Severe Combined Immunodeficiency (SCID) – Lecture Review Flashcards

Question-and-Answer flashcards summarising the definition, genetics, pathophysiology, clinical features, diagnosis and treatment of Severe Combined Immunodeficiency (SCID).

What category of immune disorder does Severe Combined Immunodeficiency belong to?

A Primary Immunodeficiency Disease (PID).

Which two lymphocyte lineages are profoundly defective or absent in SCID?

T cells and B cells.

Which primary lymphoid organs normally mature B and T cells, respectively?

Bone marrow (B cells) and thymus (T cells).

Why is proper T- and B-cell development essential for health?

They enable cell-mediated and antibody-mediated immunity, allowing the body to clear infections.

What overarching stem-cell problem characterises SCID?

Failure of lymphoid stem cells to differentiate into functional T and/or B cells.

Which two enzyme deficiencies are classic genetic causes of SCID treatable by gene therapy?

Adenosine deaminase (ADA) deficiency and Purine nucleoside phosphorylase (PNP) deficiency.

Mutations in which genes required for V(D)J recombination produce Omenn syndrome?

RAG1 and RAG2.

Defects in which cytokine-signalling genes commonly cause SCID?

JAK3 and IL-7Rα.

What immunological consequence results from TAP transporter deficiency in SCID?

Absence of MHC class I antigen presentation.

Mutation of which TCR-associated tyrosine kinase leads to SCID?

ZAP-70.

What is the most common inheritance pattern of classic SCID?

X-linked recessive.

At what age do clinical manifestations of SCID usually appear?

Early infancy.

Give three hallmark clinical features of SCID in infants.

Recurrent or chronic infections, failure to thrive, and persistent thrush (candidiasis).

Which opportunistic infection is the leading cause of death in untreated SCID?

Pneumocystis jirovecii pneumonia.

Omenn syndrome, a form of SCID, can clinically mimic which transplant-related complication?

Graft-versus-host disease (GVHD).

Name two characteristic findings of Omenn syndrome.

Generalised erythematous rash and marked eosinophilia (others include protracted diarrhoea and hepatosplenomegaly).

What family history detail is a red flag for possible SCID?

A known family history of primary immunodeficiency diseases.

How many or more new ear infections in one year signal possible PID such as SCID?

Eight or more.

Which laboratory technique is central to documenting low or absent T and B cells in SCID?

Flow cytometry of lymphocyte subsets.

What definitive test establishes the genetic subtype of SCID?

Molecular genetic testing for specific gene mutations.

What curative therapy is considered first-line for most SCID patients?

Hematopoietic stem-cell transplantation.

Which landmark gene-therapy success involved ADA-deficient SCID?

Retroviral transfer of human ADA cDNA into the patient's own T cells, leading to clinical improvement.

Why are standard oral antibiotics often insufficient for SCID infections?

Profound immune deficiency prevents effective pathogen clearance without immune reconstitution.

What systemic growth effect commonly accompanies SCID in infants?

Failure to thrive due to chronic infection and malnutrition.

Which immunoglobulin classes are usually low or absent in SCID?

Multiple classes (IgM, IgG, IgA, IgE) because of B-cell dysfunction.

What term describes SCID variants where lymphocytes are present but lack MHC expression?

Bare Lymphocyte Syndrome.

Which complement assay may be ordered in a PID work-up including SCID?

Total complement activity (CH50) test.

What laboratory test evaluates oxidative burst function in suspected PID?

Phagocytosis and respiratory-burst (oxygen radical) assay.

Which flow-cytometric functional test measures NK-cell cytotoxicity during SCID evaluation?

NK-cell cytotoxicity assay.

If left untreated, what is the typical prognosis for infants with SCID within their first year?

High mortality; the condition is often fatal.