Pennebaker has either written or co-written over 130 articles on expressive writing. One of these suggested that expressive writing has the potential to actually 'boost' the immune system, perhaps explaining the reduction in physician visits. This was shown by measuring lymphocyte response to the foreign mitogens phytohaemagglutinin (PHA) and concanavalin A (ConA) just prior to and 6 weeks after writing. The significantly increased lymphocyte response led to speculation that expressive writing enhances immunocompetence. The results of a preliminary study of 40 people diagnosed with Major Depressive Disorder suggests that routinely engaging in expressive writing may be effective in reducing symptoms of depression.