title: "Week 3: Immunology & Infectious Disease"
Week 3: Immunology & Infectious Disease
Food Allergy Overview
| Type | Timing | Symptoms | Testing |
|---|---|---|---|
| IgE-mediated | Minutes to 2 hours | Urticaria, angioedema, anaphylaxis | Skin prick, specific IgE |
| Non-IgE-mediated | Hours to days | GI (Gastrointestinal) symptoms (diarrhoea, vomiting) | Clinical diagnosis only |
Prevalence of food allergy: 1 in 10 children. Overall rising, but may have plateaued.
IgE-Mediated Food Allergy
Mechanism
- Sensitisation: Macrophage → Th2 → B cell → IgE production
- Re-exposure: Allergen + IgE on mast cells → cross-linking → degranulation → histamine release
Severity
| Mild-Moderate | Anaphylaxis |
|---|---|
| Lip/face/eye swelling | Difficult/noisy breathing |
| Hives, welts | Tongue/throat swelling |
| Tingling mouth | Wheeze, persistent cough |
| Abdominal pain, vomiting | Hoarse voice, collapse |
Adrenaline autoinjector (AU/NZ) dosing (by weight): 0.15 mg for 7.5–20 kg; 0.3 mg for >20 kg. (Also remember IM adrenaline is 0.01 mg/kg to a max of 0.5 mg.)
Source: ASCIA anaphylaxis guidelines.
Risk Factors for Severe Reaction
- Older child
- Higher allergen dose
- Poorly controlled asthma
- Nuts, shellfish, fish allergens
- Cofactors: exercise, NSAIDs, alcohol, sleep deprivation, ACE-I, beta-blockers
Investigations
Skin prick testing alone does NOT diagnose allergy. Sensitisation (positive SPT) can occur without clinical allergy; interpret results in clinical context. Wheal size indicates likelihood, not severity.
Source: ASCIA allergy testing guidance.
Do NOT test: allergen mixes, IgG to foods, random pseudoscience tests. Only test children with history of allergic reaction.
Natural History
| High chance of outgrowing | Low chance of outgrowing |
|---|---|
| Milk, egg, soy, wheat | Peanut, tree nut, fish, shellfish |
LEAP Study (2015): Early oral introduction of peanuts PREVENTS peanut allergy in high-risk infants. Window of opportunity under age 4.
Non-IgE-Mediated Food Allergy
Cow's Milk Protein Intolerance Spectrum
| Condition | Age | Key Features | Treatment |
|---|---|---|---|
| FPIAP (Proctocolitis) | First months | Blood/mucus in stool, well baby | Elimination |
| FPE (Enteropathy) | Early infancy | Mucus ± blood, may affect growth | Elimination |
| FPIES (Enterocolitis) | 6-12 months | Profuse vomiting 2-4h post-ingestion, lethargy, pallor | Ondansetron, fluids |
FPIES presents with profuse vomiting 2-4 hours after ingestion. Common triggers: rice, cow's milk, soy, oats. Usually outgrown by 2-4 years.
No allergy testing for non-IgE-mediated allergies - diagnosis is clinical!
Eosinophilic Oesophagitis (EoE)
EoE diagnosis: Symptoms of oesophageal dysfunction + ≥15 eosinophils/hpf on biopsy. Endoscopy shows rings, furrows, white exudates.
Presentations:
- Infants: feeding difficulty, FTT, vomiting
- Older children: dysphagia, food impaction
- Adolescents: chest pain when eating
Immunodeficiency
Suspect immunodeficiency with infections that are unusual, opportunistic, disseminated, or recurrent. Also: FTT, chronic diarrhoea, persistent oral Candida.
Types of Immunodeficiency
| Type | Infections | Examples |
|---|---|---|
| Humoral (antibody) | Encapsulated bacteria: S. pneumoniae, N. meningitidis, H. influenzae | XLA, IgA deficiency, CVID |
| T cell/SCID | Opportunistic: PCP, fungal, CMV, EBV | X-linked SCID, ADA deficiency |
| Phagocytic (neutrophil) | Skin/soft tissue, fungal | CGD, leukocyte adhesion deficiency |
| Complement | Neisseria (terminal), bacterial (C2) | C5-C9 deficiency, C2 deficiency |
- Recurrent bacterial → Humoral or complement
- Recurrent Neisseria → Complement (terminal)
- Opportunistic/fungal → T cell/SCID
- Staph, Pseudomonas, Nocardia → Neutrophil (CGD)
SCID
SCID = Severe Combined Immunodeficiency. Lymphopenia (mainly T cells), absent thymus on CXR (Chest X-Ray). Presents with opportunistic infections and FTT.
SCID screening: Newborn bloodspot tests for TRECs (T-cell receptor excision circles). Absent TRECs = absent thymic T-cell production.
Chronic Granulomatous Disease (CGD)
CGD: Neutrophils fail to undergo oxidative burst. Recurrent bacterial and fungal infections of skin, soft tissue, lungs, liver. Granuloma formation.
Leukocyte adhesion deficiency presents with high neutrophil count but inability to form pus. Classic sign: delayed separation of umbilical cord.
Neonatal Sepsis
| Early-Onset (< 7 days) | Late-Onset (> 7 days) |
|---|---|
| Vertical transmission (birth canal) | Horizontal (environment, lines) |
| GBS, E. coli, Listeria | Coag-negative staph, S. aureus, GNB |
Group B Strep (GBS) and E. coli are the most common causes of early-onset neonatal sepsis.
Immunisation
Vaccines are either live attenuated (MMR, varicella, rotavirus, BCG) or inactivated (whole, subunit, toxoid, conjugate).
Contraindications to vaccination: Anaphylaxis to previous dose or vaccine component. Immunocompromised patients should not receive live vaccines.
Vaccines are given at chronological age, not corrected age for premature infants.
Practice Questions
What is the first-line investigation?
What is the most likely diagnosis?