Skip to content
Explore atlas
PH-IM04

Tuberculosis

How the immune system normally clears inhaled microbes, why Mycobacterium tuberculosis survives inside macrophages, how granulomas contain it as latent infection, and how reactivation destroys lung tissue and spreads.

By the end, you can

  • Describe how alveolar macrophages and T cells normally defend the lung.
  • Explain how M. tuberculosis survives inside macrophages.
  • Explain granuloma formation, caseous necrosis and the difference between latent and active TB.
  • Explain why reactivation favours the lung apices and causes cavities.
  • Relate cough, haemoptysis, fever, night sweats and weight loss to the disease, and explain the logic of testing and treatment.
  • Recognise the classic morphology (caseating granulomas with Langhans cells, the Ghon complex, cavities and miliary spread), the tests and key numbers of TB, and explain how each main drug acts on the bacterium.

What it is

Tuberculosis is infection with Mycobacterium tuberculosis, a slow-growing bacterium spread in airborne droplets from people with active lung disease. Most infected people contain it as latent TB infection, without symptoms and not infectious. Active TB disease, most often in the lungs, develops in a minority, especially when immunity is weakened.

3 types to tell apart
Latent infection
Bacteria contained in granulomas; no symptoms, not infectious; detected by blood or skin tests.
Active pulmonary TB
Progressive lung disease; infectious when bacteria are in the sputum.
Extrapulmonary TB
Lymph nodes, pleura, bone and spine, kidneys, meninges; miliary TB when spread through the blood.

Normal function

Macrophages guard the alveoli

Small particles and microbes that reach the alveoli are engulfed by . Inside the macrophage, the engulfed material sits in a phagosome, which fuses with lysosomes whose acid and enzymes kill and digest it.

Macrophages present fragments of microbes to helper T cells in nearby lymph nodes. Th1 helper T cells return and release interferon-gamma, which activates macrophages so they kill intracellular microbes more effectively. TNF from macrophages helps organise the response.

For most inhaled microbes, this clears the infection quickly with little inflammation.

Point to it on the model