🛡️ Types of immunity −
Active immunity
- Natural: recovery from infection → memory B/T cells
- Artificial: vaccination (attenuated, killed, toxoid, subunit, etc.)
- Long-lasting; provides memory
Passive immunity
- Natural: maternal IgG across placenta; IgA in colostrum
- Artificial: antibody preparations (e.g., antivenin, IVIG, monoclonal antibodies)
- Immediate but temporary; no memory
🧬 Key concept: Primary response: IgM → IgG, 5‑10 days lag. Secondary response: rapid (1‑3 days), high‑affinity IgG, IgA, or IgE due to memory cells.
💉 Vaccine classes −
Live attenuated MMR, VZV, rotavirus
Replicate in host, strong humoral + cellular immunity. Contraindicated in immunocompromised. Usually 1‑2 doses.
Replicate in host, strong humoral + cellular immunity. Contraindicated in immunocompromised. Usually 1‑2 doses.
Killed / inactivated Rabies, IPV, HepA
Chemically inactivated; cannot replicate. Humoral response only; requires boosters.
Chemically inactivated; cannot replicate. Humoral response only; requires boosters.
Toxoid DTaP (diphtheria, tetanus)
Inactivated exotoxins. Prevent disease, not infection. Induce anti‑toxin antibodies.
Inactivated exotoxins. Prevent disease, not infection. Induce anti‑toxin antibodies.
Polysaccharide PPSV23 (adult)
Pure capsular polysaccharide → T‑cell‑independent (IgM only, no memory).
Pure capsular polysaccharide → T‑cell‑independent (IgM only, no memory).
Conjugate PCV13, Hib, MCV4
Polysaccharide + protein carrier → T‑cell‑dependent; class switching, memory, booster response.
Polysaccharide + protein carrier → T‑cell‑dependent; class switching, memory, booster response.
Component (recombinant) HBV, HPV
Immunogenic protein produced in yeast or cell culture. Highly safe, no live component.
Immunogenic protein produced in yeast or cell culture. Highly safe, no live component.
⚠️ Contraindication: Live vaccines (MMR, VZV, rotavirus, LAIV) are not given to severely immunocompromised patients (risk of disseminated disease).
Live non‑attenuated vaccine (unique)
Enteric‑coated adenovirus types 4 & 7 (U.S. military) – causes asymptomatic intestinal infection → mucosal IgA memory; protects against aerosol pneumonia.
🧪 Passive immunotherapy −
- Indications: post‑exposure prophylaxis (e.g., rabies, tetanus), snake/spider antivenin, RSV prophylaxis (palivizumab).
- Risks:
- Anaphylaxis (IgE against foreign proteins)
- Serum sickness (type III hypersensitivity) from immune complexes
- Anti‑allotype responses (even with human Ig)
- Special consideration: IgA‑deficient patients may react to IgA in IVIG; use IgA‑depleted preparations.
🧪 Humanized monoclonal antibodies (e.g., palivizumab against RSV) are engineered to reduce immunogenicity.
🤱 Maternal antibodies & neonatal immunity −
- IgG crosses placenta (active transport) → protects neonate for first few months.
- IgM does not cross; detection in newborn indicates congenital infection (e.g., TORCH).
- IgA in colostrum provides mucosal protection; infant IgA reaches ~20% of adult levels by 12 months.
- Vaccination timing: Live attenuated vaccines (MMR, VZV) are usually given after 12 months because maternal IgG can neutralize the vaccine virus. If given earlier (high‑risk situations), repeat doses are often needed.
- Infants with primary immunodeficiencies typically present after maternal IgG wanes (≈3‑6 months).
Maternal IgG →
Neonatal protection (0‑6 mo) →
IgG declines →
Vaccination & own Ig production
🧫 Bacterial vaccines (selected) −
| Organism | Vaccine | Type |
|---|---|---|
| Corynebacterium diphtheriae | DTaP (diphtheria) | Toxoid |
| Clostridium tetani | DTaP (tetanus) | Toxoid |
| Bordetella pertussis | DTaP (acellular) | Toxoid + filamentous hemagglutinin |
| Haemophilus influenzae type b | Hib | Conjugate (polysaccharide + protein) |
| Streptococcus pneumoniae | PCV13 (pediatric) / PPSV23 (adult) | Conjugate (13 serotypes) / Polysaccharide (23 serotypes) |
| Neisseria meningitidis | MCV4 | Conjugate (serogroups A, C, Y, W‑135) |
💡 High yield: Polysaccharide vaccines (PPSV23) induce IgM only and no memory; conjugate vaccines (PCV13) are T‑cell‑dependent → IgG, affinity maturation, and booster response.
🦠 Viral vaccines −
| Virus | Vaccine | Type |
|---|---|---|
| Rotavirus | RV (oral) | Live attenuated |
| Polio | IPV (Salk) / OPV (Sabin) | Inactivated / Live attenuated |
| Influenza | IIV (injected) / LAIV (intranasal) | Inactivated / Live attenuated |
| Varicella zoster | VAR (chickenpox) / Zoster | Live attenuated |
| Hepatitis A | HepA | Inactivated |
| Hepatitis B | HepB | Component (recombinant HBsAg) |
| HPV | Gardasil 9 (9v) | Component (L1 protein) |
| Measles, Mumps, Rubella | MMR | Live attenuated |
🧬 HPV vaccine: 9‑valent (types 6,11,16,18,31,33,45,52,58) prevents >90% of cervical cancers. Previously quadrivalent (6,11,16,18) covered ~70%.
⭐ High‑yield USMLE facts −
🔬 Primary vs secondary response
- Primary: lag 5‑10 d, low affinity IgM→IgG
- Secondary: lag 1‑3 d, high affinity IgG/IgA/IgE
🧪 Vaccine type memory
- Live & conjugate → T‑cell dependent → memory
- Polysaccharide & toxoid → variable; toxoid induces memory (protein), pure polysaccharide does not
⚠️ Common trap: DTaP contains tetanus and diphtheria toxoids and acellular pertussis components (toxoid + filamentous hemagglutinin) — it is not a live vaccine.
📅 Neonatal vaccination: Live vaccines delayed until >12 months due to maternal IgG interference. Exception: high‑risk exposure, but then boosters are required.
⚠️ Immunocompromised: Avoid live vaccines (MMR, VZV, rotavirus, LAIV, OPV, yellow fever). Killed, toxoid, subunit, and conjugate are generally safe.
✔️ Salk = inactivated polio
✔️ Sabin = live oral polio
✔️ PPSV23 = 23-valent polysaccharide (adult)
✔️ PCV13 = 13-valent conjugate (pediatric)