A rare, primary humoral immunodeficiency of childhood characterized by decreasing serum levels of immunoglobulin G (IgG) as maternal antibodies clear the circulation while serum levels of immunoglobulin A and immunoglobulin M remain normal or are slightly decreased. Diagnosis may be suspected after the age of six months when a child's own synthesis of IgG should accelerate but it must be confirmed retrospectively after normalization of all serum immunoglobulin levels is seen by ages 2-6. This disorder may be caused by inadequate activation of progenitor B cells, defective class-switching or may even represent a maturational variant. Typically, a normal response to protein antigens is found while there is a notably diminished response to viral and bacterial polysaccharide antigens. Clinical presentation may include recurrent infections especially those of the respiratory tract. Despite increased susceptibility to infection in childhood, this disorder is self-limited with minimal implications for a normal life span. The disease is Monarch Disease Ontology term MONDO_0015698 (transient hypogammaglobulinemia of infancy). Also known as: THI.