Type III (Immune Complex) Hypersensitivity: Mechanism and Examples
Type III hypersensitivity, also known as immune complex-mediated hypersensitivity, is one of the four distinct types of adverse immune reactions that lead to tissue injury. Unlike Type I, which involves IgE, or Type II, which targets antigens fixed to a cell surface, Type III reactions are primarily mediated by the deposition of soluble antigen-antibody complexes. These complexes form when antibodies, typically of the IgG and occasionally IgM classes, bind to soluble antigens that are freely circulating in the blood or body fluids. The resulting clinical disease arises not from the simple presence of the complex, but from its subsequent deposition in various tissues and the destructive inflammatory cascade that follows. This type of reaction can be systemic, affecting multiple organs like in autoimmune diseases, or localized, such as in certain lung or skin reactions.
The Mechanism of Immune Complex Formation and Deposition
The initiation of Type III hypersensitivity requires the formation of immune complexes that are of a specific size and proportion. When antigens and antibodies combine, they form these complexes. Under normal physiological conditions, the body’s phagocytic cells, primarily macrophages in the spleen and liver, efficiently clear these complexes from circulation. A Type III reaction occurs when there is an overwhelming quantity of complexes, often due to a persistent infection, chronic exposure to a foreign antigen, or an autoimmune attack on self-antigens. The key to the pathology lies in the immune complexes forming in slight antigen excess, resulting in small-to-intermediate sized complexes that are not easily cleared by phagocytes and thus persist in the circulation.
These circulating complexes then lodge nonspecifically within the walls of small blood vessels (vasculature). Tissues with active blood filtration and high pressure are particularly susceptible to this deposition, which is why the basement membranes of capillaries in the kidneys (glomerulonephritis), the lining of the joints (arthritis), and the skin are the most common sites of immune complex trapping. Once deposited, the complexes are no longer soluble or circulating, but are fixed in the tissue, setting the stage for the destructive effector phase of the reaction.
The Effector Mechanism of Tissue Injury
Once the immune complexes are trapped on the vascular basement membrane beneath the endothelial cells, they trigger the primary effector mechanism: the activation of the classical complement cascade. The complement system, an essential part of innate immunity, is activated by the IgG or IgM antibodies within the complex. This activation releases potent biologically active fragments, most notably C3a and C5a, which are known as anaphylatoxins and powerful chemoattractants.
Complement fragment C5a is critical, as it attracts a massive influx of inflammatory cells, particularly neutrophils, to the site of immune complex deposition. When these neutrophils arrive, they attempt to engulf the foreign material. Since the immune complexes are physically trapped on the tissue surface, the neutrophils cannot fully phagocytose them, leading to a phenomenon called “frustrated phagocytosis.” Consequently, the neutrophils discharge their lysosomal enzymes, prostaglandins, and reactive oxygen species (free oxygen radicals) into the surrounding tissue—a process of extracellular killing meant for microbes, which inadvertently causes severe, localized tissue destruction, hemorrhage, and vasculitis (inflammation of blood vessels). Furthermore, complement activation can also lead to the formation of the Membrane Attack Complex (MAC), which causes lysis of nearby tissue cells, and the immune complexes can cause platelet aggregation, leading to microthrombi and localized capillary blockage, further contributing to tissue damage and necrosis.
Systemic Examples: Serum Sickness and Autoimmune Diseases
Systemic Type III hypersensitivity occurs when the immune complexes are widely distributed throughout the body via the bloodstream, leading to generalized symptoms. The classic example is **Serum Sickness**, a transient syndrome that typically appears one to two weeks after the administration of a large dose of foreign protein (like non-human antitoxins or certain therapeutic drugs, such as penicillin or sulfonamides). The foreign protein acts as the soluble antigen. Symptoms are systemic and include fever, widespread rash (urticaria or purpura), lymphadenopathy, and arthralgia (joint pain) or arthritis, all resulting from immune complex deposition in blood vessels and joints.
Another major category is the autoimmune disease **Systemic Lupus Erythematosus (SLE)**. In SLE, the body produces autoantibodies against its own nuclear antigens (DNA, histones, etc.). These autoantigens are soluble, and the resulting autoantigen-antibody complexes circulate and are deposited predominantly in the kidneys, skin, and joints. This leads to the characteristic clinical manifestations of lupus, including severe glomerulonephritis (lupus nephritis), arthritis, and various skin lesions, showcasing the chronic, organ-damaging nature of prolonged immune complex deposition.
**Rheumatoid Arthritis** also features a Type III component, where immune complexes containing autoantibodies (like Rheumatoid Factor) are deposited in the synovium of the joints, contributing to the chronic inflammation and joint destruction.
Local Examples and Organ-Specific Conditions
Type III reactions can also be localized. The **Arthus Reaction** is a classic localized response, typically seen as a painful, necrotizing skin lesion that develops within hours at the site of a local injection of an antigen into a sensitized individual (one who already has high levels of circulating antibody). The local immune complex formation causes a severe localized vasculitis, leading to edema and sometimes tissue death.
Infections can also trigger Type III reactions. **Post-Streptococcal Glomerulonephritis (PSGN)** is a kidney disorder that occurs 1–3 weeks after a Group A streptococcal infection (strep throat or impetigo). Antigens from the bacteria circulate and form complexes that become trapped in the kidney glomeruli, causing acute inflammation, which results in symptoms like hematuria (blood in urine), hypertension, and periorbital edema. Similarly, **IgA Nephropathy** is an immune complex-mediated disease where IgA-containing complexes deposit in the kidneys.
Finally, the lung condition **Hypersensitivity Pneumonitis** (formerly known as extrinsic allergic alveolitis) is often a Type III reaction, particularly in its acute phase. Examples include Farmer’s Lung (caused by inhaled fungal spores from moldy hay) and Bird Fancier’s Disease (caused by avian proteins). The repeated inhalation of the antigen leads to the formation and deposition of immune complexes directly within the alveolar walls, resulting in inflammation of the air sacs, fever, and breathing difficulties.