Allergic Contact Dermatitis
This is a Type IV (delayed) hypersensitivity reaction. It occurs when the skin is exposed to an allergen to which the individual has been previously sensitized. T cells recognize the allergen, triggering a local inflammatory response that typically manifests hours to days after exposure. Common signs include redness, swelling, and sometimes oozing or crusting.
Irritant Contact Dermatitis
Unlike the allergic type, irritant contact dermatitis is a nonimmune reaction. It is caused by direct physical or chemical damage to the skin from substances like soaps, detergents, or solvents. Management focuses on identifying and avoiding the irritant while protecting the skin barrier.
Stasis Dermatitis
Stasis dermatitis is an inflammatory condition caused by chronic venous insufficiency, most commonly affecting the lower legs. Venous pooling leads to increased pressure, edema, and inflammation. A hallmark of this condition is brown skin discoloration caused by hemosiderin deposition.
Treatment involves improving venous return through compression therapy (e.g., stockings), leg elevation, and the use of topical corticosteroids for inflammation.
Seborrheic Dermatitis
This chronic condition affects areas rich in sebaceous (oil) glands, such as the scalp, eyebrows, and nasolabial folds. It is characterized by greasy, white-to-yellow scales. In infants, this condition is known as cradle cap.
Treatment often involves antifungal therapies (such as ketoconazole) to decrease Malassezia yeast, or topical corticosteroids for short-term inflammation control.
Chronic and Immune-Mediated Disorders
Psoriasis
Psoriasis is a chronic, relapsing, immune-mediated disorder driven by T-cell activation. This activation causes an extreme acceleration in epidermal cell turnover; instead of the normal ~28-day cycle, cells turn over in just 3–5 days. Because cells do not have time to mature and shed, they accumulate to form well-defined, raised plaques with silvery-white scales.
Treatment varies by severity:
- Mild disease: Topical corticosteroids and vitamin D analogs (e.g., calcipotriene).
- Moderate to severe disease: Systemic therapies such as methotrexate, cyclosporine, or biologics (e.g., TNF or IL-17 inhibitors).
Clinical Note: When prescribing Acitretin for psoriasis, patients must be warned that it is highly teratogenic; pregnancy must be avoided during treatment and for 3 years after stopping the medication.
Pityriasis Rosea
This is a benign, self-limiting disorder. It often begins with a single, large, oval, salmon-pink scaly lesion known as a "herald patch". This is followed days or weeks later by a widespread rash on the trunk that follows skin lines in a characteristic "Christmas tree" pattern.
Lichen Planus
Lichen planus is a chronic inflammatory disorder thought to involve a T-cell-mediated immune response. It is clinically recognized by intensely pruritic, purple/violet, flat-topped papules. It commonly affects the wrists, ankles, and genital area, and can also involve the mucous membranes of the mouth.
Rosacea
Rosacea is a chronic inflammatory disorder primarily affecting the central face of adults. It is characterized by facial flushing, persistent erythema, and visible telangiectasias (small, dilated blood vessels). In some cases, chronic inflammation can lead to rhinophyma, a thickening of the skin on the nose.
Management includes topical metronidazole or azelaic acid, and systemic doxycycline for moderate to severe cases.
Autoimmune and Acute Immune-Mediated Conditions
Lupus Erythematosus
Lupus is an autoimmune disease that frequently manifests in the skin, often exacerbated by UV exposure (photosensitivity).
- Discoid Lupus Erythematosus (DLE): Primarily affects the skin with red, scaly, disk-shaped lesions that can cause scarring.
- Systemic Lupus Erythematosus (SLE): A systemic disease that can affect multiple organs. A classic skin manifestation is the malar ("butterfly") rash across the cheeks and bridge of the nose.
Treatment includes sun protection, topical corticosteroids, and systemic medications like hydroxychloroquine.
Erythema Multiforme
This is an acute, immune-mediated disorder often triggered by infections, most notably the herpes simplex virus (HSV). It is characterized by distinctive "target" or "bull's-eye" lesions that often appear symmetrically on the extremities. While usually self-limiting, it may require antiviral therapy if HSV is the trigger.
