Introduction about the Immune System:

Immune system consists of a vast network of organs, tissues, cells, and proteins that work together to defend the body against foreign objects such as bacteria, viruses, fungi, parasites, etc. Immune cells originate from stem cells, often in the bone marrow, and develop into several types including B-lymphocytes, T-lymphocytes, NK and phagocytes. These cells and their proteins are scattered throughout the body so that they can respond quickly.

Severe Combined Immunodeficiency (SCID):

A hereditary disease affects the immune system characterized by severe malfunctions in the production of the immune T-cells and B-cells or in its functions and in some cases, there is a malfunction in the production of NK.

Causes

A defect in one of the genes responsible for the formation of the immune system.

Symptoms

Often symptoms manifest during the first months of life and includes recurrent bacterial, viral and fungal infections. The infection is often opportunistic, that it is caused by weak types of bacteria, fungus and viruses which normally do not affect normal people. This infection affects various areas and organs of the body, such as chest infections, fungal infections of the mouth, severe chronic diarrhea, and leads to severe growth deficiencies.

The incidence of cancers and autoimmune diseases in these patients is high because of malfunction of the immune system

Diagnosis

Immunological analysis is based on an assessment of the capabilities of the immune system in terms of quality and quantity. The number of T-, B- and NK- lymphocytes is estimated, and the level of immunoglobulins, the antibodies produced by B- cells, is assessed, as well as the function of the T- cells. Genetic tests are then performed to determine the type of genetic mutation that causes the disease, if possible.

If the genetic mutation is determined and the parents wish to have a child in the future, it is possible to conduct a test called “Preimplantation Genetics Screening” (PGS), which is an advanced method that relies on in vitro fertilization technique. In the process, the fertilized egg is examined in the laboratory and then planted in the uterus if it is found free from hereditary disease.

Precautions and Treatment:

In case diagnosis is confirmed, the following should be followed:

  • 1. Do not give live vaccinations to the patient, as these vaccines pose a significant risk to their health because they may activate and cause severe infections.
  • 2. The patient should see the doctor in the event of a rise in body temperature or inactivity in order to examine the patient and perform tests and cultures to make sure that there is no inflammation. The patient should take the appropriate antimicrobial.
  • 3. Circumcision should not be conducted in the case of a male patient until the disease is treated.
  • 4. Avoid contact with other patients in the family, such those showing signs of runny nose
  • 5. If blood transfusion is needed, blood is treated in a special way to avoid complications.
  • 6. Parents should bring any newborns to be tested for the disease before symptoms occur.
  • 7. The patient should be given a preventive antibiotic by mouth or vein on a regular basis as well as doses of antibodies either intravenously or under the skin.
  • 8. Members of households should not receive the oral live vaccine.

Available treatment methods:

  • 1. Stem Cell Transplant: Currently, it is the best available treatment. Stem cells are planted from a donor whose tissue is compatible with the patient.
  • 2. Enzyme Compensation: In some cases, the disease arises from a deficiency of ADA enzymes responsible for the formation of immune lymphocytes. In this case, patients are compensated by giving them enzymes until a suitable donor is available for the stem cell transplant.
  • 3. Gene therapy: The patient's stem cells are injected with a healthy gene loaded into a treated virus. When these cells proliferate, the immune dysfunction is corrected.

How the disease is inherited?

The human body consists of billions of cells that make up the various organs of the body. The cells contain the DNA, which in turn carries the genes, which are responsible for the production of proteins that specify path of the composition and different functions of the body. These genes are aligned in the chromosomes.

Different genes are exposed to mutations (changes) in their composition. Some of these mutations are harmless (do not cause a malfunction of the gene and therefore the protein is normal). Other mutations, however, cause damage to the gene, which may lead to the inability to produce protein totally or partially, or to the production of abnormal protein which is unable to perform its normal function and thus produces disease with symptoms associated with the functions of the damaged gene.

How Severe combined immunodeficiency is inherited?

Most types of this disease are inherited by recessive genetics (inheritance) and there is one type inherited by sex-linked recessive inheritance.

1. Recessive heredity

  • There are 23 pairs of chromosomes in the cell and each pair consists of a chromosome from the father and another from the mother. Each gene therefore has two copies from the father and the other from the mother. Recessive genetically transmitted disease is caused by the presence of malfunctions in both copies. As for the defect in one copy only, the person is then described as a carrier of the disease and not infected and often does not show symptoms of the disease.
  • If the mother and the father are both carrier of the disease, there will be a probability out of four for each pregnancy: a healthy copy of the gene from the father and a healthy copy of the gene from the mother are passed to the fetus, thus the fetus is healthy.
  • A defected copy of the gene from the father and a healthy copy of the gene from the mother are passed to the fetus. In this case, the fetus is a carrier of the disease and does not often show symptoms of the disease.
  • A healthy copy of the gene from the father and a defected copy of the gene from the mother are passed to the fetus. In this case, the fetus is a carrier of the disease and does not often show symptoms of the disease.
  • A defected copy of the gene from the father and a defected copy of the mother from the gene are transmitted to the fetus. In this case, the fetus is infected.
  • Accordingly, each time the mother becomes pregnant, there will be a possibility that:
  • • 25% the fetus is healthy.
  • • 50% the fetus carries the disease and does not often show symptoms.
  • • 25% the fetus is infected.

2. Sex- linked recessive inheritance

  • There are sex chromosomes that determine the sex of a person, whether male or female, namely the Y chromosome and X chromosome. When Y chromosome meets with X chromosome, a boy is formed, and when X chromosome meets with X chromosome a girl is formed. We also mentioned that each gene has two copies, but there is an exception here. The male fetus has only one copy of X chromosome and therefore has only one copy of the genes found on this chromosome. The female, however, has two copies of each gene because they have two copies of the X chromosome, so the majority of those with sex-linked genetic diseases are male. This is because the second copy of gene in female compensates for the shortage resulting from the damage of one of the two copies. In this case, the father is usually healthy while the mother carries the disease and accordingly, each time the mother becomes pregnant, there will be a probability that:
  • • 50% the male fetus is infected.
  • • 50% that the male fetus is healthy.
  • • 50% the female fetus carries the disease.
  • • 50% the female fetus is healthy.