When the body is sick or injured, muscle protein can break down faster than normal, leading to weight loss and muscle wasting. This happens because the body starts breaking down muscle proteins more than it makes new ones. Scientists have found that factors released by activated macrophages, a type of immune cell, may play a role in this process. In both experiments and studies on patients, it was observed that these factors can lead to increased breakdown of muscle proteins. However, more research is needed to fully understand how these factors trigger muscle breakdown.