The study included 587 children born between 1999 and 2001 who lived in the Faroe Islands.
The residents of the Faroe Islands, which lie midway between Norway and Iceland in the North Atlantic, have widely varying PCB exposure due to differing consumption patterns of contaminated traditional foods such as pilot whale blubber.
Routine childhood vaccinations, which feature standard antigen doses at set time points, provided an opportunity to examine immune responses in the Faroese children.
Mothers provided blood samples at 32 weeks of pregnancy and milk samples 4 to 5 days after birth.
Children received vaccinations against diphtheria and tetanus at 3, 5, and 12 months and booster vaccinations at 5 years.
Blood samples were collected from children at 12 or 18 months, before and after vaccinations at age 5, and again at age 7.
The mothers' samples were analysed for PCB content, and the children's samples were tested for PCBs and/or diphtheria and tetanus antibody concentrations.
Higher PCB concentrations, particularly in children at 18 months, were associated with lower concentrations of antibodies against diphtheria and tetanus at ages 5 and 7.
In some cases, children's antibody concentrations were below levels needed to protect against the two diseases.
Elements of the immune system critical to its response to antigens undergo crucial development during the toddler years.
Adverse impacts at this stage could have consequences beyond the diseases specific to this study.
These findings may also have implications for other environmental chemicals with toxic effects on the immune system.
The study appears online in the peer-reviewed journal Environmental Health Perspectives (EHP).