Ref: Nelson's Textbook of Pediatrics. /9th Edition, Page:Explanation:"Marked accumulations of adenosinef 22 - deoxyadenosine. and 22 -O-methyladenosine lead directly or indirectly to T-cell apoptosis, which causes the immunodeficiency " (Ref: Nelson) Severe combined ImmuntKief'iciency (SCID)Both cellular and humoral immunity affectedMC type is X-linked SCID (X-SCD) due to mutation in Common Cytokine Receptor g Chain (gC)Second most common form of SCID is absence of the enzyme adenosine deaminase (ADA)(15% patients)All patients with SCID have very small thymuses (<1 g)Thymus fail to descend from the neck, contain no thymocytes, and lack corticomeduliary distinction or Hassall corpuscles.The thymic epithelium appears histologically normal.Both the follicular and paracorticai areas of the spleen are depleted of lymphocytes.Lymph nodes, tonsils, adenoids, and Peyer patches are absent or extremely underdevelopedClinical ManifestationsAffected infants present within the 1st few months of life with Recu rrent or persistent diarrheaPneumoniaOtitis media SepsisCutaneous infectionsGrowth failurePersistent infections with opportunistic organisms including lead to deathCandida albicansPneumocystis jiroveciParainfluenza 3 virusAdenovirusRespiratory syncytial virusRotavirus vaccine virus Cytomegalovirus (CMV)Epstein-Barr virus (EBV)Varicella-zoster virusMeasles virusMMR-V vaccine virusBacillus Calmette-Guerin (BCG)Affected infants also lack the ability to reject foreign tissue and are therefore at risk for severe or fatal graft versus host disease (GVHD) from T lymphocytes in non irradiated blood products or in allogeneic stem cell transplants or less severe GVHD from maternal immunocompetent T cells that crossed the placenta while the infant was in utero.* These infants also have an absence of lymphocyte proliferative responses to mitogens, antigens, and allogeneic cells in vitro.Patients with adenosine deaminase (ADA) deficiency have the lowest absolute lymphocyte counts, usually <500/mm3.Serum immunoglobulin concentrations are low or absentNo antibodies are formed after immunizations.T cells are extremely low or absentTreatmentSCID is a true pediatric emergency.Unless immunologic reconstitution is achieved through stem cell transplantation, death usually occurs during the 1st yr of life and almost invariably before 2 yr of age.