The researchers, led by Professor Paul Hertzog, found that the progesterone hormones in the pill interacted with a protein, known as Interferon epsilon, produced in the female reproductive tract. Interferon epsilon is a type of cytokine protein but unlike other cytokine proteins that are produced by the immune system, it is made by oestrogen and progesterone.
High levels of progesterone cuts down the Interferon epsilon protein levels by as much as 10 times, lowering the body's natural immune response to infection and thus increasing the risk of a woman contracting Chlamydia. The study has been published in the journal Science.
"Cytokines are normally produced after you are exposed to an infection. [Interferon epsilon] is there to prime the system to have protective immunity before you even get infected. Since this protein boosts female reproductive tract immune responses, it's likely, although we haven't addressed it directly, that this finding will be important for other infectious diseases like HIV and HPV", Professor Hertzog said.