Louis DN, Perry A, Reifenberger G, et al: The 2016 World Health Organization Classification of Tumors of the Central Nervous System: a summary, Acta Neuropathol 131(6):803–820, 2016. [A concise summary of the most recent classification of brain tumors, including the concept of “layer” diagnoses that incorporate histologic findings as well as multiple types of molecular information in order to predict course and guide therapy.] http://ebooksmedicine.net studentconsult.com
Skin diseases are common and diverse, ranging from irritating acne to life-threatening melanoma. Many are intrinsic to the skin, but some are manifestations of diseases involving many tissues, such as systemic lupus erythematosus or genetic syndromes such as neurofibromatosis. In this sense, the skin is a uniquely accessible “window” through which numerous disorders can be recognized.
Skin is not a mere protective mantle but rather a complex organ, actually the largest in the body. It is constantly exposed to microbial and nonmicrobial antigens from the environment. Given this, it is not surprising that the skin is an active participant in immune responses. Environmentally derived antigens are processed by intraepithelial Langerhans cells, which bear their antigenic cargo to regional lymph nodes and initiate immune responses. Squamous cells (keratinocytes) help maintain skin homeostasis by providing a physical barrier to environmental insults and by secreting a plethora of cytokines that influence both the squamous and dermal microenvironments. The dermis contains resident populations of CD4+ helper and CD8+ cytotoxic T lymphocytes, some of which home to the skin by virtue of specialized receptors such as cutaneous lymphocyte antigen, as well as memory T cells, regulatory T cells (Tregs) and occasional B cells. The epidermis contains intraepithelial lymphocytes, including γ/δ T cells, while the dermis contains perivascular mast cells and scattered macrophages, all components of the innate immune system. Responses involving these immune cells and locally released cytokines account for the morphologic patterns and clinical expressions of inflammatory and infectious skin disorders.
This chapter focuses on common and pathogenically illustrative skin diseases. In considering these diseases, it is important to appreciate that the practice of
The contributions of Dr. George Murphy to this chapter in previous editions are gratefully acknowledged.
dermatopathology relies on close interactions with clinicians, particularly dermatologists, as the clinical history, gross appearance, and distribution of lesions are often as important as the microscopic findings in arriving at a specific diagnosis. Diseases of the skin can be confusing for the student, in part because dermatologists and dermatopathologists communicate using a large, “skin-specific” lexicon that students must become familiar with in order to understand skin diseases. The most important of these terms and definitions are listed in
Table 24.1
Thousands of inflammatory dermatoses exist, challenging the diagnostic acumen of even experienced clinicians. In general, acute lesions, defined as days to several weeks in duration, are characterized by inflammation, edema, and sometimes epidermal, vascular, or subcutaneous injury. Acute dermatoses are often marked by infiltrates consisting of mononuclear cells rather than neutrophils, unlike acute inflammatory disorders at most other sites. Some acute lesions may persist, transitioning to a chronic phase, while others are self-limited.
Urticaria (“hives”) is a common disorder mediated by localized mast cell degranulation, which leads to dermal microvascular hyperpermeability. The resulting erythematous, edematous, and pruritic plaques are termed wheals.
In most cases, urticaria stems from an immediate (type 1) hypersensitivity reaction (Chapter 5), in which antigens trigger mast cell degranulation by binding to http://ebooksmedicine.net
Table 24.1 Nomenclature of Skin Lesions