15.3 Drug Allergy, Desensitization & Adult Immunodeficiency

1. Drug Allergy & Desensitization

Penicillin Allergy & Cephalosporin Cross-Reactivity

• De-labeling Penicillin Allergy: Approximately 10% of the US population carries a documented penicillin allergy label; however, > 90% to 95% of these patients are NOT truly allergic when evaluated with formal Penicillin Skin Testing (PST) or direct oral Amoxicillin challenge. Mechanisms for false labeling include waning of drug-specific IgE antibodies over time (~10% per year) and childhood misattribution of viral exanthems as drug allergies.
• Cephalosporin Cross-Reactivity Biology: Cross-reactivity between penicillins and cephalosporins is determined primarily by the chemical structure of the R1 side chain, rather than the shared core four-membered beta-lactam ring.
  • First- and Second-Generation Cephalosporins (e.g., Cefazolin, Cephalexin, Cefaclor): Share similar R1 side chains with aminopenicillins; historical cross-reactivity is low (~1% to 2%). (Note: Cefazolin has a unique side chain and does not share R1 with ampicillin/amoxicillin).
  • Third-, Fourth-, and Fifth-Generation Cephalosporins (e.g., Ceftriaxone, Cefepime, Ceftazidime, Ceftaroline): Have distinct, disparate R1 side chains and demonstrate < 1% cross-reactivity with penicillins.
• Clinical Practice Algorithms:
  • Mild, Non-IgE Cutaneous Reactions (e.g., delayed maculopapular rash): All cephalosporins and carbapenems can be safely administered without prior skin testing.
  • Severe IgE-Mediated History (Anaphylaxis, Angioedema, Bronchospasm): Use a 3rd/4th generation cephalosporin with a completely distinct R1 side chain (e.g., Ceftriaxone or Cefepime), or perform Penicillin Skin Testing (PST) followed by an oral amoxicillin challenge.
  • Carbapenems (Meropenem) & Monobactams (Aztreonam): Cross-reactivity with penicillin is < 1%. Aztreonam does not cross-react with penicillins; however, Aztreonam shares an identical R1 side chain with Ceftazidime (do NOT administer aztreonam to patients with true ceftazidime allergy).

Drug Desensitization Protocols

• Definition & Mechanism: Temporary induction of clinical tolerance to a drug responsible for an IgE-mediated (Type I) immediate hypersensitivity reaction or aspirin-induced respiratory disease. Performed by administering minute, subthreshold doses of the drug (starting at 1:1,000 to 1:10,000 of the target therapeutic dose) and doubling the dose every 15 to 30 minutes across 12 to 16 sequential steps under continuous cardiac/ICU monitoring.
• Mechanism: Gradual, progressive, subthreshold antigen exposure induces slow intracellular depletion of mast cell/basophil granules and downregulates surface IgE receptors without triggering massive systemic degranulation.
• High-Yield Clinical Indications:
  1. Neurosyphilis, Congenital Syphilis, or Syphilis in Pregnancy: Penicillin G is the only proven curative bactericidal therapy (doxycycline cannot cross the placental barrier and does not achieve adequate CSF concentrations; desensitization to Penicillin G is mandatory in penicillin-allergic pregnant patients with syphilis).
  2. Severe Bacterial Infections with No Therapeutic Alternative: Enterococcus faecalis infective endocarditis (requires Ampicillin/Penicillin G).
  3. Aspirin-Exacerbated Respiratory Disease (AERD / Samter's Triad): Asthma, chronic rhinosinusitis with nasal polyps, and aspirin sensitivity. Aspirin desensitization improves asthma control, reduces polyp recurrence, and allows antiplatelet therapy after PCI.
  4. Critical Chemotherapy / Biologic Regimens: Platinum compounds (Carboplatin, Cisplatin), Taxanes, or Monoclonal antibodies (Rituximab) in patients with allergic reactions when alternative regimens are inferior.
• ABSOLUTE CONTRAINDICATIONS to Desensitization: Severe, non-IgE, T-cell-mediated Severe Cutaneous Adverse Reactions (SCAR), including:
  • Stevens-Johnson Syndrome (SJS) / Toxic Epidermal Necrolysis (TEN)
  • Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS)
  • Acute Generalized Exanthematous Pustulosis (AGEP)
  • Drug-induced liver injury, hemolytic anemia, or interstitial nephritis.
• Durability Rule: Desensitization produces temporary tolerance only. If the drug is interrupted for > 24 to 48 hours (or > 4–5 drug half-lives), mast cells resynthesize mediators and reload surface IgE; the patient reverts to full allergic sensitivity and requires repeat desensitization.

2. Adult Primary & Secondary Immunodeficiencies

Immunodeficiency in adult patients frequently presents subtly with recurrent, severe, or unusual sinopulmonary and gastrointestinal infections, bronchiectasis, autoimmune cytopenias, or granulomatous organ infiltration.

Primary Immunodeficiencies in Adults

1. Common Variable Immunodeficiency (CVID)

• Epidemiology & Genetics: The most common clinically significant primary immunodeficiency diagnosed in adulthood (bimodal presentation: childhood and young adulthood, ages 20 to 40 years). Polygenic and heterogeneous; characterized by defective terminal B-cell differentiation into antibody-secreting plasma cells.
• Diagnostic Criteria:
  1. Markedly reduced serum IgG (< 400–500 mg/dL).
  2. Markedly reduced serum IgA and/or IgM (at least 2 standard deviations below age-adjusted normal values).
  3. Impaired vaccine antibody response (demonstrated failure to generate protective specific IgG titers following polysaccharide vaccines [Pneumovax 23] or protein vaccines [Tetanus/Diphtheria toxoids]).
  4. Exclusion of secondary causes of hypogammaglobulinemia.
  • (Flow Cytometry: Total peripheral CD19+/CD20+ B-lymphocyte counts are typically normal or mildly decreased, but class-switched memory B-cells [CD27+IgD-IgM-] and plasma cells are severely reduced).
• Clinical Presentation & Complications:
  • Recurrent Encapsulated Sinopulmonary Infections: Recurrent otitis media, bacterial sinusitis, and pneumonias caused by Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis, leading to irreversible cylindrical bronchiectasis.
  • Gastrointestinal Complications: Chronic watery diarrhea, malabsorption, and weight loss caused by Giardia lamblia, Campylobacter jejuni, Salmonella, or Small Intestinal Bacterial Overgrowth (SIBO). Non-infectious CVID enteropathy mimics celiac sprue (villous blunting and intraepithelial lymphocytosis but absence of plasma cells in the lamina propria on intestinal biopsy).
  • Autoimmune Manifestations (~25–30% of patients): Immune Thrombocytopenia (ITP) and Autoimmune Hemolytic Anemia (AIHA / Evans Syndrome) are the most common autoimmune complications; rheumatoid-like polyarthritis, vitiligo, and pernicious anemia.
  • Granulomatous Disease: Granulomatous-Lymphocytic Interstitial Lung Disease (GLILD) with noncaseating granulomas on lung biopsy (mimics pulmonary sarcoidosis).
  • Malignancy Risk: Markedly elevated risk (30- to 40-fold) of Non-Hodgkin B-Cell Lymphoma and gastric adenocarcinoma.
• First-Line Treatment: Lifelong Immunoglobulin Replacement Therapy:
  • Intravenous Immunoglobulin (IVIG 400 to 600 mg/kg IV every 3 to 4 weeks) OR Subcutaneous Immunoglobulin (SCIG weekly).
  • Target trough serum IgG level: > 700 to 800 mg/dL (or higher [> 900–1000 mg/dL] in patients with established bronchiectasis).

2. Selective IgA Deficiency

• Epidemiology: The most common primary immunodeficiency disorder overall (prevalence ~1 in 300 to 1 in 600 individuals of European ancestry).
• Diagnostic Criteria: Undetectable serum IgA (< 7 mg/dL) with completely normal serum IgG and IgM levels, in a patient > 4 years of age with normal antibody responses to vaccines.
• Clinical Presentation:
  • Most patients (> 80%) are completely asymptomatic and diagnosed incidentally.
  • Symptomatic Patients: Suffer from recurrent mild viral/bacterial upper respiratory tract infections, allergic rhinitis/asthma, atopic eczema, and gastrointestinal infections (particularly Giardia lamblia due to absent mucosal secretory IgA defense).
  • Autoimmune Associations: Strongly associated with Celiac Disease (Crucial Board Pearl: Celiac screening in IgA-deficient patients requires checking IgG-tissue transglutaminase (tTG-IgG) or IgG-deamidated gliadin peptides; standard tTG-IgA screening will yield a false-negative result!).
  • Anaphylactic Transfusion Reactions: Up to 30% to 40% of patients with selective IgA deficiency develop circulating anti-IgA antibodies (specifically IgG- or IgE-class anti-IgA). When transfused with standard blood products containing trace donor IgA (packed red blood cells, platelets, fresh frozen plasma), they can develop sudden, severe, life-threatening IgE-mediated anaphylactic transfusion reactions.
  • Transfusion Management: When blood products are required, administer washed red blood cells (washed with normal saline to remove plasma IgA) or blood products collected from known IgA-deficient donors.

Secondary Immunodeficiencies