Background: Patients with Down syndrome (DS) are more likely to exhibit immunological abnormalities that predispose them to infection. Among other anomalies, individuals with DS have altered serum concentrations of some subclasses of Immunoglobulin G (IgG), particularly the IgG4 subclasses The aim of the present study was to investigate the molecular mechanism of low serum IgG4 at the level of the heavy chain gene (IGHG4). Method: Quantitative real-time polymerase chain reaction (qPCR) was carried out to measure the number of IGHG4 copies and to compare those outcomes with a reference gene (36B4). An IGHG4/36B4 ratio was considered normal (two copies of IGHG4) when between 0.8 and 1.2. We studied 44 DS patients from 8 to 57 years of age, comprising 23 DS persons carrying severe low serum IgG4 (< 0.02 g /L), with five having an IgG4 level that was not detectable, and 21 DS patients with normal serum IgG4 (level > 0.1 g /L). The patient group was compared with 38 healthy donors (controls) without DS. Result: IGHG4 heterozygous deletion was found in 16 (69.6%) DS patients with low serum IgG4 versus in two (9.5%) DS with normal serum IgG4 (P =0.0001). In the control group, deletion was found in 5.26% (2/38) of the sample. Conclusion: IGHG4 haploinsufficiency is highly correlated with low serum IgG4 in our population with DS, but other relevant factors must to be assessed in future work.