# Clinical Cases: B Cells and Antibodies

## Case 1: X-Linked Agammaglobulinemia (Bruton's)

### Patient Demographics
- **Age:** 18 months old
- **Sex:** Male
- **Ethnicity:** Caucasian

### Chief Complaint
Recurrent ear infections and pneumonia

### History of Present Illness
An 18-month-old boy is referred to pediatric immunology after his fourth episode of pneumonia. He has had multiple ear infections since 7 months of age, requiring at least 10 courses of antibiotics. He was healthy for the first 6 months of life. His current pneumonia was caused by Streptococcus pneumoniae. He has recovered normally from viral illnesses including RSV bronchiolitis.

### Past Medical History
- First 6 months: No significant infections
- Bilateral otitis media: 12+ episodes since 7 months of age
- Pneumonia: 4 episodes (8 mo, 11 mo, 15 mo, current)
- Sinusitis: 3 episodes requiring antibiotics
- No history of opportunistic infections

### Vaccination History
- Received routine vaccines including live vaccines (MMR at 12 months) without adverse events

### Family History
- Maternal uncle died at age 3 from "recurrent infections"
- One healthy older sister

### Physical Examination
- **General:** Small-appearing child, in mild respiratory distress
- **Vital Signs:** T 38.5C, HR 130, RR 32, SpO2 94% on room air
- **Growth:** Weight 10th percentile, Height 25th percentile
- **HEENT:** Bilateral TM erythema, **tonsils not visible** (absent)
- **Lymphatics:** **No palpable lymph nodes** (cervical, axillary, inguinal)
- **Lungs:** Decreased breath sounds right middle lobe, crackles
- **Abdomen:** No hepatosplenomegaly

### Laboratory Workup
| Test | Result | Reference Range |
|------|--------|-----------------|
| WBC | 12,500/uL | 6,000-17,500/uL |
| Lymphocytes | 45% | 40-70% |
| IgG | <20 mg/dL | 345-1236 mg/dL |
| IgA | <5 mg/dL | 14-159 mg/dL |
| IgM | <10 mg/dL | 43-207 mg/dL |
| **CD19+ B cells** | **<1%** (<0.1% of lymphocytes) | 14-33% |
| CD3+ T cells | Normal | -- |
| CD4+ T cells | Normal | -- |
| CD8+ T cells | Normal | -- |
| **BTK protein** | **Absent** (by flow cytometry) | Present |
| BTK gene | Pathogenic mutation | -- |

### Clinical Image
![Absent Tonsils - XLA](https://upload.wikimedia.org/wikipedia/commons/thumb/d/d5/Grade_1_tonsils.jpg/220px-Grade_1_tonsils.jpg)

*Image for comparison showing normal tonsils; in XLA, tonsils are characteristically absent. Image source: Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Grade_1_tonsils.jpg). Licensed under CC BY 3.0.*

### Diagnosis
**X-Linked Agammaglobulinemia (Bruton's Agammaglobulinemia)** due to BTK gene mutation.

### Discussion
XLA results from mutations in BTK (Bruton's tyrosine kinase), which is essential for signaling through the pre-B cell receptor. Without BTK, B cell development arrests at the pro-B to pre-B cell transition, resulting in:
- Virtual absence of mature B cells in peripheral blood (<1%)
- Profoundly low immunoglobulin levels (all classes)
- Absent lymphoid tissue (tonsils, lymph nodes) because B cells are required to populate these structures

Key clinical features:
- **Onset after 6 months** (when maternal IgG wanes)
- **Recurrent bacterial infections** with encapsulated organisms (Streptococcus pneumoniae, Haemophilus influenzae)
- **Normal recovery from most viral infections** (T cell immunity intact, though enteroviruses can cause chronic infection)
- **Absent tonsils and lymph nodes** on physical exam - important diagnostic clue
- **X-linked inheritance** (maternal uncle affected)

Distinguishing from CVID:
- XLA: Absent B cells, childhood onset, absent lymphoid tissue
- CVID: Normal B cells (dysfunction), later onset, lymphoid tissue present

### Treatment
1. **Immunoglobulin replacement:** IVIG or SCIG - lifelong, with goal trough IgG >500 mg/dL
2. **Aggressive antibiotic therapy:** For breakthrough infections
3. **Avoid live vaccines:** Although T cell-mediated immunity is normal, live viral vaccines should be avoided
4. **Prophylactic antibiotics:** May be considered for patients with recurrent infections despite adequate Ig replacement
5. **Monitoring:** Watch for chronic enteroviral infection (meningoencephalitis), bronchiectasis

---

## Case 2: Hyper-IgM Syndrome (CD40 Ligand Deficiency)

### Patient Demographics
- **Age:** 3 years old
- **Sex:** Male
- **Ethnicity:** African American

### Chief Complaint
Pneumonia and chronic diarrhea

### History of Present Illness
A 3-year-old boy presents with cough, fever, and respiratory distress for 5 days. He has had recurrent respiratory infections since infancy and chronic watery diarrhea for 1 year. His mother notes he has had several episodes of oral sores. BAL from previous pneumonia at age 2 grew Pneumocystis jirovecii.

### Past Medical History
- Pneumocystis jirovecii pneumonia at age 2
- Multiple episodes of bacterial sinusitis and otitis media
- Chronic diarrhea (1 year duration)
- Recurrent oral ulcers
- Neutropenia noted intermittently

### Family History
- Maternal uncle with recurrent infections, died at age 8

### Physical Examination
- **General:** Thin, ill-appearing child
- **Vital Signs:** T 39.0C, HR 140, RR 40, SpO2 90% on room air
- **Growth:** Weight 5th percentile
- **HEENT:** Oral ulcers, tonsils present but small
- **Lymphatics:** Palpable cervical lymph nodes
- **Lungs:** Bilateral crackles, increased work of breathing
- **Abdomen:** Hepatomegaly, no splenomegaly

### Laboratory Workup
| Test | Result | Reference Range |
|------|--------|-----------------|
| WBC | 5,500/uL | 6,000-17,500/uL |
| ANC | 800/uL | 1,500-8,500/uL |
| IgG | 45 mg/dL | 345-1236 mg/dL |
| IgA | <5 mg/dL | 14-159 mg/dL |
| **IgM** | **650 mg/dL** | 43-207 mg/dL |
| IgE | <5 IU/mL | <100 IU/mL |
| CD19+ B cells | 15% (normal) | 14-33% |
| CD3+ T cells | Normal | -- |
| **CD40 ligand expression** | **Absent on activated T cells** | Present |
| Cryptosporidium stool antigen | Positive | Negative |

### Clinical Image
![PCP on CT Chest](https://upload.wikimedia.org/wikipedia/commons/thumb/a/a9/Pneumocystis_jirovecii_chest_X-ray.jpg/220px-Pneumocystis_jirovecii_chest_X-ray.jpg)

*Image showing diffuse bilateral infiltrates characteristic of Pneumocystis pneumonia. Image source: Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Pneumocystis_jirovecii_chest_X-ray.jpg). Licensed under CC BY-SA 4.0.*

### Diagnosis
**X-Linked Hyper-IgM Syndrome (CD40 Ligand Deficiency)** with Pneumocystis pneumonia and Cryptosporidium enteritis.

### Discussion
Hyper-IgM syndromes are characterized by normal or elevated IgM with markedly decreased IgG, IgA, and IgE due to defective class switch recombination. The most common form is X-linked (CD40L deficiency), which causes combined immunodeficiency:

**Why is this a combined immunodeficiency despite being a "B cell" problem?**
- CD40L (CD154) on T cells must engage CD40 on B cells for class switch recombination
- CD40L-CD40 interaction is ALSO required for T cell-mediated macrophage activation
- Without this interaction, macrophages cannot effectively kill intracellular pathogens

**Clinical features differentiating from pure antibody deficiency:**
- Opportunistic infections (PJP, Cryptosporidium, CMV) - suggests T cell/macrophage dysfunction
- Sclerosing cholangitis from Cryptosporidium (major cause of morbidity/mortality)
- Neutropenia (mechanism unclear)

**Contrast with Type 2 Hyper-IgM (AID deficiency):**
- AID (activation-induced cytidine deaminase) is required for class switch recombination in B cells
- Type 2 affects ONLY B cells, not T cell-macrophage interaction
- Type 2 presents with bacterial infections only, NOT opportunistic infections

### Treatment
1. **Immunoglobulin replacement:** IVIG lifelong
2. **PJP prophylaxis:** TMP-SMX (required, unlike pure antibody deficiency)
3. **Cryptosporidium prevention:** Avoid contaminated water sources; no effective treatment
4. **G-CSF:** For symptomatic neutropenia
5. **Hematopoietic stem cell transplantation:** Curative, recommended due to risk of liver disease and malignancy
6. **Liver monitoring:** For sclerosing cholangitis

---

## Case 3: Selective IgA Deficiency with Transfusion Reaction

### Patient Demographics
- **Age:** 45 years old
- **Sex:** Female
- **Ethnicity:** Caucasian

### Chief Complaint
Severe reaction during blood transfusion

### History of Present Illness
A 45-year-old woman with a history of menorrhagia and iron deficiency anemia unresponsive to oral iron presents for elective hysterectomy. Preoperative hemoglobin was 8.5 g/dL. During surgery, she received 2 units of packed red blood cells. Approximately 15 minutes into the second unit, she developed urticaria, wheezing, hypotension (BP 70/40), and tachycardia. The transfusion was stopped immediately. She required epinephrine, fluids, and steroids for stabilization.

### Past Medical History
- Iron deficiency anemia (5 years)
- Recurrent sinusitis (2-3 episodes per year)
- History of "allergic reaction" to blood transfusion 10 years ago during cesarean section
- Celiac disease (diagnosed 3 years ago)
- Autoimmune thyroiditis

### Family History
- Daughter with celiac disease
- No known immunodeficiency

### Physical Examination (Post-reaction, stabilized)
- **General:** Anxious but stable
- **Vital Signs:** T 37.2C, HR 95, RR 18, BP 110/70 (after treatment)
- **Skin:** Resolving urticaria on trunk and extremities
- **Lungs:** Mild scattered wheezes
- **Cardiovascular:** Regular rhythm, no murmurs

### Laboratory Workup
| Test | Result | Reference Range |
|------|--------|-----------------|
| **IgA** | **<5 mg/dL** | 70-400 mg/dL |
| IgG | 1050 mg/dL | 700-1600 mg/dL |
| IgM | 120 mg/dL | 40-230 mg/dL |
| **Anti-IgA antibodies** | **Positive (IgE and IgG class)** | Negative |
| Tryptase (during reaction) | 25 ng/mL | <11 ng/mL |
| Direct antiglobulin test | Negative | Negative |
| Hemoglobin (post-transfusion) | 8.8 g/dL | 12-16 g/dL |

### Clinical Image
![Transfusion Reaction - Urticaria](https://upload.wikimedia.org/wikipedia/commons/thumb/4/4c/EMminor2010.JPG/220px-EMminor2010.JPG)

*Image showing urticarial reaction pattern that can occur during anaphylactic transfusion reactions. Image source: Wikimedia Commons (https://commons.wikimedia.org/wiki/File:EMminor2010.JPG). Licensed under CC BY-SA 3.0.*

### Diagnosis
**Selective IgA Deficiency** with anti-IgA antibody-mediated anaphylactic transfusion reaction.

### Discussion
Selective IgA deficiency is the most common primary immunodeficiency, affecting approximately 1:500 individuals of European descent. It is defined as:
- Serum IgA <7 mg/dL
- Normal IgG and IgM
- Age >4 years (to exclude developmental delay)

**Clinical spectrum:**
- **Most patients are asymptomatic** (85-90%)
- Increased upper and lower respiratory infections
- GI infections, Giardia susceptibility
- Associated autoimmune diseases (celiac disease, autoimmune thyroiditis, SLE)

**Transfusion reactions:**
- Some IgA-deficient patients develop anti-IgA antibodies
- IgE class anti-IgA antibodies can cause anaphylaxis when exposed to IgA in transfused blood products
- Risk is estimated at 1:20,000-1:50,000 transfusions in IgA-deficient patients

**Why did the second transfusion cause the reaction?**
- The first unit may have provided sensitization or boosted existing anti-IgA
- Anaphylaxis occurred when more IgA was introduced with the second unit
- Her previous "allergic reaction" during C-section was likely the same mechanism

### Treatment
1. **Acute anaphylaxis management:** Epinephrine, antihistamines, steroids, fluids
2. **Future transfusions:**
   - Use IgA-deficient blood products (from IgA-deficient donors)
   - If unavailable, use washed cellular blood products (removes plasma IgA)
   - Premedication with antihistamines/steroids is NOT sufficient alone
3. **Medical alert:** Patient should wear medical alert identification
4. **IVIG is contraindicated:** Standard IVIG contains IgA; if Ig replacement needed, must use IgA-depleted products
5. **Monitor associated conditions:** Celiac disease, autoimmune thyroiditis
6. **No specific treatment for IgA deficiency itself:** Ig replacement not indicated unless concomitant IgG subclass deficiency with recurrent infections