The web page for "Our Stolen Future" provides a summary of the latest information on the ability of persistent environmental contaminants to alter immune system functioning.

Pesticides may be linked to a weakening of the immune system which in turn increases susceptibility to infectious diseases and certain cancers.

Immunotoxic Exposures

Known developmental immunotoxins (studied primarily in animals) include the pesticide chlordane, TCDD, lead, the polycyclic aromatic hydrocarbon benzo[a]pyrene and diethylstilbestrol. Rather than suggesting that few environmental contaminants are immunotoxic to the young, this discrete list only reflects the lack of testing for developmental immunotoxicity on a broader range of substances.83 There are other substances suspected of being immunotoxic from human data, as will be discussed below.

Organochlorines

Several epidemiological studies have indicated an increase in incidence of infectious illnesses in the children of women who were exposed to high doses of PCBs either through accidental or occupational exposure.84 Incidence of respiratory and sinus infections and gastrointestinal and dermatological symptoms was pronounced from both transplacental and breast milk exposure to high levels of PCBs. These symptoms appear to have been confined to the earliest stages of life.

There are a number of studies currently examining the immune system effects in populations exposed to lower (i.e., environmental) levels of organochlorine contaminants, including PCBs. Altered T cell function has been associated with exposure to PCBs pre- and postnatally85 in both acute and low-level exposures in a Dutch cohort study. The most recent study of these children, now age 3 and a half, suggests that greater PCB exposure was associated with increased susceptibility to infectious diseases such as recurrent middle-ear infections and chicken pox.86

Researchers at Laval University have been following the development of Inuit children in northern Quebec who are exposed to organochlorines chemicals in utero because of the high levels of these substances in traditional food items. There is some evidence that among the Inuit children, those with highest prenatal exposure to organochlorines (such as DDE and HCB) exhibit higher rates of infectious diseases, particularly meningitis and bronchopulmonary and middle-ear infections.87

Similar to the hypothesis for reproductive effects and carcinogenicity from hormonally active agents (HAAs, i.e., endocrine disruptors), it appears that immune suppression effects from HAAs may also manifest via a developmental mechanism. This has been seen in experimental studies where DES exposure in mice is associated with lowered T helper cell and natural killer cell numbers and that the immune system deficits increased with age.88

Women who were exposed to DES as fetuses also appear to have long-term immune effects manifest as greater susceptibility to autoimmune disease. 89 These models have led to speculation that exposure to low levels of HAAs in utero or infancy may target the thymus, as is seen with a number of HAAs such as heavy metals, pesticides, PAHs and dioxin.

Pesticides

While the exact immunological effects from pesticides are not well known, it is believed that they may potentially be immunotoxic to humans. Organochlorine, organophosphate, carbamate and metal-based pesticides alter immune function in animals and cell cultures. These are hypothesized to cause immunosuppressive effects manifest as diminished host resistance to infections, as well as promotion of tumour growth.

Observations of wildlife populations appear to demonstrate that environmental exposure to contaminants, many of which are the older, bioaccumulating organochlorine type pesticides, is associated with immunotoxicity in fish, birds and marine mammals.90 An interesting prospective experiment with harbour seal pups captured from relatively unpolluted waters and housed for several years in controlled conditions determined that those fed herring from the polluted Baltic Sea exhibited substantially weaker immune responses and had a greater prevalence of infections compared to controls.91

Human data on immunosuppressive effects from pesticides is, however, limited at best. The World Resources Institute report, Pesticides and the Immune System,92 presents evidence, mainly from epidemiological studies in the former Soviet Union, suggesting that pesticides may be linked to a weakening of the immune system, which in turn increases susceptibility to infectious diseases and certain cancers.

Other Agents

Ultraviolet and ionizing radiation are two physical agents where there are human data for immune altering ability. UV-B radiation causes systemic immune suppression in both mice and humans.93 Exposure to UV radiation appears to explain some of the increases in non-Hodgkin's lymphoma (NHL, an immune system cancer) in Europe, although the positive association between these two variables is not statistically significant.94 A sample of women exposed to ionizing radiation as a result of the Chernobyl nuclear power plant accident displayed decreased immunoglobulin production and reduced monocyte phagocytic activity.95

Other compounds from which there are presumed immunosuppressive effects, despite incomplete knowledge of the precise causal mechanisms, include: air pollutants such as, ozone, nitrous oxides, environmental tobacco smoke; and, metals such as, chromium, cadmium, lead and mercury.96 For example, animal studies have shown an association between cadmium exposure and malignancies.

Heightened immune response due to contaminant exposure can lead to hypersensitivity and allergic reactions. Asthma and allergies are immediate hypersensitivity reactions that can be provoked by exposure to certain organic compounds such as isocyanates, freons, amines, anhydrides and some metals such as platinum.

Medical case reports also suggest that pesticides may be immune sensitizers for some individuals. Skin irritation or dermatitis is a delayed-type hypersensitivity response and it can occur after exposure to certain pesticides, metals, rubber compounds and chemicals such as formaldehyde. The immaturity of the young immune system appears to be somewhat protective of sensitization in very young children; however, as children achieve greater immune competence with increasing age, the hypersensitivity response can be more readily induced.

Mice prenatally exposed to cadmium exhibit altered immune system development. Postnatally these mice appear to have enhanced immune system activity and may have increased susceptibility to autoimmune disorders when older.97

Despite the fact that human data to support immune system effects are incomplete, the majority of researchers conclude that this is an important area for further research and critical evaluation of scientific evidence nonetheless.98

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