Question: What is (are) Severe combined immunodeficiency ?
Answer: Severe combined immunodeficiencies (SCID) are inherited immune system disorders characterized by abnormalities with responses of both T cells and B cells (specific types of white blood cells needed for immune system function). Common signs and symptoms include an increased susceptibility to infections including ear infections; pneumonia or bronchitis; oral thrush; and diarrhea. Due to recurrent infections, affected children do not grow and gain weight as expected (failure to thrive). SCID may be caused by mutations in any of several genes and can be inherited in an X-linked recessive (most commonly) or autosomal recessive manner. The most effective treatment is transplantation of blood-forming stem cells from the bone marrow of a healthy person. Without treatment, affected children rarely live past the age of two.
Source: GARD

Question: Is Severe combined immunodeficiency inherited ?
Answer: How is severe combined immunodeficiency inherited? Severe combined immunodeficiency (SCID) can be inherited in an X-linked recessive or autosomal recessive manner depending on the genetic cause of the condition. X-linked SCID is the most common type of SCID and is inherited in an X-linked recessive manner. A condition is X-linked if the changed (mutated) gene responsible for the condition is located on the X chromosome. The X chromosome is one of the two sex chromosomes; females have two X chromosomes and males have one X chromosome and one Y chromosome. In males, one mutated copy of the responsible gene causes signs and symptoms of the condition because they don't have another X chromosome with a working copy of the gene. In females, having one mutated copy of the gene would make them an unaffected carrier; a mutation would have to occur in both copies of the gene to cause the condition. This is why X-linked recessive disorders, including X-linked SCID, occur much more frequently in males. Because fathers only pass their Y chromosome on to their sons, fathers cannot pass X-linked conditions on to their sons. The other, less common causes of SCID are inherited in an autosomal recessive manner. These types are due to mutations in responsible genes on other chromosomes (not the sex chromosomes). In autosomal recessive conditions, a person must have mutations in both copies of the responsible gene in order to have signs or symptoms of the condition. In most cases, the affected person inherits one mutated copy of the gene from each of the parents, who are typically unaffected carriers.
Source: GARD

Question: How to diagnose Severe combined immunodeficiency ?
Answer: How is severe combined immunodeficiency (SCID) diagnosed? A diagnosis of severe combined immunodeficiency (SCID) may be suspected if a baby shows any of the following persistent symptoms within the first year of life: Eight or more ear infections Two or more cases of pneumonia Infections that do not resolve with antibiotic treatment for two or more months Failure to gain weight or grow normally Infections that require intravenous antibiotic treatment Deep-seated infections, such as pneumonia that affects an entire lung or an abscess in the liver Persistent thrush in the mouth or throat A family history of immune deficiency or infant deaths due to infections Diagnosis can be confirmed by blood tests. Blood tests show significantly lower-than-normal levels of T cells and antibodies. For further details on diagnosis see the following Web pages: The Primary Immunodeficiency Resource Center provides further details regarding diagnosis of SCID. Click on the embedded link to view the page. An article from Medscape Reference provides detailed information on the diagnosis of SCID. Click on eMedicine Journal to view the page. You may need to register to view the article, but registration is free.
Source: GARD