15. Primary Immunodeficiency Disease - combined T and B cell

Core Concepts & Clinical Presentation

• Definition: Genetic (primary) defects that cause a failure in the development or function of both T-cells (cell-mediated immunity) and B-cells (humoral immunity).

• The Pathologic Cascade: Because Helper T-cells (CD4+) are absolutely required to activate B-cells (to make antibodies), a severe T-cell defect almost always leads to a functional B-cell defect, even if the B-cells are genetically normal.

• Classic Clinical Presentation:

  • Age of Onset: Very early in life (usually within the first 3-6 months, as maternal IgG begins to wane).

  • Infections: Severe, recurrent, and opportunistic. Patients are susceptible to everything: bacterial, viral, fungal (e.g., severe oral Candida thrush), and protozoal (e.g., Pneumocystis jirovecii pneumonia).

  • Failure to thrive: Chronic diarrhea and chronic infections prevent normal growth.

  • Physical Exam/Imaging: Absent thymic shadow on chest X-ray. Lymph nodes and tonsils are hypoplastic (absent germinal centers).

• Absolute Contraindication: You must NEVER give live vaccines (like Rotavirus, MMR, or BCG) to these infants; the vaccine strain will cause a fatal disseminated infection.

Severe Combined Immunodeficiency (SCID)

This is the most severe form of primary immunodeficiency. It is a pediatric medical emergency. Without a hematopoietic stem cell (bone marrow) transplant, it is uniformly fatal in infancy.

A. X-Linked SCID (Most Common - ~60% of cases)

B. Adenosine Deaminase (ADA) Deficiency (~15% of cases)

C. RAG1 / RAG2 Mutations (Omenn Syndrome)

A. X-Linked SCID (Most Common - ~60% of cases)

• Genetics: X-linked recessive (affects almost exclusively males).

• Pathogenesis: Mutation in the IL-2Rγ chain (Interleukin-2 receptor gamma chain).

• Mechanism: This specific gamma chain is a shared, crucial component of the receptors for several interleukins (IL-2, IL-4, IL-7, IL-9, IL-15, IL-21). Without IL-7, T-cells cannot mature. Without IL-15, NK cells cannot mature.

• Cell Profile: T-cell negative, B-cell positive (but functionally useless without T-cells), NK-cell negative (T^-, B^+, NK^-).

B. Adenosine Deaminase (ADA) Deficiency (~15% of cases)

• Genetics: Autosomal Recessive (affects males and females).

• Pathogenesis: Mutation in the ADA gene. ADA is an enzyme required for purine metabolism.

• Mechanism: Without ADA, toxic purine metabolites (specifically dATP) build up inside cells. Lymphocytes (T, B, and NK cells) are extremely sensitive to dATP toxicity, which inhibits DNA synthesis and triggers apoptosis.

• Cell Profile: Pan-lymphopenia. T-cell negative, B-cell negative, NK-cell negative (T^-, B^-, NK^-).

C. RAG1 / RAG2 Mutations (Omenn Syndrome)

• Genetics: Autosomal Recessive.

• Pathogenesis: Defect in Recombination Activating Genes (RAG).

• Mechanism: RAG enzymes are responsible for V(D)J recombination—the process that creates unique T-cell receptor and B-cell antibodies. Without it, no mature T or B cells can form.

Syndromic Combined Immunodeficiencies

These feature combined immune defects alongside unique, defining physical symptoms.

A. Wiskott-Aldrich Syndrome (WAS)

B. Ataxia-Telangiectasia (A-T)

A. Wiskott-Aldrich Syndrome (WAS)

• Genetics: X-linked recessive. Mutation in the WAS gene.

• Pathogenesis: The WAS gene produces WASP (Wiskott-Aldrich Syndrome Protein), which is necessary for actin cytoskeleton reorganization in white blood cells and platelets. Without it, T-cells cannot interact properly with antigen-presenting cells.

• The Classic Triad (Highly Tested!):

  1. Thrombocytopenia (low platelets -> petechiae, purpura, bleeding).

  2. Eczema.

  3. Recurrent infections.
     (Mnemonic: WATER = Wiskott-Aldrich, Thrombocytopenia, Eczema, Recurrent infections).

B. Ataxia-Telangiectasia (A-T)

• Genetics: Autosomal Recessive. Mutation in the ATM gene.

• Pathogenesis: The ATM protein is responsible for detecting DNA double-strand breaks and halting the cell cycle for repair. Without it, mutations accumulate rapidly (especially during V(D)J recombination in lymphocytes).

• The Classic Triad:

  1. Ataxia: Cerebellar atrophy causing progressive gait unsteadiness (usually the first sign).

  2. Telangiectasia: Dilated capillary blood vessels on the skin and eyes (spider angiomas).

  3. Combined Immunodeficiency: Particularly low IgA and IgG, leading to recurrent sinopulmonary infections.

• Major Complication: Extremely high risk of lymphomas and leukemias due to the failure of DNA repair.