NBME Form 11 Practice Test

Immune complex deposition in tissues defines Type III hypersensitivity. In this type, antigen–antibody complexes form in the circulation and then lodge in various tissues, such as the kidneys, joints, and skin. Once deposited, they activate complement, which draws in neutrophils and promotes inflammation, leading to tissue damage. This pattern underlies conditions like serum sickness, the arthus reaction, and parts of systemic lupus erythematosus.

In contrast, antibodies binding directly to cell-surface or extracellular matrix antigens cause Type II hypersensitivity, leading to cell injury or dysfunction without relying on deposited immune complexes. Type I is the immediate IgE-mediated allergic reaction, and Type IV is a delayed, T-cell–mediated process.