Vaccines, Toxoids, Passive Immunoglobulins, Antivenoms & Transplant Immunosuppression

Live attenuated vaccines contain weakened, replication-competent microorganisms that have been serially passaged in non-human cell cultures so they lose virulence in healthy humans while retaining full immunogenicity. Examples include Measles-Mumps-Rubella (MMR), Varicella (chickenpox), Rotavirus (oral pentavalent RV5 or monovalent RV1), Intranasal Live Attenuated Influenza (LAIV), Yellow Fever, Adenovirus (military recruits), Oral Polio (Sabin OPV, no longer used in the US due to vaccine-associated paralytic poliomyelitis), Smallpox (vaccinia), and Bacillus Calmette-Guerin (BCG). Because live attenuated viruses infect host cells and synthesize viral proteins endogenously within the cytoplasm, their peptides are loaded onto MHC Class I molecules (activating robust CD8+ cytotoxic T lymphocytes) while extracellular virions are endocytosed into MHC Class II compartments (activating CD4+ helper T cells and germinal center B-cell IgG/IgA production). Consequently, live attenuated vaccines produce potent, durable, often lifelong cellular and humoral immunity without requiring chemical adjuvants. However, because the microbe can revert or replicate unchecked in hosts lacking intact cellular immunity, live attenuated vaccines are strictly CONTRAINDICATED in pregnant women (risk of congenital viral infection) and patients with severe immunodeficiency (SCID, DiGeorge complete thymic aplasia, active chemotherapy, high-dose corticosteroids ≥ 20 mg/day prednisone for ≥ 14 days, or anti-TNF biologics). The classic board exception is HIV: asymptomatic or mildly symptomatic HIV-positive patients CAN safely receive the MMR and Varicella live vaccines provided their CD4+ T-cell count is ≥ 200 cells/mm3 (or CD4+ ≥ 15% in children).

Whole-cell inactivated vaccines—including Inactivated Polio (Salk IPV), Hepatitis A, Rabies, and Intramuscular Quadrivalent Influenza—contain pathogens completely killed by formalin or heat; they cannot replicate or revert to virulence and are completely safe in immunocompromised and pregnant patients, but they are processed predominantly via the exogenous MHC Class II pathway, generating primarily humoral IgG titers that require periodic booster doses. Subunit and recombinant protein vaccines isolate only the protective neutralizing surface antigen: Hepatitis B vaccine contains recombinant HBsAg produced in yeast; Human Papillomavirus (HPV 9-valent) vaccine contains self-assembling L1 major capsid protein Virus-Like Particles (VLPs covering low-risk types 6, 11 and high-risk oncogenic types 16, 18, 31, 33, 45, 52, 58); Acellular Pertussis (aP) contains pertussis toxoid and filamentous hemagglutinin; and Recombinant Zoster Vaccine (Shingrix, RZV) contains varicella-zoster glycoprotein E paired with the potent AS01B liposomal adjuvant (given as a 2-dose series to adults ≥ 50 years or immunocompromised adults ≥ 19 years). Bacterial Toxoid vaccines—Diphtheria toxoid and Tetanus toxoid (combined in DTaP for children less than 7 years and Tdap/Td for adolescents/adults)—consist of formaldehyde-denatured exotoxins that retain antigenic epitopes without toxicity, eliciting neutralizing anti-toxin IgG antibodies (which wane over time, necessitating a Td or Tdap booster every 10 years, plus a Tdap dose during every pregnancy between 27 and 36 weeks gestation to pass maternal anti-pertussis IgG across the placenta). Finally, mRNA vaccines (SARS-CoV-2 spike mRNA encapsulated in lipid nanoparticles) deliver mRNA into host cytoplasm where host ribosomes translate the antigen endogenously, engaging both MHC Class I (CD8+ T cells) and MHC Class II (CD4+ TFH and neutralizing IgG) without any live virus risk.

Conjugate vaccines solve the infant polysaccharide non-responsiveness problem through the immunological Hapten-Carrier effect. In a conjugate vaccine—such as Pneumococcal Conjugate Vaccine (PCV15 or PCV20), Haemophilus influenzae type b conjugate (Hib), and Meningococcal Quadrivalent Conjugate (MenACWY)—the bacterial capsular polysaccharide (acting as the hapten-like B-cell epitope) is covalently linked to a highly immunogenic carrier protein, such as CRM197 (a non-toxic mutant diphtheria toxin), tetanus toxoid, or meningococcal outer membrane protein complex. When a naive B cell whose B-cell receptor (BCR) is specific for the bacterial polysaccharide binds the intact polysaccharide-protein conjugate, it internalizes the entire complex via receptor-mediated endocytosis. Inside the B-cell endolysosome, proteases cleave the attached carrier protein into peptide fragments and load those carrier peptides onto MHC Class II molecules! The B cell then displays the carrier peptide:MHC Class II complex on its surface to a CD4+ follicular helper T cell (TFH) that recognizes the carrier peptide. The activated CD4+ TFH cell engages CD40 on the B cell (via CD40L) and secretes IL-4, IL-21, and IFN-gamma, driving the polysaccharide-specific B cell to undergo germinal center somatic hypermutation, isotype class switching from IgM to high-affinity opsonizing IgG, and differentiation into long-lived memory B cells—even in 2-month-old infants!

Every clinical board candidate must master the age- and risk-stratified schedules for encapsulated bacterial vaccines: (1) Haemophilus influenzae type b (Hib conjugate): administered routinely to all infants at 2, 4, 6, and 12-15 months of age (preventing epiglottitis and meningitis). (2) Pneumococcal vaccination: all infants receive a 4-dose series of PCV15 or PCV20 at 2, 4, 6, and 12-15 months; for adults aged ≥ 65 years (or adults aged 19-64 years with chronic heart/lung/liver disease, diabetes, alcoholism, cochlear implants, CSF leaks, HIV, immunocompromise, or asplenia), current ACIP guidelines recommend either a single dose of PCV20 alone, OR one dose of PCV15 followed ≥ 1 year later by PPSV23 (shortened to ≥ 8 weeks in immunocompromised, asplenic, CSF leak, or cochlear implant patients). (3) Meningococcal vaccination: routine adolescent vaccination uses the quadrivalent conjugate vaccine MenACWY at age 11-12 years with a booster at age 16 years. Notably, Neisseria meningitidis serogroup B (MenB) cannot be formulated as a polysaccharide conjugate because the serogroup B capsular polysaccharide (alpha-2,8-linked sialic acid) is structurally identical to human neural cell adhesion molecule (NCAM), which would trigger autoimmune cross-reactivity or immune tolerance; therefore, MenB vaccines (Bexsero, Trumenba) are formulated as recombinant outer membrane proteins given to high-risk patients (asplenia, complement deficiency, eculizumab use, microbiologists) and via shared decision-making at ages 16-23.

Four infectious post-exposure prophylaxis (PEP) algorithms are heavily tested on board examinations: (1) Rabies PEP: indicated immediately after any bite, scratch, or mucous membrane exposure from a rabid or wild carnivore (raccoon, skunk, fox, coyote) or any physical contact with a bat (including discovering a bat in the room of a sleeping adult, infant, or intoxicated/mentally impaired person where an undetected bite cannot be excluded). In an unvaccinated person, perform thorough soap-and-water wound irrigation, infiltrate Human Rabies Immune Globulin (HRIG) directly into and around the wound margins, and administer 4 doses of inactivated Rabies Vaccine intramuscularly in the deltoid on Days 0, 3, 7, and 14 (plus Day 28 if immunocompromised). (2) Tetanus PEP: depends on wound cleanliness and whether the patient has completed ≥ 3 prior tetanus toxoid doses. For a clean, minor wound, give Tetanus toxoid (Tdap/Td) ONLY if less than 3 prior doses or if the last dose was ≥ 10 years ago (never needs TIG). For a dirty, puncture, crush, burn, or saliva/soil-contaminated wound, give Tetanus toxoid (Tdap/Td) if less than 3 prior doses or if the last dose was ≥ 5 years ago, AND add Tetanus Immune Globulin (TIG, passive anti-tetanospasmin antibody) ONLY if the patient has had less than 3 prior doses or unknown vaccination history! (3) Perinatal Hepatitis B PEP: every neonate born to an HBsAg-positive mother must receive both Hepatitis B Vaccine AND Hepatitis B Immune Globulin (HBIG) in opposite thighs within 12 hours of birth. (4) Varicella PEP: susceptible immunocompetent patients exposed to chickenpox receive the active Varicella vaccine within 3-5 days, whereas exposed pregnant women, immunocompromised patients, and premature neonates (where live vaccine is contraindicated) receive Varicella-Zoster Immune Globulin (VariZIG) within 10 days (or oral acyclovir/valacyclovir if VariZIG is unavailable).

Beyond infectious PEP, specialized passive antibody preparations prevent hemolytic alloimmunization, infant viral bronchiolitis, and neuroparalytic toxin lethality. Rho(D) Immune Globulin (RhoGAM) consists of pooled human anti-D IgG antibodies administered to Rh(D)-negative, indirect Coombs-negative pregnant women at 28 weeks gestation and again within 72 hours after delivery of an Rh(D)-positive infant (as well as after any potential fetomaternal hemorrhage event: spontaneous or induced abortion, ectopic pregnancy, molar pregnancy, chorionic villus sampling, amniocentesis, abdominal trauma, or external cephalic version). Anti-D IgG coats any Rh(D)-positive fetal erythrocytes that entered the maternal circulation and clears them via splenic macrophages before naive maternal B cells can recognize the D antigen and form memory anti-D IgG (which would cross the placenta in subsequent pregnancies and cause hydrops fetalis). Note: Once a mother is already alloimmunized (positive anti-D antibodies on indirect Coombs screen from a prior pregnancy), RhoGAM is completely ineffective and not indicated. For Respiratory Syncytial Virus (RSV) prevention, Nirsevimab is a long-acting monoclonal antibody against the prefusion RSV F (fusion) protein given as a single dose to all infants less than 8 months entering their first RSV season (unless the mother received maternal RSVpreF vaccine [Abrysvo] at 32-36 weeks gestation ≥ 14 days prior to birth), while Palivizumab is a monthly anti-RSV F-protein monoclonal reserved for high-risk premature infants or congenital heart/lung disease. Finally, Botulism is treated emergently with Heptavalent Botulinum Antitoxin (HBAT, equine) in adults/children or Human Botulism Immune Globulin (BIG-IV / BabyBIG) in infant botulism, which binds circulating extracellular botulinum toxin before it enters presynaptic cholinergic terminals.

When an allogeneic MHC molecule engages the T-cell receptor (TCR/CD3 complex, Signal 1), intracellular calcium rises and activates calmodulin and the serine/threonine phosphatase Calcineurin. Calcineurin dephosphorylates cytoplasmic NFAT (Nuclear Factor of Activated T cells), allowing NFAT to translocate into the nucleus and transcribe Interleukin-2 (IL-2). Cyclosporine (a cyclic polypeptide that binds intracellular Cyclophilin) and Tacrolimus (FK506, a macrolide that binds intracellular FKBP-12) both form drug-immunophilin complexes that directly inhibit Calcineurin, halting NFAT nuclear translocation and IL-2 transcription. Both calcineurin inhibitors cause dose-limiting acute and chronic Nephrotoxicity (via endothelin-mediated vasoconstriction of the renal afferent arteriole and striped tubulointerstitial fibrosis), hypertension, hyperkalemia, hypomagnesemia, hyperuricemia/gout, and neurotoxicity (fine resting tremor, paresthesias, Posterior Reversible Encephalopathy Syndrome [PRES]). However, their cosmetic and metabolic profiles differ sharply: Cyclosporine uniquely causes gingival hyperplasia and hirsutism (leading clinicians to prefer Tacrolimus in most modern regimens), whereas Tacrolimus carries a significantly higher incidence of post-transplant Diabetes Mellitus (direct pancreatic beta-cell insulin suppression) and alopecia. In contrast, Sirolimus (Rapamycin) and Everolimus also bind intracellular FKBP-12, but the Sirolimus-FKBP-12 complex does NOT inhibit calcineurin! Instead, it inhibits mTOR (mechanistic Target of Rapamycin) kinase downstream of the IL-2 receptor (Signal 3), arresting T and B cells at the G1-to-S phase transition in response to IL-2. Consequently, Sirolimus is NOT intrinsically nephrotoxic (making it a key calcineurin-sparing option in transplant recipients developing CNI nephrotoxicity), though its signature toxicities are severe hypertriglyceridemia/hyperlipidemia, bone marrow suppression (thrombocytopenia, leukopenia), oral aphthous ulcers, interstitial pneumonitis, and impaired surgical wound healing.

For the antimetabolite component of triple therapy, Mycophenolate Mofetil (MMF, or enteric-coated mycophenolate sodium) has largely replaced azathioprine. Active mycophenolic acid reversibly and noncompetitively inhibits Inosine Monophosphate Dehydrogenase (IMPDH), the rate-limiting enzyme in de novo purine (guanosine monophosphate) synthesis. Because proliferating T and B lymphocytes lack an effective purine salvage pathway and depend entirely on de novo purine synthesis, MMF selectively arrests lymphocyte replication without nephrotoxicity; its principal toxicities are dose-limiting gastrointestinal distress (watery diarrhea, vomiting), leukopenia/neutropenia, increased risk of CMV viremia, and severe first-trimester teratogenicity (ear/facial malformations and spontaneous abortion—must be switched to azathioprine ≥ 6 weeks prior to conception). Azathioprine is a prodrug cleaved non-enzymatically to 6-mercaptopurine (6-MP), which is converted to 6-thioguanine nucleotides that incorporate into DNA/RNA and inhibit de novo purine synthesis. Crucially, 6-MP is inactivated by two enzymes: Thiopurine Methyltransferase (TPMT, where genetic deficiency causes severe myelosuppression) and Xanthine Oxidase (XO). Therefore, administering Allopurinol or Febuxostat (xanthine oxidase inhibitors for gout) to a transplant recipient taking Azathioprine blocks 6-MP degradation and triggers life-threatening pancytopenia! Finally, adding any CYP3A4 inhibitor (voriconazole, fluconazole, ketoconazole, erythromycin, clarithromycin, diltiazem, verapamil, ritonavir, or grapefruit juice) to Tacrolimus, Cyclosporine, or Sirolimus precipitates acute drug toxicity (nephrotoxicity, tremor, hyperkalemia), whereas adding a CYP3A4 inducer (rifampin, phenytoin, carbamazepine, phenobarbital, or St. John's wort) plummets levels and precipitates acute allograft rejection.

Hyperacute Rejection occurs within minutes to hours of vascular anastomosis (often while the surgeon is still in the operating room) when Preformed Recipient Antibodies (directed against donor ABO blood group antigens or donor HLA molecules from prior blood transfusions, prior pregnancies, or previous transplants) bind immediately to donor vascular endothelium, fixing complement and triggering diffuse fibrinoid necrosis, intravascular thrombosis, and cortical infarction. Because hyperacute rejection is irreversible (requiring immediate graft nephrectomy), it is prevented entirely by preoperative ABO matching and negative complement-dependent lymphocytotoxicity / flow cytometry Crossmatching. Acute Rejection typically occurs within weeks to 6 months post-transplant (or later if the patient stops taking maintenance immunosuppressants) via two mechanisms: (1) Acute Cellular Rejection (Type IV hypersensitivity), where recipient CD8+ cytotoxic T cells and CD4+ T cells infiltrate the graft interstitium, causing lymphocytic tubulitis and endotheliitis (intimal arteritis); and (2) Acute Antibody-Mediated Rejection (AMR, Type II hypersensitivity), where de novo Donor-Specific Antibodies (DSAs) develop post-transplant and bind peritubular capillaries, diagnosed on biopsy by linear C4d complement deposition along peritubular capillaries and microvascular inflammation. Chronic Rejection develops inexorably over months to years via chronic cytokine- and antibody-mediated vascular remodeling, resulting in concentric intimal smooth muscle proliferation (transplant vasculopathy / arteriosclerosis), interstitial fibrosis, tubular atrophy, and progressive organ failure (manifesting as bronchiolitis obliterans in lung transplants and accelerated diffuse coronary vasculopathy in heart transplants).

Targeted biologics used in transplant induction, maintenance, and acute rejection rescue map directly to surface receptors on T and B lymphocytes: (1) Basiliximab is a chimeric (human-mouse) monoclonal antibody that acts as a non-depleting antagonist at the alpha-subunit of the Interleukin-2 receptor (CD25) expressed exclusively on activated T lymphocytes; it is the standard induction agent for low-to-moderate immunologic risk kidney transplants and causes minimal cytokine release or opportunistic infection risk. (2) Rabbit Anti-Thymocyte Globulin (rATG, Thymoglobulin) is a polyclonal IgG preparation produced by immunizing rabbits with human thymocytes; it binds multiple T-cell surface antigens (CD2, CD3, CD4, CD8, CD28), causing profound complement-mediated T-cell depletion used for induction in high-immunologic-risk recipients and for treating steroid-resistant acute cellular rejection (adverse effects include cytokine release syndrome, serum sickness, leukopenia, and high CMV/EBV reactivation risk). (3) Alemtuzumab is a humanized monoclonal antibody against CD52 (expressed on mature T cells, B cells, NK cells, and monocytes) that produces profound, prolonged pan-lymphocyte depletion. (4) Belatacept is a recombinant fusion protein combining the extracellular domain of human CTLA-4 with an IgG1 Fc fragment (CTLA4-Ig); it binds CD80 and CD86 (B7-1/B7-2) on antigen-presenting cells, blocking CD28 costimulation (Signal 2) as an intravenous calcineurin-free maintenance alternative, though it is strictly CONTRAINDICATED in EBV-seronegative recipients due to a high risk of CNS Post-Transplant Lymphoproliferative Disorder (PTLD). (5) Acute Antibody-Mediated Rejection (C4d+) is treated with Plasmapheresis, IVIG, Rituximab (anti-CD20 B-cell depletion), Bortezomib (proteasome inhibitor depleting antibody-secreting plasma cells), or Eculizumab (anti-C5 monoclonal blocking membrane attack complex formation).

Trimethoprim-Sulfamethoxazole (TMP-SMX, sequential folate synthesis inhibitor blocking dihydropteroate synthase and dihydrofolate reductase) is the single highest-yield prophylactic antimicrobial in transplantation. Administered daily or three times weekly for 6 to 12 months post-transplant, a single tablet of TMP-SMX simultaneously prevents FIVE major transplant infections: (1) Pneumocystis jirovecii pneumonia (PJP, diffuse bilateral ground-glass infiltrates, elevated LDH, silver stain disc-shaped cysts); (2) Toxoplasma gondii reactivation (multiple ring-enhancing brain lesions, especially in heart transplants); (3) Nocardia asteroides (cavitary pulmonary nodules and brain abscesses caused by branching, beaded, partially acid-fast aerobic actinomycetes); (4) Listeria monocytogenes meningitis; and (5) common bacterial Urinary Tract Infections (E. coli in renal allografts). In patients with severe sulfa allergy or G6PD deficiency, alternative PJP prophylaxis uses Atovaquone (which also covers Toxoplasma) or Dapsone (requires G6PD screening first; add pyrimethamine/leucovorin if Toxoplasma coverage is needed) or inhaled Pentamidine (which protects only the lungs, leaving risk for extrapulmonary PJP and zero coverage for Toxoplasma/Nocardia/UTI). For Cytomegalovirus (CMV)—especially in highest-risk Donor-Positive / Recipient-Negative (D+/R-) transplants—oral Valganciclovir (the oral prodrug of ganciclovir, phosphorylated by viral UL97 kinase to inhibit viral UL54 DNA polymerase) is administered for 3 to 6 months; its major dose-limiting toxicity is severe Neutropenia/Leukopenia, particularly when combined with mycophenolate or TMP-SMX! In contrast, BK Polyomavirus reactivation in renal transplant recipients causes BK Virus Nephropathy (tubulointerstitial nephritis with 'decoy cells' having basophilic nuclear inclusions on urine cytology, mimicking acute cellular rejection on biopsy); because antiviral drugs do not clear BK virus effectively, the definitive treatment for BK nephropathy is REDUCING maintenance immunosuppression (lowering MMF and tacrolimus doses), whereas mistakenly increasing steroids for presumed rejection accelerates graft loss!

Whereas solid-organ transplantation carries a low incidence of Graft-Versus-Host Disease (except in small bowel or liver transplants containing large donor lymphoid pools), allogeneic Hematopoietic Stem Cell Transplantation (HSCT) carries a high risk of GVHD (and may also occur after transfusion of non-irradiated blood products to an immunocompromised host). In GVHD, immunocompetent DONOR T lymphocytes recognize host MHC antigens as foreign and attack three specific recipient epithelial organs, producing the classic clinical triad: (1) Skin: painful maculopapular rash Starting on the palms, soles, and ears that can progress to generalized erythroderma and toxic epidermal necrolysis-like bullous desquamation; (2) Liver: cholestatic hepatitis with conjugated hyperbilirubinemia and elevated alkaline phosphatase (due to bile ductular destruction); and (3) Gastrointestinal Tract: profuse secretory watery/bloody diarrhea, abdominal cramping, and mucosal ulceration. GVHD prophylaxis uses methotrexate plus a calcineurin inhibitor (tacrolimus/cyclosporine) or post-transplant cyclophosphamide; acute GVHD is treated first-line with high-dose systemic Corticosteroids, and steroid-refractory acute or chronic GVHD is treated with Ruxolitinib (a selective JAK1/JAK2 inhibitor that blocks inflammatory cytokine signaling) or Ibrutinib (BTK inhibitor). Finally, chronic T-cell immunosuppression disables cytotoxic CD8+ surveillance against latent Epstein-Barr Virus (EBV) in B lymphocytes, leading to uncontrolled B-cell proliferation known as Post-Transplant Lymphoproliferative Disorder (PTLD); first-line management of PTLD is reducing immunosuppression combined with Rituximab (anti-CD20 monoclonal antibody) +/- R-CHOP chemotherapy.