The Leprosy Continuum
Leprosy exists in several different forms, although the infectious agent for all of these forms is M. leprae. Host factors such as genetic make up, individual immunity, geography, ethnicity, and socioeconomic circumstances determine which form of leprosy is contracted by a person exposed to M. leprae. Interestingly, most people who come in contact with the bacterium—about three-fourths—never develop leprosy, or develop only a small lesion on the trunk or extremity that heals spontaneously. Most people, then, are not susceptible to M. leprae, and their immune systems function effectively to neutralize the bacteria. But one-fourth of those exposed to M. leprae come down with the disease, which may manifest itself in various ways.
Five forms of leprosy are recognized, and a person may progress from one form to another. The least serious form is tuberculoid leprosy. In this form, the skin lesions and nerve damage are minor. Tuberculoid leprosy is evidence that the body's cellular immune response—the part of the immune system that seeks out and destroys infected cells—is working at a high level of efficiency. Tuberculoid leprosy is easily cured with antibiotics.
If tuberculoid leprosy is not treated promptly, or if a person has a less vigorous cellular immune response to the M. leprae bacteria, the disease may progress to a borderline leprosy, which is characterized by more numerous skins lesions and more serious nerve damage. The most severe form of leprosy is lepromatous leprosy. In this form of leprosy, the skin lesions are numerous and cause the skin to fold, especially the skin on the face. This folding of facial skin leads to the leonine (lion-like) features typical of lepromatous leprosy. Nerve damage is extensive, and people with lepromatous leprosy may lose the feeling in their extremities, such as the fingers and toes. Contrary to popular belief, the fingers and toes of people with this form of leprosy do not spontaneously drop off. Rather, because patients can not feel pain because of nerve damage, the extremities can become easily injured. Sometimes these injuries are severe, and fingers and toes are cut off by sharp objects which the patient cannot even detect.
Lepromatous leprosy occurs in people who exhibit an efficient antibody response to M. leprae but an inefficient cellular immune response. The antibody arm of the immune system is not useful in neutralizing intracellular pathogens such as M. leprae; therefore, people who initially react to invasion by M. leprae by making antibodies may be at risk for developing more severe forms of leprosy. Researchers are not sure what determines whether a person will react with a cellular response or an antibody response; current evidence suggests that the cellular immune response may be controlled by a special gene. If a person has this gene, he or she will probably develop the less severe tuberculoid leprosy if exposed to M. leprae.