Unforgiveness has been framed as a stress reaction (Worthington & Scherer,2004). The negative health consequences of chronic stress and the physiological wear and tear of a hyper-aroused stress response have been observed in traumatized populations and in people who have endured extreme and/or chronic stressors.
The results [of the study] have been mixed (e.g., Futterman, Kemeny, Shapiro, & Fahey, 1994) but generally support the idea that positive affect enhances and negative affect compromises immune functioning (e.g., Labott, Ahleman, Wolever, & Martin, 1990). [Handbook on Forgiveness by Alex H. S. Harris & Carl E. Thoresen P.322]
From David Scudder's Sermon "Stopping the Hurt Through Forgiveness"