Severe combined immunodeficiency disease (SCID)

Severe combined immunodeficiency disease (SCID) is an inherited primary immunodeficiency disorder that presents with opportunistic infections caused by bacteria, viruses, fungi, and protozoa since birth.

Types:-
1. Mutation of IL-2 receptor (X-linked recessive; Most common subtype of SCID).
2. Deficiency of adenosine deaminase (autosomal recessive disorder; Deficiency of adenosine deaminase causes accumulation of toxic metabolites that inhibits DNA synthesis and repair and subsequently apoptosis of lymphocytes causing severe lymphopenia; Least common subtype of SCID).

Presents with recurrent viral, bacterial, fungal, and protozoal infections since birth; Characterized by B-cells (i.e. absent lymph nodes and tonsils) and T-cells (i.e. absent thymic shadow) deficiency.

Diagnosis:-
1. Low WBC (i.e. Low T cells and B cells).
2. Low serum levels of immunoglobulins of all classes.
3. Chest x-ray shows absent thymic shadow.
4. Lymph node biopsy shows aplasia or hypoplasia of the cortex and paracortex (i.e. B-cell and T-cell zone) of lymph nodes.
5. Flow cytometry shows low T cells, B cells and NK cells.
6. Genetic testing.

Management:-
1. Antibiotics, Antifungals, Antivirals and Antiprotozoal.
2. IVIG.
3. Hematopoietic stem cell transplant (HSCT).
4. Enzyme replacement for adenosine deaminase (ADA) deficiency.