Immunology has roughly been split into two major subdivisions: cellular immunity, which comprises all different immunocompetent cells, and humoral immunity, a collection of all molecules involved in immune processes, such as cytokines, immunoglobulins, complement factors, and many others. Serum studies revealed the existence of various functional factors that were able to immobilize bacteria (immobilisins) or to precipitate bacterial components (precipitins). We now know that these factors are specific subsets of antibodies. Knowledge of both immune response–related cells and molecules expanded during many decades, and at the time immunology was recognized as a separate scientific entity, the subdivision into cellular and humoral immunity was generally accepted. One branch is innate or natural immunity, which encompasses a variety of natural defense mechanisms, exemplified by phagocytosis and intracellular killing of microorganisms. That mechanism is greatly enhanced by binding of specific antibodies to surface structures of these pathogens (opsonization). The other major branch is adaptive or acquired immunity that deals with specific immune responses in which specificity for antigens is represented by T-cell receptors (TCRs), B-cell receptors (BCRs), and immunoglobulins secreted by plasma cells.
[...] Adaptive Immunity and the Skin Immune System In vivo evidence for the presence of acquired immunity in SIS is exemplified by the concept of immunosurveillance. When acquired immunity mechanisms are inhibited, as occurs in iatrogenic immunosuppression in transplant patients and in patients with HIV-induced T-cell loss, malignancies occur more frequently. Viral and tumor escape mechanisms are enhanced by diminished immune responses, and oncogenic viruses related to tumors as well as other malignancies arise uncontrolled. As compared to the physiology of acquired immunity responses in normal human skin, the pathology that results from ill-directed acquired immune responses in human skin is more familiar and better studied. [...]
[...] Innate Immunity and the Skin Immune System A variety of physical, humoral, and cellular elements together form the innate part of SIS. The general concept is that PRRs, which can be regarded as any receptor recognizing pathogen-associated molecular patterns (PAMPs) and capable of triggering antimicrobial function in leukocytes, are present on cell membranes or located within the cell. The PAMPs are defined as highly conserved structures of common pathogens that are essential for microbe survival and include, for example, components of bacterial cell walls such as lipopolysaccharide, peptidoglycan and lipoteichoic acid, fungal cell-wallcomponent zymosan, and viral double-stranded RNA. [...]
[...] In this process, the activation of immature dermal dendritic cells or epidermal Langerhans' cells can be further enhanced by factors of the innate subsystem, such as proinflammatory cytokines TNF-α and IL-1α, which are produced by keratinocytes upon contact with pathogens. The immature skin resident dendritic cells become activated when they take up (foreign) antigens from the skin environment and they then undergo maturation as they migrate to the skindraining lymph nodes to initiate adaptive immune responses. Maturation includes the downregulation of phagocytosis activity and the upregulation of human leukocyte antigen (HLA) expression and of co-stimulatory molecules, which are required for efficient priming of effector T lymphocytes. [...]
[...] Innate and Adaptive Compartments Linked by Dendritic Cells Innate or natural immunity is evolutionarily old, and it is the only defense system in invertebrates. In vertebrates, an adaptive or acquired system of immune responses has developed. Innate immunity of the integument is represented by a number of physical, biologic, and cellular factors. Essentially, innate immunity instantly responds to microorganisms and thus serves as an instantaneous defense mechanism against invading pathogens. The adaptive subsystem, however, requires several days to develop and is primarily characterized by its specificity, which is mediated by TCRs, BCRs, and immunoglobulins. [...]
[...] Neutrophils form part of the recruited compartment of innate immunity, and they can phagocytose particular matter. Eosinophils may also phagocytose and are classically known to play a role in the expulsion of parasitic infestations from the gut. Basophils have many similarities with mast cells. It may be argued that mast cells and basophils that bear Fcε receptors, and thus can bind antigenspecific immunoglobulin E have a role in acquired immunity, such as in IgE-mediated allergic inflammation and immediate (cutaneous) hypersensitivity. [...]
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