Pathophysiology of Dermatological and Immune Disorders

A primary example of this mechanism is Systemic Lupus Erythematosus (SLE). In SLE, the deposition of these complexes can lead to severe organ damage, most notably lupus nephritis, which manifests clinically through hematuria (blood in the urine) and proteinuria (protein in the urine).

Type IV: Delayed-Type Hypersensitivity

Unlike the first three types, Type IV is T-cell-mediated rather than antibody-mediated. The reaction is "delayed" because it takes time for T-cells to migrate to the site of the antigen and initiate an inflammatory response. A common manifestation is contact dermatitis, such as the skin irritation experienced after touching poison ivy.

Exam Tip: To differentiate hypersensitivities, focus on the mediator. If it's IgE, think Type I; if it's antibodies attacking a cell, think Type II; if it's complexes depositing in tissue, think Type III; and if it's T-cells, think Type IV.

Dermatological Pathophysiology

Skin disorders often serve as visible windows into complex underlying immunological or vascular processes.

Psoriasis and Psoriatic Arthritis

Psoriasis is not merely a skin condition but a systemic inflammatory disease driven by T-cell-mediated autoimmune inflammation. Specifically, the Th1 and Th17 pathways play central roles in the disease process. This immune activation triggers the accelerated hyperproliferation of keratinocytes, which results in the characteristic erythematous (red) plaques covered with silvery scales.

Because psoriasis is a systemic inflammatory state, patients face increased risks for comorbidities, including metabolic syndrome, obesity, and cardiovascular disease. Furthermore, many patients develop psoriatic arthritis, a seronegative inflammatory arthritis that frequently targets the distal interphalangeal (DIP) joints.

Rosacea

Rosacea is characterized by chronic inflammation and vascular dysregulation. The pathophysiology involves persistent vasodilation, which leads to the clinical presentation of facial flushing and the development of permanent telangiectasias (visible small blood vessels).

Malignant Melanoma

Malignant melanoma arises from the uncontrolled, malignant proliferation of melanocytes. This type of cancer has a high potential for metastasis. Clinicians use the ABCDE criteria to assess the risk of melanoma:

• Asymmetry
• Border irregularity
• Color variation
• Diameter (increasing)
• Evolving

Risk factors include fair skin and intermittent, intense exposure to UV radiation.

Immune Suppression and Deficiency

While hypersensitivity involves an overactive immune system, other conditions are defined by the failure or depletion of critical immune components.

HIV and CD4 T-cell Destruction

The pathophysiology of Human Immunodeficiency Virus (HIV) is centered on the progressive destruction of CD4 T-helper cells. Since CD4 cells are the "conductors" of the immune response, their depletion severely impairs cell-mediated immunity, leaving the body vulnerable to opportunistic infections.

Neutropenia

Neutropenia is a deficiency in neutrophils, the primary white blood cells responsible for fighting bacterial infections. This condition is frequently seen as a side effect of chemotherapy. A significant reduction in neutrophil count drastically increases a patient's susceptibility to acute bacterial infections.

Summary Table: Comparison of Hypersensitivity Types