That nagging cough and congestion just won’t quit, even though you kicked the smoking habit months ago. New research offers an explanation—smoking may inflict lasting damage to your immune system, leading to inflammation that persists up to 15 years after your last cigarette.

“This is the first time we’ve shown smoking’s long-term effects on immune responses,” Darragh Duffy, who leads the translational immunology unit at the Institut Pasteur and is a study co-author, said in a press release.

How Smoking Affects Immunity

The study, published in the journal Nature, analyzed data from 1,000 healthy French adults between 20 and 69 years of age enrolled in the Milieu Interieur project, a population based study examining how genetics and the environment affect the immune system. Participants provided blood samples and answered 136 questions about diet and various lifestyle factors.

Researchers stimulated production of 13 cytokines, which are inflammation-regulating proteins. They enable the body to mount an immune defense when viruses, bacteria or other pathogens invade and pose a threat to health.

The results suggest smoking impairs the body’s innate and adaptive immune systems. The innate system acts quickly against new threats, while the adaptive system is built up over time and it targets specific pathogens. Though the innate system recovered faster after quitting, inflammation persisted in the adaptive system.

As a result, people with a history of smoking may struggle to fend off viruses their body has previously encountered, since their adaptive immunity remains impaired by inflammation.

“This is a major discovery elucidating the impact of smoking on healthy individuals’ immunity and also, by comparison, on the immunity of individuals suffering from various diseases,” Violaine Saint-Andre, research scientist at Institut Pasteur and first author of the study said.

Persistent Inflammation Impairs Defense Mechanisms

Cytokine activity declined in those who were overweight, had herpesviruses, or smoked

Smokers showed heightened inflammatory responses compared to non-smokers. While inflammation decreased after quitting smoking, it persisted for years at lower levels.

That persistent inflammation hindered the cytokines’ ability to establish a robust defense system. Consequently, people with a smoking history could fall ill from the same virus instead of developing immunity to it.

The researchers hypothesize smoking may alter immunity by changing cell structure through DNA methylation. This is a process in which a small molecule called a methyl group is added to DNA, proteins, or other molecules within the body, altering the behavior of these molecules.

Smoking may modify sequences involved in metabolism and signaling, directly impacting cytokine function.

Smoking Kills 480,000 People a Year in the US

While smoking rates have dropped since the 1960s, it remains the leading preventable cause of death in the U.S. In fact, the risk of dying from cigarette smoke has increased over the past 50 years.

In addition to potential DNA changes through methylation, cigarette smoke damages nearly every bodily organ, contributes to many diseases, and deteriorates overall health.

Smoking causes over 480,000 U.S. deaths annually, accounting for nearly one in five deaths, according to the U.S. Centers for Disease Control and Prevention. It is responsible for about 90 percent of lung cancer fatalities and 80 percent of deaths from chronic obstructive pulmonary disease.