n WHAT IS HEMOPHAGOCYTIC LYMPHOHISTIOCYTOSIS (HLH)? Hemophagocytic lymphohistiocytosis (HLH) is a very rare disorder of the body’s defense system during which the body’s normal infection-fighting process becomes out of control. Instead of fighting invading cells from infection, the disorderly immune system begins to attack the body’s normal cells. When the body’s infection-fighting process does not work correctly, the patient usually becomes very sick. n WHAT CAUSES HLH? HLH occurs as a result of overreaction of the immune system. It is usually triggered by an infection. Nor- mally, when an infection occurs, the body’s defense system (immune system) goes into action, releasing infection-fighting cells to control the infection. To help the infection-fighting cells do their job, a lot of special chemicals are also released in a process called inflammation . This process occurs in a very orderly fashion, and when the infection is finally under control, the cells and chemicals of the immune system go back to their usual quiet, inactive state. However, because there is a defect in the immune system with HLH, it cannot turn off. Therefore, the cells and chemicals that were activated to attack the “invaders” begin to affect normal cells instead. As a re- sult, many normal cells are destroyed in a process called hemophagocytosis , which can cause a decrease in the amount of red blood cells, white blood cells, and platelets. Another effect of this out-of-control immune system is an abnormal production of the chemicals that cause inflammation, which can result in fevers, rashes, and problems with bleeding. One important part of the defect in the immune system that results in HLH involves a special infection- fighting cell called a natural killer cell.
n WHAT IS A NATURAL KILLER CELL? Natural killer (NK) cells are an important part of the immune system. Every cell in the human body carries a marker showing that it is a cell that belongs to that specific person. NK cells are like watch dogs. They reside in tissues all over the body, and their job is to constantly check the identification of every cell they meet to be sure that it carries the correct marker and is not a foreign invader. When an NK cell meets a cell without proper identification, it sounds the alarm to other cells of the body’s defense system. Then it latches on to the foreign cell and releases little particles that puncture the foreign cell’s outer cover and make it easier for other immune system cells to destroy the invader. Patients who develop HLH have either a decreased amount of NK cells or NK cells that do not work correctly. In either case, when an infection occurs, the cells that are supposed to be serving as watch dogs are not able to sound the alarm,
Figure 1. Natural killer cell attaching to an unidentified cell
which can allow a serious infection to develop. This then stimulates the body to respond with a second wave of defense—the inflammatory process. In patients with HLH, however, there is an overresponse of inflammation, which results in symptoms such as fevers, rashes, and enlargement of the liver or spleen.
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