Common variable immunodeficiency (CVID) is a USMLE favorite because it sits right at the intersection of “recurrent infections” and “lab pattern recognition”—and the answer choices almost always include other immunodeficiencies that are nearly right. The trick is to let the vignette and immunoglobulin pattern do the heavy lifting, then use the distractors to prove to yourself why CVID is the best fit.
Tag: Immunology > Immunodeficiencies
The Q-bank–style vignette
A 22-year-old woman has had multiple episodes of sinopulmonary infections (sinusitis and pneumonia) over the past 2 years. She also reports chronic watery diarrhea and 5-lb weight loss. She has no history of severe infections as a child. Physical exam is notable for mild cervical lymphadenopathy. Labs show:
- IgG: low
- IgA: low
- IgM: low
- B-cell count: normal
- Poor response to prior vaccinations (low specific antibody titers)
Which diagnosis best explains these findings?
Correct answer: Common variable immunodeficiency (CVID)
Why CVID fits best
CVID is characterized by:
- Hypogammaglobulinemia (classically low IgG plus low IgA and/or IgM)
- Normal (or sometimes reduced) B-cell numbers, but impaired B-cell differentiation into plasma cells
- Poor vaccine responses (low specific antibody production)
- Later presentation (often adolescence or adulthood) compared with many congenital immunodeficiencies
Typical clinical picture (what Step questions love)
Recurrent sinopulmonary infections with encapsulated bacteria:
- Streptococcus pneumoniae
- Haemophilus influenzae
Chronic GI disease
- Giardia lamblia (classic association → chronic diarrhea, malabsorption)
- Sometimes Campylobacter, norovirus, etc.
Noninfectious associations (high-yield)
- Autoimmune disease (ITP, autoimmune hemolytic anemia, autoimmune thyroid disease)
- Lymphadenopathy/splenomegaly (benign lymphoproliferation)
- Increased risk of lymphoma and gastric cancer
Pathophysiology in one line
Defective B-cell maturation → ↓ plasma cells → ↓ antibody production → low immunoglobulins + poor opsonization.
Key labs to memorize
| Feature | CVID |
|---|---|
| Age at presentation | Variable; commonly teens/adults |
| Immunoglobulins | ↓ IgG, often ↓ IgA and/or ↓ IgM |
| B cells | Often normal number, but dysfunctional |
| Vaccine response | Poor (low specific titers) |
| Lymph nodes/tonsils | Often present/enlarged (unlike XLA) |
Management (Step-relevant)
- IVIG replacement (or subcutaneous IG)
- Treat infections promptly; sometimes prophylactic antibiotics
- Avoid live vaccines if significant humoral immunodeficiency is present (question-dependent)
Why every other answer choice is wrong (and how to spot it fast)
Below are common distractors that show up in CVID questions. Use them as a checklist.
Distractor 1: X-linked (Bruton) agammaglobulinemia
Why it’s tempting: recurrent bacterial/enteroviral infections + low immunoglobulins.
Why it’s wrong here:
- Timing: presents after 6 months of age (loss of maternal IgG), typically in male infants, not a 22-year-old woman.
- B cells: absent/very low CD19+ B cells.
- Physical exam: absent tonsils and lymph nodes (no germinal centers).
High-yield discriminator
- CVID: B cells present but not functioning
- XLA: B cells absent due to BTK mutation (failure of pre-B → mature B)
Distractor 2: Selective IgA deficiency
Why it’s tempting: recurrent sinopulmonary infections + diarrhea/giardiasis.
Why it’s wrong here:
- Selective IgA deficiency has isolated low IgA with normal IgG and IgM.
- Many patients are asymptomatic; when symptomatic, infections tend to be mucosal (resp/GI).
- Vaccine responses are typically normal (IgG-mediated).
High-yield extras
- Associated with anaphylaxis after blood transfusion (anti-IgA antibodies).
- Increased risk of autoimmune disease (e.g., celiac).
Quick rule
- If IgG is low, it’s not “selective” IgA deficiency.
Distractor 3: Hyper-IgM syndrome (class switch recombination defect)
Why it’s tempting: recurrent infections and abnormal immunoglobulins.
Why it’s wrong here:
- Hyper-IgM classically has ↑/normal IgM with ↓ IgG, IgA, IgE due to impaired class switching.
- Often presents earlier in life.
- Key association: opportunistic infections (e.g., Pneumocystis jirovecii, Cryptosporidium) because many forms also impair macrophage activation (especially CD40L deficiency on Th cells).
High-yield discriminator
- CVID: all low (IgG low + IgA/IgM low)
- Hyper-IgM: IgM high
Mechanism clue
- Most classic: X-linked CD40L mutation (Th cell can’t signal B cell) → no class switching, no germinal centers.
Distractor 4: Severe combined immunodeficiency (SCID)
Why it’s tempting: “immunodeficiency” with infections.
Why it’s wrong here:
- SCID presents in early infancy with severe recurrent infections: bacterial, viral, fungal, protozoal.
- Classic findings: chronic diarrhea, thrush, failure to thrive.
- Absent thymic shadow on CXR (T-cell deficiency).
- Low T cells (and often B cell dysfunction too).
High-yield discriminator
- SCID = T-cell problem first (combined)
- CVID = primarily humoral problem with later presentation
Distractor 5: DiGeorge syndrome (22q11 deletion)
Why it’s tempting: T-cell issues can cause infections; sometimes “immunology” distractor.
Why it’s wrong here:
- DiGeorge is a thymic aplasia → T-cell deficiency.
- Comes with a classic triad:
- Cardiac defects (conotruncal: truncus arteriosus, TOF)
- Abnormal facies
- Thymic aplasia → low T cells
- Plus hypocalcemia (low PTH) → tetany/seizures
- The vignette focuses on hypogammaglobulinemia + poor vaccine response in an adult.
High-yield clue
- If you see hypocalcemia + conotruncal defect, think DiGeorge—not CVID.
Distractor 6: Chronic granulomatous disease (CGD)
Why it’s tempting: recurrent infections, “immune defect.”
Why it’s wrong here:
- CGD is a phagocyte NADPH oxidase defect → impaired respiratory burst.
- Hallmark infections: catalase-positive organisms (PLACESS):
- Pseudomonas, Listeria, Aspergillus, Candida, E. coli, Staph aureus, Serratia
- Not a hypogammaglobulinemia disorder; immunoglobulin levels are typically normal.
High-yield tests
- Abnormal dihydrorhodamine (DHR) flow cytometry (preferred) or NBT test.
Distractor 7: Chediak-Higashi syndrome
Why it’s tempting: recurrent infections.
Why it’s wrong here:
- Defective microtubule function → impaired phagolysosome fusion.
- Clues: partial albinism, peripheral neuropathy, nystagmus; pyogenic infections (often staph/strep).
- Again, not characterized by low immunoglobulins across the board.
The “pattern” that should trigger CVID on test day
1) Timing
- Not severe infant infections
- Yes: recurrent issues beginning in teens/adulthood
2) Type of infections
- Sinopulmonary (encapsulated bacteria)
- GI infections, especially Giardia
3) Labs
- Low IgG (must-have)
- Often low IgA and low IgM
- B cells present but poor vaccine response
If those three pillars are present, CVID should jump to the top of your differential.
Ultra–high-yield takeaways (rapid review)
- CVID = most common symptomatic primary immunodeficiency in adults
- ↓ IgG + ↓ IgA and/or ↓ IgM
- Normal B-cell count, but impaired plasma cell differentiation
- Poor response to vaccines (low specific antibody titers)
- Recurrent sinopulmonary infections (S. pneumo, H. flu) + Giardia diarrhea
- Associated with autoimmune disease and increased risk of lymphoma