Introduction
Selective IgG subclass deficiency is one of the 4 most common Inborn Errors of Immunity; being underdiagnosed, especially IgG3 deficiency as it is 4% of total serum IgG, so this pathology going unnoticed.

Presentation of a case:
A 5-year-old female with recurrent infections from 6 months of age with a total of 8 per year, CRUP episodes associated with Cocksackie virus; without complications. In February 2022, after the CRUP episode, with an adequate response to the antibiotic, she went to an allergist to assess these infections, who requested the application of the 23V vaccine and the collection of pneumococcal antibodies 6 weeks later, blood count, immunoglobulins, and IgG subclasses. On physical examination, no abnormalities.
Laboratories: 05.22: Hemoglobin 12.3g/dl, Leukocytes 2.6×102/mm3, Lymphocytes 1248uL, Neutrophils 1222uL.
10.22: Leukocytes 6100×102/mm3, Lymphocytes 4514uL, Neutrophils 1464uL, IgA 136mg/dl, IgM 138mg/dl, IgE 585.7UI/ml, IgG 928mg/dl, IgG1 636mg/dl, IgG2 247mg /dl, IgG3 8.47mg/dl, IgG4 29.7mg/dl.
Study of antibodies against pneumococcal polysaccharides within normal limits.
Discussion:
The patient presents selective deficiency of isolated IgG subclasses since she does not have another affected subclass or antibody deficiency.
The suspicion arose secondary to the number of infections presented per year, despite being within the normal range for daycare attendance.
The prognosis is favorable, since the infections have been treated and self-limited, with normal laboratories despite having decreased IgG3, without requiring human Immunoglobulin G.
Conclusion:
Selective IgG3 deficiency is an Inborn Error of Immunity, due to deficiency of predominant antibodies, presence of recurrent respiratory infections and prophylactic antibiotic therapy as treatment; in the presence of recurrent infections with complications, human Immunoglobulin G is indicated. Finally, Inborn Error of Immunity should be suspected in patients with complicated recurrent infections