This book reviews the role of each cell subset in the skin, providing the basics for understanding skin immunology and the mechanisms of skin diseases. The skin is one of the immune organs and is continually exposed to foreign antigens and external stimuli that must be monitored and characterized for possible elimination. Upon exposure to foreign antigens, the skin can elicit a variety of immune responses in harmony with skin components that include keratinocytes, dendritic cell subsets, mast cells, basophils, fibroblasts, macrophages, gamma-delta T cells, neutrophils, myeloid-derived suppressor cells, vascular and lymphatic cells, hair follicles, platelets, and adipose tissues, among others. In the past 10 years, knowledge of immunology has expanded drastically in areas such as innate immunity (Toll-like receptors, C-type lectins), and host defenses to bacteria and viruses, and this increased knowledge has led to the development of more effective treatment of psoriasis and other skin diseases. This book provides updates on the mechanisms of skin diseases including contact dermatitis, atopic dermatitis, psoriasis, urticaria, drug eruption, bullous diseases, anaphylaxis, graft-versus-host disease, rosacea, lymphoma, photodermatology, and collagen vascular diseases. Understanding the basics of skin immunology will help clinicians and dermatologists use new therapeutics such as biologics efficiently. Serving as an intermediary between basic science and clinical medicine, this book gives readers the opportunity to understand and marvel at the mystery and fascination of skin immunology.