The competence of the immune system is largely predictable based on the self-perception of an individual. A high self-rating toward good health is an indication of a strong immune system while a low self-rating toward poor health is likely an indication of a susceptible immune system.

Psychology Professor Sheldon Cohen of the Carnegie Mellon University relates that among healthy individuals aged 18-55,  "Poor self-ratings of health have been found to predict poor health trajectories in older adults, including an increased risk for mortality." 

The associations remained consistent even when compared to objective indicators as physical examinations, medical health records and hospital admissions.

The premise for these associations is how people base self-rating on their knowledge of the benefits and the consequences of their regular habits and activities.
Those who engage in regular exercise, live a healthy lifestyle and have healthy social ties and emotional health. These yield a healthy self-rating. They are more likely to have stronger immune system and live longer lives.

Professor Cohen says, "We wanted to examine whether self-rated health predicted effective immune response in younger adults selected for their good health and whether this association was dependent on health practices and socioemotional factors"

A group of 360 healthy individuals with an average age of 33 were asked to perform self-rating of their health.  The ratings used were excellent, very good, good, fair and poor.  Two percent of the self-rating was of fair health and zero reported poor self-rating.  All the participants were exposed to a common colds virus subsequently and were observed for five days. One-third were affected by the virus and developed colds.

The researchers determined that the individuals who self-rated as excellent had stronger immune systems and were more resistant to the virus than those who self-rated with fair, good and very good. Neither the socioemotional factors nor health practices could account for this difference.

Professor Cohen suggests that the self-rating by the participants actually reflect pre-morbid indicators (sense, feeling, diffuse) regarding any decline in the immune system. These indicators enable the person to 'sense' that the body's immune system is not in top condition.

He attributes this to the fact that individuals know things about their bodies that may not be apparent to the examining physicians. Based on the data, the self-rating consistently predicted the immune system's reaction to infectious agents.

The research, which was funded by the National Center for Complementary and Integrative Health, National Institute of Allergy and Infectious Diseases, National Heart, Lung and Blood Institute and National Institutes of Health and published in Psychosomatic Medicine, were acknowledged by Hyong Jin Cho and Michael Irwin of the UCLA School of Medicine as a possible and unique reference for a simple cost-effective tool for assessing susceptibility to infectious and inflammatory disorders