Immunoglobulin A (IgA)

Selective IgA deficiency is the most common primary immunodeficiency. NIDDK puts its general-population frequency at roughly 1 in 400 to 1 in 800. Many people who have it never know, because IgA is the one antibody class the body can partly do without. Others notice a pattern of stubborn mucosal infections, the sinus and gut and chest complaints that keep coming back because the surface coating is thin.

The number that defines it is small. StatPearls sets selective IgA deficiency at a serum IgA below 7 mg/dL, with normal IgG and IgM, in a person older than 4 once other causes are excluded. Below that threshold the standard lab method cannot measure IgA accurately, so a report may simply read as the bottom of the scale.

Here is where the power cut matters. The first-line celiac screen, the tissue transglutaminase IgA antibody (tTG-IgA), is itself an IgA molecule. If you barely make IgA, you barely make that antibody, so the test can read negative even when celiac disease is active. NIDDK reports that about 2% to 3% of people with celiac disease are IgA-deficient, well above the rate in the general population. So among exactly the people most likely to have celiac disease, the routine test is most likely to miss it. You flip the lamp, the room stays dark, and the conclusion looks like "no celiac" when the truth is "no IgA to detect it."

This is the gap that a reassuring tTG-IgA can leave wide open: a person with real symptoms, a normal-looking celiac result, and a quietly low IgA that explains both.

A high IgA is read differently and more loosely. MedlinePlus describes quantitative immunoglobulin testing as a way to measure gamma globulin proteins, where abnormal totals can point to infection, immune deficiency, chronic inflammatory disease, or certain cancers. An elevated IgA is a prompt to look at the wider clinical picture rather than a diagnosis on its own, and it does not carry the testing trap that a low value does. In myeloma the malignant plasma cells often raise an immunoglobulin such as IgA alongside beta-2 microglobulin, which tracks the underlying cell turnover.

The fix for the masking effect is procedural, and it is worth knowing the words before the appointment.

The point of the swap is simple. If the street has no power, you stop testing your own lamp and bring in a different circuit. An IgG-based celiac test does not rely on the IgA you lack, so it can register disease that the IgA test cannot.

IgA rarely travels alone on a report. It sits in a quantitative immunoglobulin panel beside IgG, the dominant antibody in blood, and IgM. That grouping is what makes selective IgA deficiency recognizable: low IgA against normal IgG and IgM. It is a different molecule from IgE, the antibody tied to allergy and parasites, even though the names look almost identical.

Because immunoglobulins are proteins, IgA also folds into broader chemistry. It is one contributor to globulin, the protein fraction reported on a metabolic panel, and through that to total protein. A meaningfully low IgA can nudge those summary numbers without being the headline on the report.

If your IgA reads low, the single most useful next move is the one NIDDK spells out: make sure any celiac antibody test was either paired with total IgA or run as an IgG-based version. For more on how a result like this fits with the rest of your panel, see our guide to reading blood test results. One quiet number can decide whether a different test told you the truth.