Severe combined immunodeficiency (SCID) is a group of rare disorders caused by mutations in different genes involved in the development and function of infection-fighting immune cells. Infants with SCID appear healthy at birth but are highly susceptible to severe infections.

Severe combined immunodeficiency – A deadly combination of defects of T & B lymphocytes + Natural killer cells

Its a primary immunodeficiency (inherited in the family) affecting T lymphoocytes, B lymphocytes and Natural killer cells in different combinations.

These children die within one year without a bone marrow transplantation.There have been cases like “David the bubble boy” who survived for 12 years in a sterile chamber isolated from pathogens.

Patients are small babies that suffers from fungal, bacterial, or viral infections since their immune system is not functioning as it should.

The symptoms can be:
recurrent infections resistant to antibiotics,
failure to thrive with abnormal growth curves,
diarrhea,
diaper rash,
bronchitis,
pneumonia,
otitis media,
liver abscess,
morbilliform rash,
oral candidiasis

Doctors can check the sequence of DNA to determine any mutations that would cause this disease.
One have to remember that the levels of antibodies decline after about 6 month of age if the baby fails to produce new ones for himself, since the antibodies from the mother are depleted.
So these babies may appear healthy for about 6 months.
Therefore we can check blood values of lymphocytes, natural killer cells and antibodies to easier determine which type of Severe combined immunodeficiency we are dealing with.

What are the types of this disease?
Mainly X linked recessive (IL2RG gene mutation at Xq13.1) and autosomal recessive .

The autosomal recessive have many types:
adenosine deaminase deficiency,
Jak3 intracellular kinase mutation,
Purine nucleoside phosphorylase mutation,
Omenn syndrome with RAG gene mutation,
Bare lymphocyte syndrome with MHC 2 gene mutation.

The adenosine deaminase deficiency cause an accumulation of dATP, which blocks the enzyme Ribonucleotide reductase (converts NDP to dNDP). Without dNDP there is no DNA synthesis and therefore no production of cells like T & B lymphocytes.

You can imagine that without T & B cells there is no real immunity against infections. We can see depleted zones in the hypoplastic lymphoid organs. One example being a small thymus lacking lymphocytes and Hassall’s corpuscles. On X-ray there is an abscence of thymic shadow.

The treatment?
Bone marrow transplantation and gene therapy. About 1 patient out of 5 have a risk of getting acute T cell leukemia when treated with bone marrow transplantation.

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