The human body is a complicated system which operates much like a self-contained city. Some organs produce new cells, others use cells to perform their jobs, and eventually certain scavenger cells arrive to remove dead cells from the system. In the case of the human body, these scavengers are specialized white blood cells called macrophages. Macrophages remove dead cells essentially by eating them, which helps to explain why the word macrophage means "big eater" in Greek.
When external skin cells die, there are a number of mechanical and chemical methods used to slough them off. Exfoliants and scrub brushes are often employed to remove dead cells and encourage new cell turnover. But dead cells within the human body are not so easily removed. They go through a much more complicated elimination process, which is not always as efficient or thorough as one might hope.
Living cells become dead cells through two different processes. Many body cells are programmed to die at a prescribed time, through a process called apoptosis. Red blood cells, for example, are programmed to die after 120 days of service. Other cells, such as white blood cells, may be programmed to die an apoptotic death after only a few days. These dead cells may continue to flow through the body's bloodstream or collect in various organs, but they are clearly no longer contributing to the system.
The other process for creating dead cells is called necrosis. Necrotic cell death usually occurs after a trauma or infection or other shock to the system. When cells become necrotic, they may be removed through surgery or other medical intervention, but often they enter the bloodstream in the same way as apoptic cells. The body cannot function well with an overabundance of dead cells, so macrophages take on the mission of breaking down the excess.
A macrophage cell can literally detect dead cells through smell, much like a scavenger bird detects dead animals. Whenever dead cells reach the part of the bloodstream patrolled by a macrophage, the macrophages surround them and convert them into easily removed components. At the same time, the macrophage covers the dead cells with a substance known as an antigen. This action tags the cells for further attack from other types of cells in the body's immune system. Ideally, the macrophages and killer T-cells should render both dead cells and foreign invaders harmless enough to re-enter the bloodstream for elimination.
When macrophages become overwhelmed, however, they may allow some dead cells to pass through unprocessed. The DNA from those cells may trigger an inflammatory reaction as the dead cells combine with other substances. This process is the basis for many autoimmune diseases such as Crohn's disease or lupus. Bolstering the body's macrophages is often a course of treatment recommended for autoimmune diseases and even some forms of cancer.
Once the macrophages and other immune system components have essentially digested the body's dead cells, the cells make their way through the bloodstream and eventually into the digestive system for final disposition. This means that the solid waste we call fecal matter is largely composed of dead cells sloughed off by the various organs and processed for elimination by macrophages.