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.
Yes, it's produced primarily by the adipocytes of white adipose tissue, but it's also manufactured by your bone marrow, brown adipose tissue, liver, mammary epithelial cells, ovaries, pituitary, placenta, skeletal muscle and the stomach. The amount of leptin in the body is proportional to the amount of fat in the body.