Residency · Residency · Dermatology

Cutaneous Immunology and the Skin Immune System

Overview

The skin is the body's largest immune organ, harboring a complex network of innate and adaptive immune cells collectively termed skin-associated lymphoid tissue (SALT). Understanding cutaneous immunology is foundational to inflammatory, autoimmune, infectious, and neoplastic dermatology — because the immune architecture of the skin determines how it responds to everything from a contact allergen to a melanoma.

Innate Immune Components

Physical and Chemical Barriers

The first line of immune defense is structural. The stratum corneum physically prevents pathogen entry. The acid mantle (pH 4.5 to 5.5) inhibits bacterial colonization. Antimicrobial peptides (AMPs) provide chemical defense: cathelicidin (LL-37) is a broad-spectrum antimicrobial that is upregulated in psoriasis but deficient in atopic dermatitis; human beta-defensins include the constitutively expressed hBD-1 and the inducible hBD-2 and hBD-3; psoriasin (S100A7) is active against E. coli and constitutively expressed; and dermcidin is produced by eccrine sweat glands. Complement activation also occurs at the skin surface.

Keratinocytes as Immune Sentinels

Keratinocytes are far more than passive structural cells. They express pattern recognition receptors (PRRs) that detect danger signals: Toll-like receptors (TLR1 through 6 and TLR9) recognize pathogen-associated molecular patterns such as bacterial lipoproteins, double-stranded RNA, LPS, flagellin, and CpG DNA. NOD-like receptors (NLRs) serve as intracellular sensors, and NLRP3 inflammasome activation is particularly important. RIG-I-like receptors detect cytosolic viral RNA. Upon activation, keratinocytes produce a range of cytokines including IL-1alpha and IL-1beta (via the inflammasome), TNF-alpha, IL-6, and IL-8 (CXCL8). Of particular importance in atopic dermatitis, damaged keratinocytes release the epithelial-derived "alarmins" TSLP, IL-25, and IL-33, which drive type 2 immunity. Under inflammatory conditions (especially IFN-gamma stimulation), keratinocytes can also express MHC class II molecules, allowing them to participate directly in antigen presentation.

Langerhans Cells

Langerhans cells are the principal dendritic cells of the epidermis, comprising 2 to 5 percent of epidermal cells. They derive from fetal liver monocytes and maintain themselves as a self-renewing population. Their pathognomonic ultrastructural feature is the Birbeck granule — a pentalaminar, "tennis racket"-shaped structure visible on electron microscopy, created by the protein langerin (CD207). Functionally, Langerhans cells capture and process antigens in the epidermis, migrate to draining lymph nodes via afferent lymphatics, and present antigens to naive T cells with cross-presentation capability. Depending on the context, they can play either immunogenic or tolerogenic roles. UV radiation depletes Langerhans cells, which contributes to UV-induced immunosuppression. In Langerhans cell histiocytosis, a clonal neoplastic proliferation of these cells occurs, with BRAF V600E mutations found in over 50 percent of cases.

Dermal Dendritic Cells

The dermis contains multiple dendritic cell subsets. Conventional type 1 DCs (cDC1, CD141+) specialize in cross-presentation and are important for anti-tumor immunity. Conventional type 2 DCs (cDC2, CD1c+) promote Th2 and Th17 responses. Plasmacytoid DCs (pDCs) are the major source of type I interferons (IFN-alpha) and are critical in antiviral defense and lupus pathogenesis. Inflammatory dendritic epidermal cells (IDECs), which express the high-affinity IgE receptor FcepsilonRI, are found in atopic dermatitis lesional skin.

Mast Cells

Mast cells reside in the dermis, particularly around blood vessels. They express the high-affinity IgE receptor (FcepsilonRI), and their degranulation releases preformed mediators — histamine, tryptase, and TNF-alpha — while they also synthesize prostaglandins, leukotrienes, and cytokines de novo. Mast cells play a central role in urticaria, anaphylaxis, and mastocytosis. Serum mast cell tryptase serves as a marker for systemic mastocytosis and anaphylaxis.

Innate Lymphoid Cells (ILCs)

ILCs are the "innate counterparts" of T helper subsets, lacking antigen-specific receptors but producing the same signature cytokines. | ILC Subset | Signature Cytokines | T Helper Parallel | Key Disease Associations |

ILC1 cells produce IFN-gamma (paralleling Th1). ILC2 cells produce IL-5 and IL-13 (paralleling Th2) and are expanded in AD skin, where they respond to the alarmins TSLP, IL-25, and IL-33. ILC3 cells produce IL-17 and IL-22 (paralleling Th17) and play roles in psoriasis and mucosal defense. ILCs contribute to early inflammatory responses before adaptive immunity has time to engage.

Adaptive Immune Components

T Cells in the Skin

The skin contains approximately 20 billion T cells — twice the number circulating in the blood. The majority are tissue-resident memory T cells (TRM), which express CD103 (binding E-cadherin on keratinocytes) and CD69 on CD8+ cells, and provide rapid local immune surveillance without needing to recirculate. TRM cells are critical for defense against HSV recurrence, underlie fixed drug eruptions (recurring at the same site because TRM cells persist there), and may drive recurrent psoriasis at previously affected sites.

Circulating T cell subsets that infiltrate skin include Th1 cells (producing IFN-gamma and TNF-alpha, driving contact dermatitis and lichen planus), Th2 cells (producing IL-4, IL-5, and IL-13, driving atopic dermatitis), Th17 cells (producing IL-17A, IL-17F, and IL-22, driving psoriasis), Th22 cells (producing IL-22, promoting epidermal hyperplasia), regulatory T cells (FOXP3+, maintaining immune tolerance, enriched in hair follicles), and follicular helper T cells (supporting B cell responses). T cells home to the skin by expressing cutaneous lymphocyte antigen (CLA) and chemokine receptors CCR4 and CCR10.

Gamma-Delta T Cells

In mice, dendritic epidermal T cells (DETCs) are well-characterized gamma-delta T cells, though no exact human equivalent exists. Human gamma-delta T cells reside in the dermis and contribute to wound healing and antimicrobial defense, producing both IL-17 and IFN-gamma.

B Cells and Immunoglobulins

B cells are not normally abundant in the skin but infiltrate in specific conditions such as cutaneous B cell lymphoma, lupus erythematosus, and Borrelia-associated lymphocytoma. Locally produced antibodies are important in autoimmune blistering diseases. In atopic dermatitis, IgE is bound to mast cells and to FcepsilonRI on Langerhans cells, amplifying the allergic response.

Skin-Associated Lymphoid Tissue (SALT)

Concept

The SALT concept, proposed by Streilein in 1983, holds that the skin functions as an integrated immune organ whose components — keratinocytes (sentinels and cytokine sources), Langerhans cells and dermal DCs (antigen presentation), T cells (effector and regulatory functions), dermal macrophages, endothelial cells (trafficking via adhesion molecules E-selectin, ICAM-1, and VCAM-1), and draining lymph nodes — work as a coordinated system. Lymphocyte homing to the skin requires CLA binding to E-selectin on dermal postcapillary venules.

Immune Surveillance and Tolerance

The skin constantly samples environmental antigens, maintaining a delicate balance between immunity (against pathogens and tumors) and tolerance (to commensals and self-antigens). When tolerance breaks down, autoimmune skin diseases result. UV-induced immunosuppression — which depletes Langerhans cells, induces regulatory T cells, and shifts immune responses from Th1 to Th2 — helps explain both the therapeutic benefit of phototherapy in inflammatory dermatoses and the increased risk of UV-induced carcinogenesis.

Immunopathology Patterns in Dermatologic Disease

Type 2 Inflammation (Th2/ILC2)

Atopic dermatitis is the paradigmatic type 2 inflammatory skin disease, driven by IL-4, IL-13, IL-31 (the key pruritogen), and TSLP. Therapeutic targets include dupilumab (anti-IL-4Ralpha, blocking both IL-4 and IL-13), tralokinumab (anti-IL-13), and nemolizumab (anti-IL-31 receptor).

Type 17 Inflammation (Th17/ILC3)

Psoriasis is driven by the IL-23/Th17 axis, where IL-23 drives Th17 differentiation and IL-17A/F and IL-22 are the effector cytokines. Therapeutic targets include secukinumab and ixekizumab (anti-IL-17A), and guselkumab and risankizumab (anti-IL-23).

Type 1 Inflammation (Th1)

Contact dermatitis (in its effector phase) and lichen planus are Th1-mediated, driven by IFN-gamma and TNF-alpha. Cytotoxic CD8+ T cells mediate the interface dermatitis characteristic of these conditions.

Mixed/Complex

Some diseases involve more complex immune patterns. Lupus erythematosus is driven by type I interferon (via pDC activation) with Th1 contribution. Dermatomyositis shares a type I interferon signature. Drug reactions like SJS/TEN involve cytotoxic T cells and granulysin.

<image>Comprehensive illustration of the skin immune system showing the epidermis with Langerhans cells extending dendrites between keratinocytes, the dermis with dermal dendritic cells, mast cells around blood vessels, tissue-resident memory T cells, and innate lymphoid cells. Show the afferent lymphatic vessel carrying antigen-loaded dendritic cells to the draining lymph node. Label all cell types and include key surface markers (CD207 on Langerhans cells, CLA on T cells, FcepsilonRI on mast cells). Use a cross-sectional skin diagram as the framework.</image>

<image>Diagram comparing the three major immunopathologic patterns in inflammatory skin disease: Type 2 (atopic dermatitis) showing Th2 cells, ILC2, IL-4/IL-13/TSLP, spongiotic epidermis with eosinophils; Type 17 (psoriasis) showing Th17 cells, IL-23/IL-17/IL-22 axis, acanthotic epidermis with neutrophilic microabscesses; and Type 1 (contact dermatitis/lichen planus) showing Th1/CD8+ cells, IFN-gamma, interface dermatitis with basal cell damage. Use side-by-side panels with color-coded cytokine networks.</image>

<image>Illustration of the antigen presentation pathway in skin: a pathogen breaches the stratum corneum, is captured by a Langerhans cell via pattern recognition receptors, processed internally, and presented on MHC class II. The activated Langerhans cell migrates through the dermis into the afferent lymphatic, arriving at the paracortical T cell zone of the draining lymph node where it presents antigen to naive T cells, driving their differentiation into Th1, Th2, or Th17 effector cells that then home back to skin via CLA and CCR4/CCR10.</image>

Clinical Pearls

The skin contains twice as many T cells as the peripheral blood (approximately 20 billion), and most are tissue-resident memory T cells providing local surveillance. TSLP, IL-25, and IL-33 are "alarmins" released by damaged keratinocytes that drive type 2 immunity; TSLP is now a therapeutic target (tezepelumab in asthma, with exploration in AD). Plasmacytoid dendritic cells are the primary source of type I interferons and are pathogenically important in lupus erythematosus, where their activation by self-DNA/LL-37 complexes drives the interferon signature. LL-37 (cathelicidin) plays a dual role: antimicrobial defense and driving autoimmunity in psoriasis, where it forms complexes with self-DNA that activate pDCs via TLR9. UV radiation is immunosuppressive — it depletes Langerhans cells, induces regulatory T cells, and isomerizes urocanic acid to the immunosuppressive cis form — explaining both the therapeutic benefit of phototherapy and the risk of UV-induced carcinogenesis. JAK inhibitors work across multiple inflammatory dermatoses because JAK-STAT signaling mediates the downstream effects of many key cytokines, including IL-4, IL-13, IL-23, IFN-gamma, and IL-6.

References

• Clark RA, et al. The vast majority of CLA+ T cells are resident in normal skin. J Immunol. 2006;176(7):4431-4439.
• Kabashima K, Honda T, Ginhoux F, Egawa G. The immunological anatomy of the skin. Nat Rev Immunol. 2019;19(1):19-30.
• Nestle FO, Di Meglio P, Qin JZ, Nickoloff BJ. Skin immune sentinels in health and disease. Nat Rev Immunol. 2009;9(10):679-691.
• Streilein JW. Skin-associated lymphoid tissues (SALT): origins and functions. J Invest Dermatol. 1983;80(Suppl):12s-16s.
• Werfel T, et al. Cellular and molecular immunologic mechanisms in patients with atopic dermatitis. J Allergy Clin Immunol. 2016;138(2):336-349.

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