Why a missing immune system makes the world a lethal place for newborns

Nature · Published 3 October 2026 Swipe more like this →

In short: Severe combined immunodeficiency, or SCID, is a group of rare, inherited disorders that leave infants without a functional immune system. Because they lack the T-cells and B-cells necessary to fight off common germs, even minor infections can become life-threatening. Without intervention, most affected babies do not survive infancy.

SCID is a collection of genetic conditions characterized by the absence or malfunction of specialized white blood cells known as lymphocytes. These cells—T-cells, B-cells, and natural killer cells—are essential for identifying and neutralizing foreign invaders like viruses, bacteria, and fungi. In children with SCID, a mutation in one of several possible genes prevents the body from producing the proteins required for these cells to develop or function correctly. Because the defect impairs both the T-cell and B-cell responses, the condition is termed 'combined.'

Babies born with SCID often appear healthy for the first few weeks of life, as they are temporarily protected by antibodies passed from their mother across the placenta. However, as this protection fades, the first signs typically emerge between three and six months of age. Infants may suffer from persistent thrush, chronic diarrhoea, failure to thrive, and recurrent, severe infections such as pneumonia. Germs that are harmless to healthy individuals, such as certain fungi or common viruses, can cause fatal illness in these children.

Diagnosing SCID often relies on identifying a low lymphocyte count during routine investigations triggered by a child's failure to gain weight or repeated infections. Once suspected, infants are referred to specialist immunology centres for confirmation and to determine the specific genetic cause. While the condition was historically fatal, modern medicine offers hope. Haematopoietic stem cell transplantation is the primary treatment, and gene therapy is currently being explored for specific forms of the disorder. In England, an evaluation of newborn blood spot screening began in September 2021 to determine if early detection can improve outcomes on a national scale.

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