Transient Hypogammaglobulinemia of Infancy
Transient hypogammaglobulinemia of infancy (THI) is defined by low serum IgG levels in infants, after excluding other known immunodeficiencies such as X-linked agammaglobulinemia (XLA) and secondary causes, such as protein-loss enteropathy. In THI, serum IgG levels usually reach normal range during childhood.
The International Union of Immunological Societies (IUIS) defines THI when a child presents with serum IgG and IgA levels lower than two standard deviations below the mean for age, with conserved ability to develop antibodies to vaccines (Bousfiha et al. 2018). The European Society for Immunodeficiencies (ESID) has a working definition of THI used for registry purposes (Seidel et al. 2019), for patients who present IgG below age-related normal value in the first 3 years of life, exclusion of defined causes of hypogammaglobulinemia, and spontaneous resolution at about the fourth birthday.
Because most children with THI would not achieve normal serum IgG...
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