How Your Cycle Influences the Immune System

The female cycle doesn’t just affect fertility, it also has a significant impact on the immune system. Many women notice physical changes throughout their cycle, including increased susceptibility to infections during certain phases. These observations are now well-supported by science: the immune response varies depending on hormone levels and cycle phase, influencing the risk of infections, autoimmune conditions, and allergies.

Throughout the roughly 28-day menstrual cycle, hormone levels fluctuate significantly — particularly estrogen, progesterone, and testosterone. These hormones don’t just regulate fertility; they also interact directly with immune cells, shaping how the body responds to pathogens. Estrogen, for example, binds to immune cell receptors and boosts immune activity. That’s why, in the first half of the cycle — especially during the follicular phase — the immune system tends to be more active and effective at fighting off infections.

After ovulation, however, things shift. Progesterone levels rise and estrogen drops. During this luteal phase, the immune system is deliberately dialed down to allow for the possibility of embryo implantation. But this dampened immune response comes with a cost: viruses and bacteria may find it easier to gain a foothold. In fact, research over the past two decades confirms that the immune system is consistently less active in the second half of the cycle

Even though your immune system naturally shifts throughout the menstrual cycle, there’s a lot you can do to support it:

• Eat a balanced diet rich in fruits, vegetables, whole grains, healthy fats, and proteins.
• Move your body regularly – ideally outdoors.
• Minimize stress – it can disrupt your hormones and weaken your immune response.
• Get enough sleep, especially during the luteal phase.
• Track your cycle – for example, with a cycle computer like Daysy or Lady-Comp. This helps you understand each phase of your cycle and adjust your lifestyle accordingly, from targeted nutrition and exercise to planned rest during times when your body may be more vulnerable.

Unfortunately, no study results are yet available that allow conclusions to be drawn about when girls and women should ideally get vaccinated. It would be interesting to learn whether the immune response and/or vaccination reactions differ depending on whether the shot was given in the luteal or follicular phase.

Research is also still needed into the effect of hormonal contraception on the immune response. Contraception affects the immune response as the "pill" contains artificial hormones. Do permanently elevated levels of sex hormones mean fewer infections? Or, on the contrary, are there more of them? There are indications that some pills containing only progestin, an artificial progesterone, throttles inflammatory responses in the body in such a way that the tendency towards infection increases. Here, too, however, many questions remain unanswered.