Earlier studies on the subject showed that the immune system changes during pregnancy and after childbirth, as well as throughout the course of the menstrual cycle. The study from the scientists at Indiana University is one of the first to show that sexual activity affects these changes. Compared with women who remain sexually abstinent, sexually active women had observable differences in immune function.

The results were drawn from data collected during the Kinsey Institute’s WISH Study, which stands for Women, Immunity, and Sexual Health. The study gathered data throughout the menstrual cycle from 30 healthy women, half of who engaged in regular sexual activity. The other half remained abstinent. Professors Heiman and Demas were co-researchers on the study.

The findings actually stem from two separate papers. In the first, professor Lorenz and colleagues found that sexually active women underwent greater changes in helper T cells, as well as the proteins these cells use to communicate with the rest of the body. The second paper compares the differences in antibodies between the two groups of women.

Helper T cells play a key role in the body’s immune system. They activate the cells that attack foreign microbes, antibodies called immunoglobulins. These are secreted by white blood cells and play a key part in protecting the body from invasive pathogens.

The immune system and pregnancy are intertwined in a unique way, Lorenz explains. A woman’s body needs to have a fully functioning immune system to protect against bacteria and viruses that could very easily make her sick. At the same time, however, conception requires genetic material from two separate partners. If a woman’s immune system were to attack sperm or an embryo, then it would be terribly difficult to complete a pregnancy.

The key to understanding the findings is in the different types of helper T cells and immunoglobulins. Type 1 helper T cells primarily guard the body against external threats like a virus or bacterial infection. Type 2 helper T cells, however, tell the immune system what to attack and what to leave alone – “explaining” to the body that sperm or an embryo are not actually pathogens.

Another antibody, immunoglobulin A, is typically found in the mucus of the female reproductive tract and can prevent the movement of sperm and ova during fertilization. Immunoglobulin G antibodies, the type that are commonly present in the blood, help ward off diseases without bothering the processes of the uterus.

Professor Lorenz and colleagues discovered that type 2 helper T cells were significantly more abundant in sexually active women who were not yet pregnant, but had entered the luteal phase of their menstrual cycle – the point when the lining of the uterus thickens to prepare for pregnancy. They also found that type 1 helper T cells were much more abundant in the same women during the follicular phase of the menstrual cycle, the period when the follicles of the ovaries mature.