Fat cells just under a person's skin may be the first responders to invading bacteria, buying time until the white blood cells arrive at a wound site, according to a new study.

The new findings suggest that the task of fighting infections is not solely the responsibility of the immune system, the researchers said. Moreover, fat cells may fight infections by producing antimicrobial compounds, lab experiments in mice and human fat cells showed.

"That was totally unexpected," study co-author Dr. Richard Gallo, chief of dermatology at the University of California, San Diego School of Medicine, said in a statement. "It was not known that [fat cells] could produce antimicrobials, let alone that they make almost as much" as immune system cells, he said.

In the study, the researchers exposed mice to the bacterium methicillin-resistant Staphylococcus aureus (MRSA), a type of staph bacteria that is found on human skin and is resistant to several antibiotics. MRSA can cause hard-to-treat — and even deadly — infections in people. [6 Superbugs to Watch Out For]

When the skin is cut and exposed to pathogens, the immune system sends specialized white blood cells, such as neutrophils, to the wound site to kill the bacteria entering the skin. But it takes time for these cells to reach the site of injury.

In the experiments in mice, the researchers found that the layer of fat under the skin at the site of an infection thickened, and that the fat cells produced an antimicrobial compound called cathelicidin. These findings suggest that these fat cells can directly sense the staph bacteria and respond accordingly, the researchers said.

In addition, mice that lacked healthy fat cells under the skin suffered more frequent and severe infections with MRSA, according to the study, published today (Jan. 2) in the journal Science.

However, the findings don't mean that having too much fat in the body will bring better protection against infections. In fact, obesity or insulin resistance could result in having fat cells that don't respond as they should, and may actually lower a person's defense against infections, the researchers said. This could explain the results of previous studies that found that obese people may be at higher risk for skin infections, the researchers said.

Email Bahar Gholipour. Follow Live Science @livescience, Facebook & Google+. Originally published on Live Science.