Hypersensitivity reaction

Types of hypersensitivity reactions

Erythroblastosis fetalis

• Mother: Rh-negative
• Father: Rh-positive
• Fetus: may be Rh-positive

Pathogenesis

1. Sensitization of the Rh-negative mother

• At delivery
• During abortion, miscarriage, ectopic pregnancy
• Following antepartum hemorrhage or abdominal trauma
• After invasive procedures, such as amniocentesis
• Following incompatible Rh-positive blood transfusion

2. Formation of maternal anti-D IgG and memory cells

3. Transplacental transfer of IgG

4. Fetal hemolysis

• Hemolytic anemia
• Increased erythropoiesis
• Release of immature nucleated red cells, or erythroblasts, into fetal blood, hence the name erythroblastosis fetalis
• Hepatosplenomegaly due to extramedullary hematopoiesis
• High-output cardiac failure
• Generalized edema and hydrops fetalis
• In severe cases, intrauterine death or stillbirth

Prevention

1. Antenatal blood grouping and antibody screening

• Determine maternal ABO and Rh(D) blood group.
• Screen maternal serum for clinically significant red-cell antibodies using the indirect Coombs test.
• Repeat antibody screening later in pregnancy according to local protocol.

2. Anti-D immunoglobulin prophylaxis

• Routinely during late pregnancy, commonly at about 28 weeks or through a local routine antenatal anti-D prophylaxis regimen.
• Within 72 hours after delivery of an Rh-positive infant.
• After any potentially sensitizing event, such as miscarriage, induced abortion, ectopic pregnancy, antepartum hemorrhage, abdominal trauma, external cephalic version, or invasive prenatal procedure.

3. Avoid sensitizing transfusions

Control and treatment of an affected pregnancy or newborn

1. Assess antibody level and fetal risk by serial maternal antibody titers and paternal/fetal Rh typing where available.
2. Monitor fetal anemia, commonly with middle cerebral artery Doppler assessment.
3. Treat severe fetal anemia with specialist-guided intrauterine transfusion and plan delivery at an appropriate gestation.
4. Treat the newborn with monitoring of hemoglobin and bilirubin, phototherapy, intravenous immunoglobulin in selected cases, and exchange transfusion when severe hemolysis or hyperbilirubinemia warrants it.
5. Provide compatible antigen-negative blood for fetal or neonatal transfusion.

Mechanism

1. Abnormal valves cause turbulent blood flow
   Conditions such as rheumatic valvular disease, mitral valve prolapse, bicuspid aortic valve, calcific stenosis, congenital defects, and prosthetic valves alter normal flow. High-velocity jets traumatize the valvular endothelium.
2. Endothelial injury exposes a thrombogenic surface
   The damaged endocardium promotes deposition of platelets and fibrin, producing small sterile thrombi called nonbacterial thrombotic vegetations.
3. Transient bacteremia seeds these deposits
   Bacteria entering the bloodstream, for example after dental infection or procedures, skin infection, intravenous drug use, or invasive devices, can adhere to the platelet-fibrin deposits. Less virulent organisms such as viridans streptococci particularly need this damaged surface for colonization.
4. Vegetations protect bacteria
   Organisms become embedded in layers of fibrin and platelets. This relatively avascular environment limits access of phagocytes, antibodies, complement, and antibiotics. The organisms proliferate and form the characteristic infected vegetation.
5. Progressive valve destruction follows
   The vegetation may erode or perforate valve leaflets, cause regurgitation, form abscesses, and embolize.

Bacterial spore

Important spore-forming bacteria

• Bacillus species: aerobic or facultative anaerobic
  Examples: Bacillus anthracis, B. cereus
• Clostridium/Clostridioides species: anaerobic
  Examples: Clostridium tetani, C. botulinum, C. perfringens, C. difficile

Structure

• Core: bacterial DNA, ribosomes, enzymes, and very little water
• Calcium dipicolinate: stabilizes core components and contributes to heat resistance
• Inner membrane
• Cortex: specialized peptidoglycan layer
• Spore coat: tough proteinaceous outer covering

Properties

• Heat and drying
• Cold
• Radiation
• Chemicals and many disinfectants
• Desiccation and starvation
• Extremes of pH

Sporulation

Germination

1. The spore is activated by suitable stimuli such as heat, pH change, or nutrients.
2. It takes up water and loses its resistance.
3. The spore coat breaks down.
4. A normal vegetative bacterial cell emerges and multiplies.

Importance

• Spores allow bacteria to survive in soil, dust, food, and hospital environments for long periods.
• They make sterilization difficult. Proper autoclaving and appropriate sporicidal agents are required.
• They are important in diseases such as anthrax, tetanus, botulism, gas gangrene, and C. difficile infection.

Carriers of communicable disease

Elements of carrier state

1. Presence of the infectious agent in the body
2. No recognizable signs or symptoms of disease
3. Shedding of the agent in secretions or excretions, thus acting as a source of infection

Classification of carriers

1. According to stage or type

2. According to duration

3. According to portal of exit

Pseudo-carrier

Public-health importance

• Carriers maintain infection in the community and can initiate outbreaks.
• They may contaminate food, water, or fomites.
• Food handlers, water-supply workers, and healthcare workers who are carriers may pose particular risk.
• Control includes early detection, laboratory confirmation, treatment where possible, health education, hand hygiene, safe food handling, and restriction from high-risk occupations until non-infectious.

Laboratory diagnosis of viral infections

1. Specimen collection and transport

2. Direct detection of virus

A. Detection of viral nucleic acid

• PCR detects viral DNA.
• RT-PCR detects RNA viruses after conversion of RNA to DNA.
• Real-time PCR can provide rapid detection and, in some infections, estimate viral load.
• Multiplex PCR panels can detect several respiratory, gastrointestinal, or central nervous system viruses simultaneously.
• Sequencing or genotyping may be used for strain identification, outbreak investigation, and antiviral-resistance testing.

B. Detection of viral antigen

• Direct or indirect immunofluorescence
• Enzyme immunoassay or ELISA
• Rapid immunochromatographic antigen tests
• Immunohistochemistry in tissue sections

C. Virus isolation in cell culture

• Cytopathic effect, such as cell rounding, syncytium formation, or cell lysis
• Haemadsorption or haemagglutination
• Antigen detection or PCR from the cultured cells

D. Microscopy and cytology

• Light microscopy may show characteristic cytopathic changes or inclusion bodies.
  • Multinucleated giant cells and intranuclear inclusions: herpesvirus infections
  • “Owl’s-eye” inclusions: cytomegalovirus
  • Negri bodies: rabies
• Electron microscopy can directly demonstrate virus particles in selected samples, but it is expensive and not routinely used.

3. Serological diagnosis

Tests used

• ELISA
• Neutralization test
• Haemagglutination-inhibition test
• Complement-fixation test
• Immunofluorescence assay
• Western blot or immunoblot in selected infections

Interpretation

• Virus-specific IgM suggests recent or acute infection, but may persist for weeks to months and may occasionally be falsely positive.
• IgG seroconversion or a significant rise, usually fourfold or more, in antibody titre between acute and convalescent sera supports recent infection.
• A single IgG-positive sample generally indicates past infection, immunity, or previous vaccination rather than acute disease.

4. Other supportive investigations

• Total and differential leukocyte count, often showing leukopenia or lymphocytosis
• Liver-function tests in viral hepatitis
• CSF examination in viral meningitis or encephalitis, usually lymphocytic pleocytosis with relatively normal glucose
• Histopathology of biopsy or autopsy tissue
• Immunohistochemistry or in-situ hybridization to localize viral antigen or nucleic acid in tissue

Summary

1. Virus isolation in cell culture
2. Antigen detection
3. Nucleic-acid detection by PCR/RT-PCR
4. Serology for virus-specific IgM or rising IgG titre
5. Cytology, histopathology, and electron microscopy in selected cases

Coccidian parasitic infections

• Cryptosporidium spp.
• Cyclospora cayetanensis
• Cystoisospora belli formerly Isospora belli
• Sarcocystis spp.

Mode of transmission

• Contaminated drinking or recreational water
• Contaminated food, especially raw vegetables and fruits
• Contaminated hands and fomites
• Contact with infected animals, particularly for some Cryptosporidium species

General life cycle

• Cryptosporidium oocysts may be infective when passed, allowing direct person-to-person spread and autoinfection.
• Cyclospora and Cystoisospora oocysts require maturation or sporulation in the environment before becoming infective. Thus immediate direct faeco-oral spread is less likely.

Important infections

Laboratory diagnosis

• Examine multiple freshly collected stool specimens, since oocyst excretion may be intermittent.
• Routine ova-and-parasite examination can miss these parasites. The laboratory should be specifically asked to test for intestinal coccidia.
• Use stool concentration methods and modified acid-fast staining.
• Cyclospora oocysts show autofluorescence under ultraviolet microscopy.
• Antigen detection tests and PCR are particularly useful for Cryptosporidium.
• In persistent disease with negative stool testing, duodenal aspirate or intestinal biopsy may demonstrate Cystoisospora.

Prevention

• Safe drinking water, including boiling or adequately filtering water where indicated
• Proper disposal of faeces and sanitary sewage systems
• Handwashing after toilet use, diaper changes, animal contact, and before food handling
• Wash raw fruits and vegetables thoroughly
• Avoid untreated water and unpasteurized products during outbreaks or travel to high-risk settings
• Avoid swimming while diarrhoeal, particularly in suspected or confirmed cryptosporidiosis
• Early HIV diagnosis and antiretroviral treatment reduce severe and recurrent disease in people with HIV.

i. Likely organism

ii. Pathogenesis of Staphylococcus aureus abscess

1. Colonization and entry

• S. aureus commonly colonizes the anterior nares, skin, axilla, and perineum.
• It enters the skin through minor trauma, hair follicles, injections, wounds, or other breaks in the epithelial barrier.

2. Adherence to host tissues

• Surface adhesins, including fibronectin-binding proteins and teichoic acid, help the organism attach to damaged skin and tissue.
• In diabetes, impaired neutrophil function and vascular insufficiency facilitate bacterial multiplication.

3. Evasion of host defenses

• Capsule/slime layer inhibits phagocytosis and assists adherence.
• Protein A binds the Fc portion of IgG, thereby interfering with opsonization and phagocytosis.
• Coagulase converts fibrinogen to fibrin, producing a fibrin barrier around bacteria. This helps localize infection and protects organisms from phagocytes.
• Clumping factor promotes attachment to fibrinogen and fibrin.

4. Tissue invasion and abscess formation

• Cytotoxins, particularly alpha toxin and leukocidins, damage leukocytes and tissue cells.
• Enzymes such as hyaluronidase, lipases, proteases, and staphylokinase promote tissue spread and destruction.
• Peptidoglycan and bacterial products induce acute inflammation with neutrophil infiltration.
• Local tissue necrosis, liquefaction, fibrin deposition, and accumulation of neutrophils form pus and an abscess.

iii. Laboratory diagnosis

1. Specimen

• Collect pus aspirate from the depth of the abscess before antibiotic treatment, preferably by needle aspiration or from material obtained at incision and drainage.
• A superficial swab is less satisfactory because it may yield contaminating skin flora.
• Take blood cultures if there is fever, systemic toxicity, suspected bacteremia, or deep infection.

2. Direct microscopy

• Numerous pus cells
• Gram-positive cocci in irregular, grape-like clusters

3. Culture

• Blood agar: smooth, round, opaque, golden-yellow colonies, often with beta-hemolysis
• Nutrient agar: golden-yellow pigmented colonies may be seen
• Mannitol salt agar: growth in high salt concentration with mannitol fermentation, producing yellow colonies/medium
• MacConkey agar: usually no growth or poor growth, since it is mainly selective for Gram-negative bacteria

4. Identification tests

5. Antimicrobial susceptibility testing

• Methicillin-resistant S. aureus (MRSA), usually screened with cefoxitin or oxacillin susceptibility testing
• Inducible clindamycin resistance using the D-test, where indicated
• Susceptibility to agents used for serious infections, such as vancomycin, linezolid, and others according to local laboratory protocol

Report

Staphylococcus aureus isolated from pus, with antimicrobial susceptibility pattern, including MRSA/MSSA status.

Role of cellular immune response in human infections

1. Defence against intracellular microorganisms

• Viruses
• Intracellular bacteria, such as Mycobacterium tuberculosis, Listeria monocytogenes, Salmonella and Brucella
• Some intracellular protozoa, such as Leishmania and Toxoplasma gondii
• Many fungi, especially in immunocompromised persons

2. Role of CD4+ T helper cells

Th1 cells and macrophage activation

• Interferon-gamma (IFN-gamma): activates macrophages and enhances intracellular killing of microbes.
• Tumour necrosis factor-alpha (TNF-alpha): promotes inflammation, macrophage activation, and granuloma maintenance.
• Interleukin-2 (IL-2): promotes T-cell proliferation and activation.

Granuloma formation

3. Role of CD8+ cytotoxic T lymphocytes

• Release of perforin, which forms pores in the target-cell membrane
• Release of granzymes, which induce apoptosis
• Fas ligand-Fas interaction, which also induces apoptosis

4. Role in viral infections

• CD4+ T cells help coordinate inflammatory and antibody responses.
• CD8+ T cells destroy virus-infected cells.
• IFN-gamma inhibits viral replication and activates macrophages.
• Memory T cells enable a faster and stronger response upon reinfection.

5. Role in parasitic and fungal infections

• Th1-mediated macrophage activation is important against intracellular protozoa, such as Leishmania and Toxoplasma.
• CD4+ T-cell responses and macrophages are important in protection against fungi, including Cryptococcus, Histoplasma, and Pneumocystis jirovecii.
• Th2 cells activate eosinophils and promote IgE production, helping defense against helminths that are too large to be phagocytosed.

6. Delayed-type hypersensitivity

• Tuberculin skin test
• Contact dermatitis
• Granulomatous inflammation in tuberculosis and leprosy

7. Harmful effects of cellular immunity

• Caseation and cavitation in pulmonary tuberculosis
• Nerve damage in tuberculoid leprosy
• Hepatocyte injury in viral hepatitis
• Tissue damage in chronic granulomatous infections

8. Consequences of impaired cellular immunity

• HIV/AIDS
• Corticosteroid or immunosuppressive treatment
• Organ transplantation
• Hematological malignancy
• Severe malnutrition
• Congenital T-cell defects

Summary

1. Activating macrophages to kill intracellular microbes
2. Killing virus-infected and other infected host cells
3. Forming granulomas to contain persistent organisms
4. Coordinating inflammatory responses through cytokines
5. Producing immunological memory

Identification

Pathogenesis of H. pylori infection

1. Entry and colonization of gastric mucosa

• Flagella provide motility, allowing the organism to penetrate gastric mucus.
• It uses adhesins to attach to gastric epithelial cells and resist clearance by mucus turnover.

2. Survival in acidic environment: urease

3. Mucosal injury and inflammation

• Ammonia produced by urease
• Proteases and phospholipases that damage mucus and epithelial cells
• Reactive oxygen species and enzymes released from recruited neutrophils
• Persistent inflammation with lymphocytes and plasma cells

4. Major virulence factors

• CagA protein: injected into gastric epithelial cells by a type IV secretion system. It alters intracellular signaling, promotes inflammation, disrupts cell junctions, and is associated with more severe gastritis, peptic ulcer disease, and increased gastric-cancer risk.
• VacA cytotoxin: causes epithelial-cell vacuolation, apoptosis, and impaired immune-cell function.
• Adhesins: facilitate attachment to gastric epithelium.
• Lipopolysaccharide: contributes to inflammation and can help immune evasion.

5. Formation of peptic ulcer

Rapid diagnostic tests for H. pylori

A. Rapid urease test

• Gastric antral and corpus biopsies are obtained during endoscopy.
• The biopsy is placed in a medium containing urea and a pH indicator, commonly phenol red.
• If H. pylori urease is present, urea is broken down to ammonia, the pH rises, and the indicator changes color, usually from yellow/orange to pink/red.

B. Urea breath test

• The patient ingests urea labelled with carbon-13 or carbon-14.
• Bacterial urease breaks labelled urea into ammonia and labelled carbon dioxide.
• Labelled CO₂ is detected in exhaled breath.

C. Stool antigen test

• Detects H. pylori antigen in faeces by enzyme immunoassay or immunochromatographic methods.
• It indicates current infection and can be used to confirm eradication.

D. Serology

• ELISA detects serum IgG antibodies to H. pylori.
• It may be rapid and useful for epidemiological surveys or when other tests are unavailable.

E. Rapid molecular tests

Summary table

Pathogenesis of carcinoma colon

1. Conventional adenoma-carcinoma sequence

Normal mucosa → adenoma → carcinoma

Molecular events

1. APC gene inactivation, early event
   • Both copies of the APC tumour-suppressor gene on chromosome 5q are lost or inactivated.
   • APC normally degrades β-catenin.
   • Loss of APC causes accumulation of β-catenin in the nucleus.
   • β-catenin activates Wnt-signaling target genes such as MYC and cyclin D1, causing increased epithelial proliferation and formation of an early adenoma.
   • Germline APC mutation causes familial adenomatous polyposis (FAP). A second somatic hit produces numerous adenomas, with a very high risk of carcinoma.
2. KRAS activation, intermediate event
   • Activating mutation of KRAS promotes cell proliferation and reduces apoptosis.
   • It is associated with growth of small adenomas into larger, more dysplastic adenomas.
3. Loss of tumour-suppressor genes on chromosome 18q
   • Loss of SMAD2, SMAD4, and sometimes DCC impairs the growth-inhibitory TGF-beta pathway.
   • This facilitates progression to late adenoma.
4. TP53 inactivation, late event
   • Loss of TP53 permits survival and proliferation of genetically damaged cells.
   • It is an important event in transformation from adenoma to invasive carcinoma.

APC loss → β-catenin accumulation → early adenoma → KRAS mutation → large adenoma → 18q/SMAD loss and TP53 loss → carcinoma

2. Microsatellite instability pathway

• Normally, mismatch repair genes correct DNA replication errors.
• Inherited mutation of mismatch repair genes causes Lynch syndrome or hereditary nonpolyposis colorectal cancer.
• The main genes are MLH1, MSH2, MSH6, and PMS2.
• Loss of repair function leads to mutations in repetitive DNA sequences called microsatellites, producing microsatellite instability, especially MSI-high status.

• TGFBR2, leading to loss of TGF-beta-mediated growth inhibition
• BAX, reducing apoptosis

3. Serrated or CpG-island methylator phenotype pathway

• There is widespread promoter hypermethylation of CpG islands, called the CIMP phenotype.
• BRAF mutation is common.
• Hypermethylation of the MLH1 promoter silences MLH1 expression.
• This creates mismatch repair deficiency and frequently produces an MSI-high carcinoma.

Sessile serrated polyp → BRAF mutation + CpG island hypermethylation → MLH1 silencing → MSI-high colorectal carcinoma

Role of environmental and inflammatory factors

• Diet high in fat, refined carbohydrates, and processed/red meat
• Low dietary fibre intake
• Obesity and sedentary lifestyle
• Long-standing ulcerative colitis or Crohn colitis
• Family history and inherited syndromes, particularly FAP and Lynch syndrome

Summary

Alcohol-associated liver disease

Fatty liver (steatosis) → alcoholic steatohepatitis/hepatitis → fibrosis → cirrhosis → hepatocellular carcinoma

Pathogenesis

Alcohol metabolism

1. Ethanol → acetaldehyde by alcohol dehydrogenase
2. Acetaldehyde → acetate by aldehyde dehydrogenase
3. With chronic drinking, the microsomal ethanol oxidizing system, especially CYP2E1, becomes important.

Mechanisms of liver injury

• Increased NADH from ethanol metabolism inhibits fatty-acid oxidation and promotes triglyceride synthesis, causing fatty change.
• Acetaldehyde is directly toxic. It forms protein adducts, damages mitochondria and cytoskeleton, impairs protein secretion, and triggers immune responses.
• CYP2E1 generates reactive oxygen species, resulting in oxidative stress, lipid peroxidation, and hepatocyte injury.
• Alcohol increases gut permeability, allowing endotoxin such as lipopolysaccharide to reach the liver through portal blood.
• Endotoxin activates Kupffer cells via TLR4, producing TNF-alpha and other inflammatory cytokines.
• Cytokines, acetaldehyde, and oxidative stress activate hepatic stellate cells, which deposit collagen and cause pericellular and perisinusoidal fibrosis, especially around central veins.

Morphology and clinical forms

1. Alcoholic fatty liver

• Earliest and most common lesion
• Liver is enlarged, soft, yellow, and greasy
• Histology: macrovesicular steatosis, initially in centrilobular zone 3
• Usually asymptomatic and reversible with abstinence

2. Alcoholic steatohepatitis or alcoholic hepatitis

• Hepatocyte ballooning and necrosis, mainly in zone 3
• Neutrophilic infiltration
• Mallory-Denk bodies: eosinophilic cytoplasmic aggregates of damaged keratin intermediate filaments
• Fatty change
• Pericellular “chicken-wire” fibrosis and perivenular fibrosis

• Jaundice
• Fever
• Tender hepatomegaly
• Anorexia, nausea, vomiting, weight loss
• Ascites and encephalopathy in severe cases

3. Alcohol-associated cirrhosis

• Progressive fibrosis links portal tracts and central veins, producing regenerative nodules and architectural distortion.
• In advanced disease, the liver may become small, firm, and nodular.
• Complications include portal hypertension, ascites, variceal hemorrhage, splenomegaly, spontaneous bacterial peritonitis, hepatic encephalopathy, hepatorenal syndrome, and hepatocellular carcinoma.

Diagnosis

• AST elevated more than ALT, often AST:ALT ratio >2
• Aminotransferases are usually only moderately elevated
• Raised bilirubin, INR, and alkaline phosphatase in severe disease
• Low albumin and thrombocytopenia in advanced cirrhosis
• Raised gamma-glutamyl transferase may support alcohol exposure but is nonspecific

Management

1. Complete abstinence from alcohol is the most important intervention at every stage.
2. Treat alcohol-use disorder with counseling, behavioural therapy, family support, and addiction services. Selected patients may receive medicines such as acamprosate or baclofen under medical supervision.
3. Provide nutritional support, including adequate calories and protein, and correct thiamine, folate, electrolyte, and vitamin deficiencies.
4. Manage complications of cirrhosis, including ascites, varices, encephalopathy, and infection.
5. Severe alcohol-associated hepatitis needs specialist assessment. Prednisolone may be used in carefully selected patients after excluding uncontrolled infection and other contraindications; response is reassessed with a prognostic score.
6. Liver transplantation may be considered for selected patients with decompensated cirrhosis or severe hepatitis after specialist multidisciplinary assessment.

Prognosis

• Simple steatosis has a good prognosis if alcohol is stopped.
• Continued alcohol use increases the risk of steatohepatitis, cirrhosis, liver failure, and hepatocellular carcinoma.
• Malnutrition, obesity, chronic viral hepatitis, genetic susceptibility, and smoking can worsen progression.