Farmer's Lung: Gell and Coombs Type-III Classification

Farmer's lung is an example of an immune-mediated disease affecting the lungs, often due to exposure to organic dusts like mold spores found in hay or straw. The Gell and Coombs classification system categorizes hypersensitivity reactions into four main types based on the underlying immunological mechanisms.

Understanding Hypersensitivity Types

The Gell and Coombs classification describes different ways the immune system can overreact:

• Type I: Immediate reactions mediated by IgE antibodies.
• Type II: Cytotoxic reactions involving IgG or IgM antibodies binding to cells.
• Type III: Immune complex reactions where antigen-antibody complexes deposit in tissues.
• Type IV: Delayed reactions mediated by T-cells.

Farmer's Lung Mechanism

In Farmer's lung:

• Inhaled antigens (e.g., thermophilic actinomycetes from moldy hay) trigger an immune response.
• Antibodies (primarily IgG) are produced against these antigens.
• These antibodies bind to the antigens, forming circulating immune complexes.
• These immune complexes deposit in the small airways and alveoli of the lungs.
• The deposition of immune complexes activates the complement system and attracts inflammatory cells (like neutrophils).
• This leads to inflammation and lung tissue damage, characteristic of an immune complex disease.

Classification Determination

Because Farmer's lung is caused by the deposition of antigen-antibody complexes leading to inflammation, it fits the definition of a Type-III hypersensitivity reaction according to the Gell and Coombs classification.