy malnutrition (PEM) and by catabolic illnesses such as sepsis and trauma, which in turn cause PEM. Specific trace element and vitamin deficiencies can also alter the immune state. However, overnutrition and obesity can also influence immune mechanisms. Obesity can promote the development of diabetes, which can alter the immune state. Finally, immunity becomes less effective with ageing and this process is enhanced by associated malnutrition.