Questions: Immunopathology of Infectious Diseases: Protective vs. Pathogenic Immunity

Question 1

Patients infected with hepatitis B virus who mount a strong CD8+ T cell response often develop severe hepatitis, while those with weak immune responses may become asymptomatic chronic carriers. What does this pattern reveal about the cause of liver damage in hepatitis B?

• A. Hepatitis B virus produces toxins that directly destroy hepatocytes, and a strong immune response clears the virus before too much damage occurs
• B. The liver damage is caused by CD8+ T cells attacking infected hepatocytes — the virus itself is noncytopathic and does not directly kill cells
• C. A strong Th2 response drives eosinophilic infiltration of the liver, causing the characteristic inflammation
• D. High viral load in immunocompromised patients directly kills hepatocytes through metabolic exhaustion

Answer: (B)

Hepatitis B virus is noncytopathic — it does not directly kill the cells it infects. The liver damage characteristic of clinical hepatitis is caused by the CD8+ cytotoxic T lymphocyte (CTL) response, which attacks and kills infected hepatocytes. Patients who mount strong CTL responses clear the virus but experience significant liver injury in the process. Patients who fail to mount an effective CTL response become chronic carriers with little liver damage — they live with the virus because the immune system is not attacking it. This is a paradigm case of immunopathology: the disease is caused by the immune response, not the pathogen.

Question 2

A patient who recovered from dengue serotype 1 becomes infected with dengue serotype 2 and develops dengue hemorrhagic fever — a much more severe illness than her first infection. What mechanism best explains this worsening?

• A. Dengue serotype 2 is inherently more virulent than serotype 1 and causes severe disease in all patients
• B. Immunosuppression caused by the first infection left the patient unable to fight the second
• C. Antibodies from the first infection bind serotype 2 without neutralizing it, facilitating enhanced viral uptake by Fc-receptor-bearing immune cells
• D. The patient developed autoimmune antibodies during the first infection that cross-react with her own vascular endothelium

Answer: (C)

Antibody-dependent enhancement (ADE) in dengue occurs when antibodies generated against one serotype bind a different serotype without fully neutralizing it. The antibody-virus complex is recognized by Fc receptors on macrophages and monocytes, which take up the complex more efficiently than they would take up free virus. This dramatically increases viral replication inside these immune cells and drives a massive inflammatory response. The cruel irony is that the patient's prior immunity — the antibodies she produced — actively makes her second infection worse. This is why dengue vaccine development is so challenging: a vaccine that generates non-neutralizing antibodies could prime ADE.

Question 3

A higher antibody titer against a pathogen typically indicates better protection, because antibodies work by binding and neutralizing pathogens before they can infect host cells.

• T. True
• F. False

Answer: False

This is the central misconception addressed by immunopathology. Antibody-dependent enhancement in dengue is the clearest counterexample: pre-existing antibodies from a prior serotype infection enhance uptake of a new serotype, making illness worse rather than better. Quality, specificity, and balance of the immune response matter as much as magnitude. Non-neutralizing antibodies — those that bind but do not block pathogen infectivity — can actually facilitate pathogen entry into Fc-receptor-bearing cells. More immunity is not always better immunity.

Question 4

Granulomas in tuberculosis serve a dual function: they both contain the pathogen (protective) and cause tissue damage (pathological), illustrating that the immune response can simultaneously defend the host and injure it.

• T. True
• F. False

Answer: True

TB granulomas are the paradigm of double-edged immunopathology. The Th1/IFN-γ response activates macrophages that wall off M. tuberculosis in organized structures — the granuloma — containing but not eliminating the pathogen. The same inflammatory response causes caseous necrosis at the granuloma center. If the granuloma breaks down (as can happen in immunosuppression or advanced disease), the necrotic center liquefies and spills bacteria into the airways, causing the cavitation and hemoptysis characteristic of active TB. The patient's immune response is both what keeps them alive and what causes their symptoms.

Question 5

Explain why the statement 'a stronger immune response is always better for fighting infection' is incorrect. Use at least one specific example.

Model answer: The immune response has to be calibrated — too weak allows pathogen overgrowth; too strong causes immunopathology. In dengue hemorrhagic fever, antibody-dependent enhancement means that a pre-existing antibody response from a prior serotype infection actively worsens the outcome by enhancing viral uptake into immune cells, triggering a more severe inflammatory cascade. In tuberculosis, the Th1 granulomatous response is necessary to contain the pathogen but also produces caseous necrosis and, if unchecked, tissue destruction. In hepatitis B, the CTL response that clears the virus is also the cause of liver damage. A 'stronger' response in each case means more damage, not less. The goal of a successful immune response is not maximum intensity but appropriate specificity, magnitude, and resolution.

The key insight is that immunity is a physiological process operating under a tradeoff: sufficient response to control pathogens vs. collateral damage to host tissue. Evolution has not produced 'always maximize immunity' — it has produced regulation, tolerance mechanisms, and anti-inflammatory pathways precisely because unchecked immunity kills the host.