Severe combined immunodeficiency vs Other primary and mixed immunodeficiency syndromes
How to tell Severe combined immunodeficiency and Other primary and mixed immunodeficiency syndromes apart when a board question makes them look alike.
SCID is combined T- and B-function failure; the comparison syndromes localize to B-cell maturation, class switching, platelets/eczema, thymic T-cell development, or late hypogammaglobulinemia.
Side by side
SCID causes severe T-cell dysfunction with impaired cellular and humoral immunity, chronic diarrhea, thrush, opportunistic infections, and absent thymic shadow.
Bruton causes absent mature B cells; Hyper-IgM causes failed class switching; Wiskott-Aldrich causes thrombocytopenia/eczema; DiGeorge is 22q11.2 T-cell deficiency with hypocalcemia; CVID causes recurrent sinopulmonary infections.
SCID causes combined cellular and humoral failure, classically from IL2RG common gamma-chain defects or ADA deficiency, so infants develop opportunistic infections, thrush, chronic diarrhea, and absent thymic shadow.
The comparison side separates by the broken immune compartment: BTK blocks B-cell maturation, CD40L blocks class switching, WAS affects cytoskeletal signaling, and 22q11.2 thymic aplasia causes T-cell deficiency.
Very low T cells with poor vaccine responses; B cells may be low or nonfunctional; absent thymic shadow supports SCID.
Bruton: low immunoglobulins/absent B cells. Hyper-IgM: high IgM with low IgG/IgA. Wiskott-Aldrich: small platelets. DiGeorge: low T cells plus hypocalcemia.
The trap
SCID is combined T and B vulnerability; named B/mixed syndromes separate by B cells, class switching, platelets, thymus/calcium, or adult hypogammaglobulinemia.
The repair move
Before naming the answer, force the stem through the separator: SCID is combined T and B vulnerability; named B/mixed syndromes separate by B cells, class switching, platelets, thymus/calcium, or adult hypogammaglobulinemia.
Try a free question
CoreA 4-month-old boy has had recurrent thrush, chronic diarrhea, failure to thrive, and a Pneumocystis jirovecii pneumonia. He has no palpable lymph nodes and an absent thymic shadow on chest X-ray. Flow cytometry shows markedly decreased T cells, with B cells present but nonfunctional (no class-switched antibody). Genetic testing reveals an IL2RG mutation (X-linked SCID). Which condition is most likely?
Show answer and explanation
Answer: A. Severe combined immunodeficiency. SCID is severe combined T and B cell dysfunction; most common cause is X-linked IL2RG (common gamma chain) mutation. Other causes: ADA deficiency, JAK3, RAG1/2. Features: opportunistic infections (PJP, CMV, fungal), thrush, chronic diarrhea, failure to thrive, absent thymic shadow. Treatment: hematopoietic stem cell transplant. Newborn screening uses T-cell receptor excision circles (TRECs).
Why the other choices are tempting
- B. X-linked agammaglobulinemia has ABSENT mature B cells but normal T cells; presents with bacterial sinopulmonary infections after 6 months, not opportunistic infections at 4 months.
- C. Hyper-IgM syndrome has elevated IgM with low IgG/IgA but normal T cell numbers.
- D. Wiskott-Aldrich features thrombocytopenia (small platelets), eczema, and immunodeficiency, not isolated SCID phenotype.
- E. CVID typically presents in young adulthood with hypogammaglobulinemia, not infantile SCID.
Flashcards
Tap a card to flip it.
What clue supports Severe combined immunodeficiency?
Choose Severe combined immunodeficiency when the stem gives this pattern: SCID causes severe T-cell dysfunction with impaired cellular and humoral immunity, chronic diarrhea, thrush, opportunistic infections, and absent thymic shadow.
What clue supports B-cell and mixed immunodeficiency syndromes?
Choose B-cell and mixed immunodeficiency syndromes when the stem gives this contrasting pattern: Bruton causes absent mature B cells; Hyper-IgM causes failed class switching; Wiskott-Aldrich causes thrombocytopenia/eczema; DiGeorge causes T-cell hypocalcemia; CVID causes recurrent sinopulmonary infections.
Quick answers
What is the fastest way to tell Severe combined immunodeficiency from Other primary and mixed immunodeficiency syndromes?
SCID is combined T- and B-function failure; the comparison syndromes localize to B-cell maturation, class switching, platelets/eczema, thymic T-cell development, or late hypogammaglobulinemia.
What is the key clue for Severe combined immunodeficiency?
SCID causes severe T-cell dysfunction with impaired cellular and humoral immunity, chronic diarrhea, thrush, opportunistic infections, and absent thymic shadow.
What is the key clue for Other primary and mixed immunodeficiency syndromes?
Bruton causes absent mature B cells; Hyper-IgM causes failed class switching; Wiskott-Aldrich causes thrombocytopenia/eczema; DiGeorge is 22q11.2 T-cell deficiency with hypocalcemia; CVID causes recurrent sinopulmonary infections.
What is the common trap with Severe combined immunodeficiency vs Other primary and mixed immunodeficiency syndromes?
SCID is combined T and B vulnerability; named B/mixed syndromes separate by B cells, class switching, platelets, thymus/calcium, or adult hypogammaglobulinemia.