Leptin, a key biomarker linked to weight management and immune system ageing, plays a pivotal role in both appetite regulation and immune responses. Produced by fat cells, leptin not only controls hunger but also steers inflammatory cells in fighting infections. High leptin levels, often seen in obesity, can disrupt the balance of these cells, highlighting the intricate connection between body fat, immune function, and ageing. Thus, measuring leptin levels is essential for gaining insights into immune system ageing and overall health management.