Malformations in reproductive organs, fewer young or even complete failures to reproduce are some of the detrimental signs of high contaminant levels. One of the underlying causes of failure to reproduce is that some of the chemicals interfere with sex hormones. Such hormone disrupters can mimic or block hormones because they are similar enough in structure to fit into the body's biochemical receptors. Contaminants that block the estrogen receptor can inhibit the growth of the reproductive tract and the mammary glands. In fish the same receptor stimulates the production of a precursor to egg yolk.
Sex hormones are important to the normal sexual development of young animals.
In polluted temperate environments, high levels of hormone disrupters have been connected to malformations in the reproductive organs, change of sex in some species, and abnormal mating behavior. The immune system is very sensitive One of the most sensitive targets for organic contaminants may be the immune system, the body's primary defense against disease. The thymus, which normally produces antibodies to fight infectious agents, can waste away and cease to function. There are signs that animals with a high load of organic contaminants are more susceptible to infections. POPs also limit cell-mediated immunity, the branch of the immune system that fights cancer cells and parasites.