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Immunoglobulin A (IgA)

SI UNITS (recommended)

CONVENTIONAL UNITS

Synonyms

  • IgA
  • Immunoglobulin A
  • Serum IgA
  • Secretory IgA (sIgA) – mucosal form
  • Total IgA
  • α-heavy chain immunoglobulin

Units of Measurement

  • µmol/L
  • g/L
  • mg/dL
  • mg/100 mL
  • mg%
  • mg/mL

Molecular Weight

Serum IgA (monomeric): ~160 kDa
 Secretory IgA (dimeric): ~385 kDa
 Reference ranges use total serum IgA (monomeric).

Key Conversions

Using MW ~160,000 g/mol for IgA:

1 g/L ≈ 6.25 µmol/L
 1 µmol/L ≈ 0.16 g/L
 1 mg/dL = 0.01 g/L
 mg% = mg/dL = mg/100 mL
 1 mg/mL = 1 g/dL = 10 g/L

Description

Immunoglobulin A (IgA) is the second most abundant serum immunoglobulin and the dominant antibody of mucosal surfaces, including:

  • GI tract
  • Respiratory tract
  • Urogenital tract
  • Saliva
  • Tears
  • Breast milk

IgA exists in two major forms:

  • Serum IgA → monomeric
  • Secretory IgA (sIgA) → dimeric with a secretory component

IgA plays a central role in mucosal immunity, protecting epithelial barriers against infections.

Physiological Role

1. Mucosal Immunity

Secretory IgA prevents:

  • Pathogen adherence
  • Toxin absorption
  • Viral entry

2. Neutralization

Binds and neutralizes:

  • Bacteria
  • Viruses
  • Toxins

3. Non-inflammatory Protection

IgA protects mucosa without activating complement, avoiding tissue damage.

4. Immune Regulation

Interacts with microbiome and maintains gut immune balance.

Clinical Significance

High IgA (Hyper-IgA)

1. Chronic Infections

  • Respiratory infections
  • GI infections
  • Chronic sinusitis

2. Autoimmune Diseases

  • Rheumatoid arthritis
  • Systemic lupus erythematosus (SLE)
  • IgA vasculitis (Henoch–Schönlein purpura)

3. Liver Disease

  • Alcoholic liver disease (classic high IgA)
  • Cirrhosis
  • Portal hypertension

4. Monoclonal Gammopathies

  • IgA myeloma (marked elevation)
  • MGUS (IgA type)

5. Inflammatory Bowel Disease

Especially ulcerative colitis.

Low IgA (Hypo-IgA)

1. Selective IgA Deficiency (sIgAD) - most important

Most common primary immunodeficiency.

Diagnostic criteria:

  • Serum IgA <0.07 g/L (<7 mg/dL)
  • Normal IgG and IgM
  • Age >4 years

Clinical features:

  • Recurrent respiratory infections
  • Recurrent GI infections
  • Allergies
  • Autoimmune diseases (celiac, thyroid disease)
  • Risk of anaphylaxis to blood products containing IgA

2. Common Variable Immunodeficiency (CVID)

Low IgA + low IgG (often IgM low).

3. Protein-Losing Conditions

  • Nephrotic syndrome
  • Protein-losing enteropathy

4. Medications

  • Immunosuppressants
  • Anti-epileptics
  • Rituximab

5. Newborns

Low until 6 months (immature immune system).

Reference Intervals

Adults

  • 0.7 – 4.0 g/L
     (= 70 – 400 mg/dL)

Children

AgeIgA (mg/dL)
0–1 year1 – 83
1–3 years20 – 100
3–6 years30 – 140
6–12 years40 – 230
>12 yearsAdult range

Selective IgA Deficiency

  • <7 mg/dL (<0.07 g/L) = diagnostic

Diagnostic Uses

1. Screening for Celiac Disease

IgA is required for:

  • tTG-IgA (Tissue transglutaminase IgA)
  • EMA-IgA (Endomysial IgA)

Low IgA → use IgG-based tests (tTG-IgG, DGP-IgG).

2. Evaluate Recurrent Infections

Especially:

  • Sinusitis
  • Otitis media
  • Pneumonia

3. Diagnose Immunodeficiency

  • Selective IgA deficiency
  • CVID

4. Diagnose/Monitor IgA Myeloma

Monoclonal IgA spike on electrophoresis.

5. Autoimmune Disease Workup

Elevated IgA may support inflammatory/autoimmune pathology.

6. Liver Disease Evaluation

IgA elevation characteristic in alcoholic liver disease.

Analytical Notes

  • Measure by nephelometry or turbidimetry
  • Serum preferred
  • Hemolysis has minimal impact
  • IgA stable after collection
  • Check for monoclonal bands if IgA is very high

Clinical Pearls

  • In celiac disease screening, ALWAYS check total IgA; if IgA is low, IgA-based tests will be falsely negative.
  • IgA deficiency is associated with increased autoimmune diseases.
  • IgA myeloma often causes hyperviscosity syndrome.
  • Breast milk contains high secretory IgA → protects infants.
  • Low IgA with normal IgG and IgM is almost always selective IgA deficiency.

Interesting Fact

Secretory IgA (sIgA) is the most abundant antibody produced daily in the human body (~60–70 mg/kg/day), defending all mucosal surfaces continuously.

References

  1. Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Immunoglobulins.
  2. ESID/AAAAI Primary Immunodeficiency Guidelines.
  3. Mayo Clinic Laboratories - Immunoglobulin A.
  4. ARUP Consult - Immunodeficiency Evaluation.
  5. NIH – Immunoglobulin Physiology.
  6. MedlinePlus / NIH - IgA Test.

Last updated: January 26, 2026

Reviewed by : Medical Review Board

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