Why can congenital immunodeficiency syndromes remain unnoticed until exposure to certain infections?

Congenital immunodeficiency syndromes can remain unnoticed until exposure to specific infections primarily because newborns are often protected by maternal antibodies transferred across the placenta or through colostrum. These passive antibodies provide temporary immunity, effectively masking the infant's own inability to mount an immune response.12

As the infant grows, these maternal antibodies undergo natural metabolic breakdown and become diluted. Once this passive protection wanes, the infant becomes increasingly susceptible to pathogens. Because the development of the child's own acquired immunity is a gradual process, the underlying congenital disorder often only becomes clinically apparent when the child encounters environmental microbes that their compromised immune system cannot combat. Consequently, the inability to resist these infections serves as the primary indicator of the underlying immunologic defect.123