Little is known about the relationship between immunology and skin appearance; however, it is certain that the immune system plays an important role in the health of the skin. Work is ongoing to help elucidate how this vital system interacts with the largest organ of the body. It is very likely that this segment of research, as it pertains to the cosmetic dermatology arena, will offer significant potential for discovery of new therapeutics and procedures in the next several years. This chapter will serve as a brief introduction to the skin as an immune organ and how the immune response plays a role in cosmetic dermatology.
In the past, the skin was viewed primarily as a barrier mechanism to prevent invading pathogens and other environmental toxins, including UV radiation, from penetrating into internal organs. However, we now know that the skin essentially acts as an immense and integral immune organ and first point of contact with the environment, capable of initiating an intricate series of events leading to host defense. A basic review of skin immunology, including the role of cytokines and growth factors, will be provided as an important part of this discussion. Mechanisms of various immune responses found in skin disease, the interplay between innate immunity and extracellular matrix synthesis, as well as emerging immune-based treatments will also be highlighted. Finally, the relevance of the local immune system and its relationship to skin aging, particularly photoaging, will be briefly reviewed.
SKIN—AN INNATE IMMUNE ORGAN
The immune response can be divided into innate and adaptive immunity. Innate immune response occurs rapidly and the cells of the innate immune system use pattern recognition receptors (PRRs) to secrete soluble factors that can lead to both inflammation and host defense. The adaptive immune response, on the other hand, occurs slowly and activation of adaptive immune cells, such as B and T cells, requires that receptors undergo gene rearrangements. The adaptive immune system can mount either humoral immunity (B cells, which make antibodies) or cell-mediated immunity (T cells). Furthermore, the adaptive immune system is also responsible for immune memory, which confers long-term protection to the host. Although the two systems appear distinct, they are not separate, and in fact can act synergistically, insofar as the innate immune system instructs the adaptive immune response and the adaptive immune system influences the innate system.
In the epidermis, the two main innate cells are the keratinocytes and Langerhans cells. In addition, neutrophils, macrophages, and dendritic cells present within the dermis also play a role in innate immunity. When a foreign substance is encountered, activation of innate cells occurs through PRRs, including the Toll-like receptors (TLRs), which are reviewed below. Upon activation, the innate cells become capable of inducing a direct antimicrobial response by producing factors that can help protect the host from external insults. These factors include reactive oxygen and nitrogen intermediates (also known as ...