When do B cell deficiencies typically present in infants?

B cell deficiencies often manifest after 6 months of age due to the physiology of immunoglobulin levels in infants. At birth, infants possess maternal antibodies that provide passive immunity. These maternal antibodies can last for several months, protecting the infant from infections. However, as these maternal antibodies wane, the infant’s immunologic system begins to rely on its own ability to produce antibodies, which is facilitated by functioning B cells.

In conditions such as X-linked agammaglobulinemia or other forms of B cell deficiency, the ability to produce adequate immunoglobulin responses is compromised. Therefore, symptoms typically do not present until after this period, around 6 months, when the maternal antibodies have decreased and the infant's own B cell function is critical for adequate immune defense. This timing can help clinicians differentiate between transient conditions and more serious congenital immunodeficiencies.