The underlying theory of blood type diets is that people with different blood types digest lectins differently, and that if people eat food that is not compatible with their blood type, they will experience many health problems.
Leptin is a potent stimulator of the immune system. Leptin receptors on monocytes cause the release intracellularly of STAT3 which cause the monocytes to release IL-6, IL-12 and TNF alpha. The IL-6 goes to the liver and increases production of CRP.
White adipose tissue (WAT) is the main site of leptin synthesis, but it is now evident that the hormone is also produced in other tissues as well. There is some evidence that brown adipose tissue (BAT) is also a site of leptin production. A major issue is the physiological role of leptin produced by brown fat.