LEP and obesity due to melanocortin 4 receptor deficiency: Leptin levels in the serum of young obese individuals are comparable to those in the serum of elderly lean individuals (68), and we have recently demonstrated that incubation of B cells from young lean individuals with leptin decreases class switch and influenza vaccine-specific IgG antibodies, similar to the levels observed in B cells from young obese and from elderly lean individuals, further supporting the concept that obesity accelerates age defects.