All subjects · Biology · WBI14 (Unit 4)

How tuberculosis and HIV cause disease

Mycobacterium tuberculosis survives inside the very cells sent to destroy it, and HIV destroys the cells that coordinate the immune response.

Tuberculosis (TB)

1. *Mycobacterium tuberculosis* is spread in airborne droplets when an infected person coughs; it is inhaled into the lungs.

2. Macrophages engulf the bacteria, but the bacteria survive inside them: their thick, waxy cell wall (rich in mycolic acid) resists damage, and they stop the phagosome fusing properly with lysosomes.

3. The immune system walls off infected cells in granulomas (tubercles) with a dead centre. The bacteria can stay dormant there for years: a latent infection with no symptoms.

4. If the immune system is weakened (for example by HIV, malnutrition or old age), the bacteria become active, destroy lung tissue and may spread. Symptoms include a persistent cough (sometimes with blood), fever, night sweats and weight loss.

TB is treated with a long course of several antibiotics.

HIV and AIDS

1. HIV is transmitted in body fluids: unprotected sex, shared needles, infected blood, and from mother to baby.

2. The glycoprotein gp120 on the viral envelope binds to CD4 receptors on T helper cells (and some macrophages). The envelope fuses with the cell membrane and the capsid enters.

3. Reverse transcriptase makes DNA from the viral RNA. Integrase inserts it into the host DNA as a provirus.

4. The provirus may stay latent, or be transcribed to make new viruses, which bud from the cell. Infected T helper cells are destroyed.

5. Over years the T helper cell count falls. With too few T helper cells, B cells and T killer cells are not activated, so the person cannot fight infections: AIDS, in which opportunistic infections (such as TB and pneumonia) and some cancers develop.

Why TB and HIV are linked

HIV weakens the immune system, so latent TB is far more likely to become active. TB is a leading cause of death in people with HIV.

Worked example

A person's T helper cell count falls from 1000 to 250 cells per mm³. Calculate the percentage decrease.

  1. Decrease = 1000 - 250 = 750
  2. Percentage = 750 / 1000 x 100 = 75%

Answer: 75% decrease.

Key idea: TB: droplets, engulfed by macrophages but survives (waxy wall), walled off in granulomas, latent until immunity weakens. HIV: gp120 binds CD4 on T helper cells, reverse transcriptase, provirus; falling T helper count leads to AIDS and opportunistic infections.

Check yourself

  1. A T helper cell count falls from 800 to 200 cells per mm³. Calculate the percentage decrease.
    Show answer

    75 %. 600 / 800 x 100 = 75%.

  2. Which cells does HIV mainly infect?
    • T helper cells with CD4 receptors
    • Red blood cells
    • Plasma cells only
    • Neurones in the spinal cord
    Show answer

    T helper cells with CD4 receptors. gp120 binds CD4 on T helper cells.

  3. What does reverse transcriptase do?
    • Makes RNA from DNA
    • Digests the host cell membrane
    • Makes DNA using viral RNA as a template
    • Joins amino acids into proteins
    Show answer

    Makes DNA using viral RNA as a template. It reverses the usual DNA to RNA direction.

  4. How does Mycobacterium tuberculosis survive being engulfed by macrophages?
    • It is too large to be engulfed
    • Its waxy cell wall resists damage and it prevents normal fusion with lysosomes
    • It kills all macrophages instantly
    • It has no cell wall
    Show answer

    Its waxy cell wall resists damage and it prevents normal fusion with lysosomes. It persists inside the macrophage.

  5. What is latent TB?
    • An active infection with severe coughing
    • Infection by a virus
    • TB that cannot be treated
    • Dormant bacteria walled off in granulomas with no symptoms
    Show answer

    Dormant bacteria walled off in granulomas with no symptoms. Bacteria survive but are inactive.

  6. Why do people with AIDS develop opportunistic infections?
    • HIV directly kills all bacteria
    • Too few T helper cells means B and T killer cells are not activated
    • They have too many antibodies
    • Their red blood cells are destroyed
    Show answer

    Too few T helper cells means B and T killer cells are not activated. Loss of T helper cells cripples the specific immune response.

  7. How is TB usually spread?
    • Mosquito bites
    • Contaminated food only
    • Sharing towels
    • Airborne droplets from coughs and sneezes
    Show answer

    Airborne droplets from coughs and sneezes. TB is an airborne disease of the lungs.

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Last updated 7 November 2026. Spotted a mistake? Tell us.