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Severe Congenital Immune Deficiencies: Overview

What are immune deficiencies?

These are a group of genetic diseases in which a child is born without a normal ability to fight infection. This condition is called an immune deficiency or severe combined immune deficiencies (SCID).

When present, SCID and other congenital severe immune deficiencies, like Wiskott Aldrich Syndrome, are life-threatening diseases because the child cannot effectively fight off common infections.

Children with severe immune deficiencies often need to be hospitalized and can die from common respiratory viruses like flu, RSV or adenovirus, other common viruses like chickenpox, or opportunistic infections that only affect the severely immune-compromised (people with weak immune systems) like certain types of pneumonia.

What is severe combined immune deficiency (SCID)

Because the human immune system is so complex, there are many possible genetic mutations that can result in severe congenital immune deficiency. The most severe is Severe Combined Immune Deficiency (SCID), where both the disease-fighting immune system T-cells and B-cells are absent or broken. Once discovered, a child with SCID is usually should be kept in protective isolation to prevent catching a possibly fatal viral infection.

At this time the only known effective and curative therapy is hematopoietic stem cell transplantation (HSCT) from a healthy donor.

It is important to know that severe congenital immune deficiencies are very different from acquired immune deficiencies such as HIV and AIDS, which cannot be cured by a standard allogeneic HSCT.

Finally, there is a family of diseases called the Histocytic Disorders, which are life-threatening diseases that are due to an inherited immune defect.

What causes severe congenital immune deficiencies?

The word "congenital" means that children are born with a condition. All severe congenital immune deficiencies are caused by genetic mutations. The abnormal changes in the genetic code that result in severe congenital immune deficiencies are usually recessive or "weaker" genes. There are two main patterns of SCID inheritance autosomal X-linked recessive:

Autosomal Recessive: Each parent could carry a rare SCID gene, which is "hidden" because their normal gene provides the instruction code for normal. The chance of a parent donating the defective gene to their child is 50% for each egg or sperm. If a child inherits only 1 defective gene from parents, they are normal, but are a carrier and could pass the SCID to their children if the other parent also carried a SCID defect in the same gene. If the child inherits neither of the defective genes, they are normal and cannot pass the SCID to their children.

X-Linked Recessive: Only the mother carries a "hidden" SCID gene on her X chromosome and she is normal because of a normal gene on her other X chromosome (females have 2 X chromosomes, and males have one X and one Y chromosome). If she is the mother of a boy (XY) and he receives the X chromosome with the defective gene, he will have a SCID. If the boy receives the normal X from mom, he will not have SCID. If a mother donates her abnormal X with the SCID gene to a daughter, that daughter will be a carrier and could pass SCID on to her boys (50% chance). If the girl gets mom’s normal X, she will be normal and cannot pass the SCID to her children.

Who gets SCID, SCID variants or histiocytic disorders?

Fortunately, all combined severe congenital immune deficiencies are very rare diseases. Each form of SCID is therefore very rare.

In autosomal recessive SCID, both parents have a rare mutated gene, but this is usually not predictable based on family history. Sharing the same parents or grandparents is a known risk factor and, if culturally common, there may be a SCID history.

In X-linked recessive SCID, there is often a family history of increased early fetal loss or death in the first year of life in male children. However, if the defective gene is new in mom or there are not many other relatives, there may not be any history.

Currently, the genetic mutations causing SCIDs can be identified in about 75% of children with severe congenital immune deficiency. This is important because some children with SCIDs may have other medical conditions or increased susceptibility to cancer or problems with chemotherapy or radiation. Also there may be other medical issues to counsel them about that may not be corrected by stem cell transplants. Finally, knowing the exact genetic defect allows for appropriate genetic counseling and the possibility of diagnosis before birth.

Helpful resources for immune deficiencies