Interestingly, all the ligands for the tyrosine kinase receptors expressed by adipocytes, such as IGF-1, Protein S, Gas6, or growth differentiation factor 3, which also promotes adipocyte growth [62], are known to be produced by engulfing macrophages, and their loss is associated with a decreased sensitivity to HFD-induced obesity. This evidence concerns the gene NTRK1 and obesity due to melanocortin 4 receptor deficiency.