Dr Ira Shah
Consultant Pediatrician, Nanavati Hospital and B.J.Wadia Hospital for Children, Mumbai
Editor - Pediatric Oncall

What is severe combined immunodeficiency (SCID)?
SCID is a rare primary immunodeficiency disorder in which there is combined absence of T-lymphocyte and B-lymphocyte function. It can lead to extremely serious infections which can even be fatal.

What is the cause of severe combined immunodeficiency?
There are currently 12 known types of SCID based on various genetic mutations. These are

1.	Deficiency of the common gamma chain of six different cytokine receptors - This is the commonest form of SCID and seen only in males. It has very low T-lymphocytes and NK-lymphocytes but B-lymphocyte count is high though they do not function properly.
2.	Adenosine Deaminase (ADA) deficiency - This type of SCID occurs due to mutation in gene that forms ADA. ADA deficiency is the second most common cause of SCID and children have low T cells, B cells and NK cells. Both boys and girls are affected.
3.	Deficiency of the Alpha Chain of the IL-7 receptor. Affected children have no T cells but have normal B cells and NK cells. However B cells do not function properly.
4.	Deficiency of the Janus Kinase 3 (JAK 3) - Patients have no T & NK cells but normal B cells which do not function properly.
5.	Deficiencies of CD3 chains:- Three types are seen
6.	Deficiencies of CD45
7.	Recombinase activating genes 1 and 2 (RAG 1 and RAG 2) deficiency. It is also known as Ommen’s syndrome. Patients have absent T & B cells but normal NK cells.
8.	Artemis deficiency - Patients have absent T & B cells
9.	Ligase 4 deficiency - Patients have absent T & B cells

There are other SCID causing mutations that have not been identified.

What are the symptoms of severe combined immunodeficiency (SCID)?
Symptoms of SCID are recurrent serious infections starting right in infancy. Infections by unusual organisms such as pneumocystis jiroveci (PCP), chickenpox, cytomegalovirus (CMV) are commonly seen. Patients commonly have serious bacterial, viral and fungal infections and may succumb to one of the infections.

Some children may also have a rash due to reaction caused by the mother's T cells in the baby's blood circulation leading to reaction against the baby's organs. This is known as graft versus host disease.

The doctor will notice that inspite of serious infections, the child has no glands enlarged and tonsils are also absent (Lymph nodes and tonsils are where the lymphocytes are stored). Chest X-Ray will also show absence of thymus.