Smoking is the factor that most alters defenses, even years after quitting. It is the environmental factor that has the most influence of the 136 analyzed, followed by body mass index and latent infections by cytomegalovirus.

The authors of the study, from the Pasteur Institute in Paris, have focused on the production of cytokines, proteins that are released in cells when they detect a pathogen. They have not yet studied the relationship between this alteration of white blood cells and tumors.