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Your immune system has a Dr. Jekyll and Mr. Hyde complex when it comes keeping you healthy.  Traditionally, the immune system is seen as a good guy that kills off viruses and eats up bacteria attacking your body. However, it can also turn against you and cause disease.

To fight infection, the cells of your immune system produce proteins called cytokines that cause the infected area to swell, turn red, heat up and hurt. This reaction is called inflammation, and it helps you recover from viral and bacterial invaders relatively quickly.

Cytokines can also reduce inflammation, allowing for a system of checks and balances that prevents inflammation from lasting too long and hurting you instead of helping you. For example, the immune cell called “natural killer T cell” produces cytokines that cause inflammation and is kept in check by the immune cell “regulatory T cell”, which produces cytokines that counteract inflammation. Some immune cells can even produce both types of cytokines and switch roles. The immune cell “macrophage” can go from causing inflammation to kill the infection to preventing inflammation to help the injury heal.

Inflammation becomes bad when it lasts too long or is turned on at the wrong time. The cytokines can cause changes to your body that lead to disease, including stroke, heart disease, obesity and diabetes. Scientists are still unsure why the immune system can ignore its own safeguard. For obesity and diabetes, scientists at Harvard Medical School think the culprit is a hormone called leptin.

The researchers noticed that in obese people and obese mice immune cells found in fat called “mast cells” had five times more leptin than their lean counterparts. Moreover, mice that did not have leptin in their mast cells did not become obese or diabetic when put on a high-fat diet. Interestingly, obese mice lost weight and improved blood sugar levels when they were injected with mast cells with no leptin. This happened because mast cells with no leptin produced inflammation-reducing cytokines, while mast cells with leptin produced inflammation-causing cytokines, the research team said. In obesity and diabetes, leptin turned the immune system into a bad guy.

This is the first study to show that simply reducing the amount of leptin in mast cells can prevent, and even reverse, obesity and diabetes caused by a high-fat diet. Understanding the details of how our immune system causes disease can lead to new and better treatments to help people stay healthy.

Dao H. Ho, PhD, is a biomedical research physiologist at Tripler Army Medical Center.