Zinc is found in all body tissues, but the highest concentrations are found in the eye, bone, and male reproductive organs. Zinc is involved in RNA and DNA synthesis and is essential in the process of tissue repair. It is also required for the formation of collagen and the production of active vitamin A (for the visual pigment rhodopsin). Zinc also functions as a chelating agent to protect the body from lead and cadmium poisoning. Zinc is absorbed from the small intestine. Its absorption and excretion seem to be through the same sites as those for iron and copper. The body does not store zinc as it does copper and iron. Untreated zinc deficiency in infants may result in a condition called acrodermatitis enteropathica. Symptoms include growth retardation, diarrhea, impaired wound healing, and frequent infections. Adolescents and adults with zinc deficiency exhibit similar adverse effects on growth, sexual development, and immune function, as well as altered taste and smell, emotional instability, impaired adaptation to darkness, impaired night vision, tremors, and a bullous, pustular rash over the extremities.