Anti-keyhole limpet hemocyanin (KLH) IgG isotype (a, b, and c), anti-KLH IgG1 isotype (d, e, and f) and anti-KLH IgG2α isotype (g, h, and i) responses in young, middle-aged, and aged rats, respectively. The rats were fed controlled diet or diet supplemented with SDS. Antibody levels were determined as described in “Materials and methods”. Values are medians of measured optical density (OD) at 450 nm ± SE of medians. ***p < 0.001, **p < 0.01, and *p < 0.05, the differences of comparing SDS group to control group