Obesity is one of the major health challenges of our day. We now understand that the immune system is intimately involved in regulating body weight. Obesity also affects the immune system negatively. Hormones also regulate the immune system – and impact on body weight in this way. The environment is a major driver of obesity – at work, home, school and within society. This lecture will outline what we now know about the bodies robust metabolic and behavioural defence against weight loss. The nature of this defence against weight loss by the immune system will be explored. We will look at how the female hormone system regulates immune function and weight and how HIV interacts with this system.