"Obesity is related to a higher rate of infections and some types of cancer," study co-author Alfredo Halpern of the University of Sao Paulo, Brazil, said in a statement.

Halpern and colleagues studied the effect of weight loss on immune function in 28 morbidly obese patients who had traditional "stomach-stapling," or Roux-en-Y gastric bypass surgery. The 20 women and eight men lost an average of 78.5 pounds six months after gastric bypass. The operation permanently reduces the size of the stomach and bypasses part of the intestines, thus restricting food intake and food absorption.

Before and six months after the surgery, the researchers evaluated patients' blood samples for natural killer cells, which help control infections and cancer as well as cytokines -- proteins essential to the immune response.

Although the number of natural killer cells did not increase after surgery, their activity changed, Halpern said. Before surgery, natural killer cells mounted a weak immune defense, but after surgery their activity increased by nearly 79 percent.

The findings were presented at the Endocrine Society's 90th annual meeting in San Francisco.