As the name implies, photoimmunology is the study of the effects of photons on the immune system. As a scientific discipline, it is only a little more than 10 years old, and thus it represents one of the more recent subspecialties in photobiology. Photoimmunology combines certain elements of three scientific disciplines, namely photobiology, immunology, and dermatology. Dermatology is included because all of the known effects of photons on the immune system are initiated in the skin, either from a direct effect of UV radiation on immune cells within the skin or by means of indirect effects of UV radiation on other components of the skin that secondarily affect immune cells. The photons of most interest in photoimmunology are those with wavelengths in the UV region of the spectrum, particularly in the UV-B (28c320 nm) range. Wavelengths in the UV-C (20@ 280 nm) range are also immunologically active, but they have received much less attention because of their absence from natural sunlight. The second component of photoimmunology, the immune system, is the primary defense mechanism that protects the body against infectious disease. Its importance in this regard is easily visualized by considering the unfortunate fate of people who lack one or more components of the immune system, such as those with AIDS or other immunodeficiency diseases. The immune system is a complex, interactive system of checks and balances designed to rid the body of foreign substances and maintain homeostasis.