Joe Dispenza is not medically qualified to give out medical advice about the human immune system.
However,
according to the Cleveland Clinic, a reduction of chronic stress helps the functioning of the immune system.
Likewise, the
Harvard Medical School reports the benefit of invoking the relaxation response in order to reduce chronic stress.
The Harvard Medical School also reports the following:
The idea of boosting your immunity is enticing, but the ability to do so has proved elusive for several reasons. The immune system is precisely that β a system, not a single entity. To function well, it requires balance and harmony. There is still much that researchers don't know about the intricacies and interconnectedness of the immune response. For now, there are no scientifically proven direct links between lifestyle and enhanced immune function.
https://www.health.harvard.edu/staying-healthy/how-to-boost-your-immune-system
From the Wexner Medical Center of Ohio State University:
Janice Kiecolt-Glaser, PhD, has dedicated her research career to understanding how stress alters the endocrine and immune systems. Her Stress and Health Lab, part of The Ohio State University Medical Center's Institute for Behavioral Medicine Research (IBMR) of which she is director, has been at the forefront of this field.
Dr. Kiecolt-Glaser's research reveals how chronic stress, such as spousal care giving, can have negative effects on the caregiver's immune system and can increase the person's risk for age-related diseases. Her work reveals that chronic stress can impair vaccine response and her lab was the first to demonstrate that stress slows the rate at which wounds heal.
https://wexnermedical.osu.edu/blog/making-connection-between-chronic-stress-immune-response
In short, qualified medical scientists have found a link between chronic stress and and the functioning of the immune system, but they do
not say that one's immune system will be improved by 50% if one focuses on gratitude 10 minutes a day for 4 days.