Principles of the innate and adaptive immune systems

The skin as an immune organ

The skin functions as an active immune organ, not merely a passive physical barrier - both innate (keratinocytes, Langerhans cells, physical/chemical barrier) and adaptive (resident and recruited T cells) immune components operate within the skin to provide surveillance and defence
Understanding cutaneous immune function underpins the pathogenesis of essentially every inflammatory, infective, and immune-mediated dermatological condition

Innate: the stratum corneum provides a physical barrier; keratinocytes themselves produce antimicrobial peptides and pro-inflammatory cytokines in response to injury/infection; Langerhans cells (epidermal dendritic cells) capture antigen and migrate to lymph nodes to initiate adaptive responses
Adaptive: skin-resident memory T cells provide localised, rapid-response immune surveillance without needing to be recruited fresh from the circulation each time - a mechanism relevant to why some skin infections/inflammatory conditions recur at the same anatomical site

Not directly tested clinically; understanding is inferred from disease patterns and treatment response (e.g. a T cell-targeted therapy's efficacy supports the adaptive immune contribution to a given disease)
This framework explains the mechanism of action of most dermatological immunomodulatory treatments - topical corticosteroids and calcineurin inhibitors dampen local T cell activity; phototherapy modulates Langerhans cell function and induces local immunosuppression; biologics target specific cytokines produced by this cutaneous immune network (e.g. IL-17/IL-23 in psoriasis, IL-4/IL-13 in atopic dermatitis)

Psoriasis (Th17-driven), atopic dermatitis (Th2-driven), contact dermatitis (a classic delayed-type/Th1 hypersensitivity reaction), skin-resident memory T cells and disease recurrence at the same site

Recognising the skin as an active immune organ (not just a barrier) explains why topical and systemic immunomodulatory treatments work, and why understanding the dominant immune pathway in a given dermatosis (Th2 in atopic dermatitis, Th17 in psoriasis) increasingly determines which targeted biologic is selected

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