Classification and Disease Associations
IgA nephropathy is a member of the broader group of mesangioproliferative glomerulonephritides. It is not a standalone disease entity, but rather a collective term grouping pathologies with a shared morphological pattern. Alongside Berger disease, this group includes prolonged primary acute glomerulonephritis, the mesangial form of systemic lupus erythematosus (SLE), infective endocarditis, rheumatoid arthritis, hepatitis A, IgM nephropathy, and several other conditions.
Typically, Berger disease presents as isolated renal involvement. However, it shares many features with Henoch-Schönlein purpura (an IgA-mediated systemic vasculitis more common in children). Additionally, secondary forms of the disease can develop in association with underlying hepatic and intestinal disorders.
Pathogenesis: How the Disease Develops
Under normal physiological conditions, immunoglobulin A is the primary protective antibody of mucosal secretions. It is present in low concentrations in the serum, predominantly as monomers, while polymeric forms are rapidly catabolized by the liver.
In IgA nephropathy, this physiological balance is disrupted:
- Serum levels of polymeric IgA₁ rise sharply (while IgA₂ levels remain unchanged).
- Circulating immune complexes (CIC) containing this abnormal IgA₁ form in a subset of patients.
- These complexes are trapped and deposited within the glomerular mesangium.
- The complement system is activated via the alternative pathway, as evidenced by the presence of the C₃ fraction and properdin in the glomeruli, in the complete absence of C₁q and C₄ components.
The underlying etiology is believed to stem from congenital or acquired immune regulation abnormalities. In response to environmental triggers (viruses, bacteria, dietary antigens) entering via the respiratory or gastrointestinal tracts, mucosal tissues overproduce IgA.
Morphology and Diagnosis
Light microscopy alone is insufficient for a definitive diagnosis, as biopsy findings under light microscopy only allow for a clinical suspicion. The gold standard for verification is immunohistochemistry or immunofluorescence.
- Light Microscopy: The hallmark feature is widening of the mesangial matrix due to mesangial cell proliferation. Changes may be diffuse (classic mesangioproliferative glomerulonephritis) or segmental (focal proliferative glomerulonephritis, involving only a subset of glomeruli, with occasional focal necrosis and fibrin deposition). Crescentic glomerulonephritis is significantly less common. Over time, scarring of focal lesions can lead to focal segmental glomerulosclerosis.
- Immunofluorescence Microscopy: Characteristic staining reveals prominent IgA deposits within the mesangial matrix. These are frequently accompanied by the C₃ complement fraction and properdin, and occasionally small amounts of IgG or IgM. Other complement components are typically absent.
- Electron Microscopy: High-magnification ultrastructural analysis (e.g., ×18,000) reveals dark, electron-dense deposits in the mesangium, corresponding to aggregated immune complexes.
Clinical Manifestations and Prognosis
The leading clinical symptom of Berger disease is recurrent gross hematuria or microscopic hematuria (blood in the urine). Proteinuria is usually mild. Nephrotic syndrome can occur but is not a mandatory feature.
In most cases, the disease follows a relatively stable clinical course, though it can rarely progress to a severe complication: rapidly progressive glomerulonephritis (RPGN).