Pathology - Infections of the lung

Question 1

What are the 4 types of pulmonary infection?

Answer 1

1. Nodular (tuberculosis granuloma)
2. Diffuse parenchymal (airspace (lobar) pneumonia)
3. Patchy parenchymal (bronchopneumonia)
4. Interstitial (interstitial pneumonia)

Question 2

What is the body’s first reaction to tuberculosis?

Answer 2

Acute inflammatory reaction (neutrophils and other leukocytes) - however, your leukocytes cannot effectively kill Mycobacteria (it is not an effective response).

Macrophages are more effective at killing the MTB than neutrophils, but still not very effective

Question 3

The adaptive immune system produces specialized cells that kill and contain the MTB infection more effectively. Name these cells and describe the process by which they are formed.

Answer 3

Macrophages containing MTB antigens travel to the lymph nodes, where T cells activate and transform macrophages into more specialized cells with enhanced ability to kill the MTB. Upon further activation, some of these specialized macrophages become epithelioid histiocytes which are much better at killing MTB.

Question 4

What is the result of the inflammatory response to MTB? Under the microscope, what do we see?

Answer 4

• Necrotic (dead) lung tissue
• Accumulation of epithelioid histiocytes (with characteristic elongated nuclei)

Question 5

What are the 2 main defence mechanisms our body has against MTB?

Answer 5

• epithelioid histiocytes (specialized immune cells that kill the bacteria)
• granuloma formation (infection is walled off by a layer of fibrous connective tissue and epithelioid histiocytes surrounding an area of caseous necrosis to prevent further spread)

Question 6

MTB often gain access to the blood. How?

Answer 6

• By entering the lymphatics and draining into the bloodstream
• Direct invasion of pulmonary blood vessels

Question 7

What happens when MTB enters the blood and spreads throughout the body?

Answer 7

Most of the time, the body is really good at killing organisms that enter the blood. However, sometimes this can trigger a tuberculous infection in other tissues and organs (disseminated tuberculosis).

Question 8

After primary MTB infection, if the disease progresses, the disease is called…

Answer 8

progressive tuberculosis

Question 9

What is a “Ghon focus”?

Answer 9

It is the initial site of tuberculosis disease in the lung

Question 10

Where does the MTB usually spread during primary infection (3 common zones)

Answer 10

1. Initial infection focus (“Ghon focus”)
2. Regional lymph nodes
3. Apex of the lung

Question 11

Sometimes, latent TB becomes active after many years. Which zone of primary infection does this usually occur in? Why?

Answer 11

Where: MTB usually becomes active after years of latency in the apical foci of the lungs (reactivation in the upper lobe).
Why: The upper lobes are areas of high oxygen tension, where MTB thrives.

Question 12

What are the two main outcomes of reactivated pulmonary tuberculosis?

Answer 12

1. Miliary tuberculosis
2. Cavitary tuberculosis

Question 13

Miliary tuberculosis

Answer 13

A severe, disseminated form of tuberculosis which can result from reactivation of apical foci - MTB spread through the blood and form infection foci throughout the lung and the body.
Characterized by tiny little nodules - thousands of little TB foci (seed-like granulomas) - of infection in the lungs (seen on CXR and CT).
This happens because there are millions of little capillaries all throughout the lung through which the infection spreads.

Question 14

Cavitary tuberculosis

Answer 14

In some cases of reactivation, especially in the apical foci where the MTB thrive, the caseous, necrotic tissue within the granuloma liquefies and forms an air-filled cavity in the lung. When a cavity communicates with the bronchial tree (airways), the individual will start coughing up the caseous material and necrotic tissue, as well as blood (hemoptysis). Cavitary TB is therefore associated with high transmission rates, because bacteria are coughed up along with the necrotic material.

Note that the necrotic tissue at the centre of the granuloma liquefies an breaks down faster than the body’s ability to produce sufficient fibrotic tissue to contain it

Question 15

What is the histologic appearance of reactivated tuberculosis?

Answer 15

The same as regular active tuberculosis:
* Caseous necrosis
* Epithelioid histiocytes
* Fibrosis

Question 16

Before we developed drug therapy for tuberculosis, what would eventually happen to the lungs of TB-infected patients?

Answer 16

The infection would shrink and destroy entire lobes of the lung (usually upper lobe).

Question 17

What type of pulmonary infection is tuberculosis?

Answer 17

Nodular pulmonary infection (granuloma)

Question 18

Other than TB, what is another example of nodular pulmonary infection?

Answer 18

Fungal infection by histoplasmosis

*progression to active disease is extremely rare

Question 19

What is airspace (lobar) pneumonia? Which of the 4 types of infection?

Answer 19

Airspace (lobar) pneumonia is an example of diffuse parenchymal infection.
It is a type of pneumonia that primarily affects the alveoli (airspaces) within a lobe of the lung. It is typically caused by bacterial infections.

Question 20

Where does disease start in airspace (lobar) pneumonia

Answer 20

In the periphery of the lung

Question 21

What type of bacterial infection usually leads to airspace (lobar) pneumonia (name the bacterium)?

Answer 21

Streptococcus pneumoniae

Question 22

Describe the infection seen in airspace (lobar pneumonia). How do the organisms spread?

Answer 22

The alveolar spaces near the the foci of infection become filled with exudate (fluid, pus and inflammatory cells). This fluid spreads easily, leading to consolidation of large areas of the lung. This fluid rapidly spreads through the pores of Kohn and fills interconnected alveoli, carrying the microorganisms with it.

Question 23

What happens when the airspace (lobar) pneumonia spreads to the pleura? (3)

Answer 23

1. Fibrinous pleuritis: Inflammation of the pleura characterized by the deposition of fibrin-rich exudate on the pleural surfaces. This process often results in a reddish discoloration of the pleura due to the inflammatory response and increased blood flow (hyperemia) in the affected area.
2. Pleural effusion: The inflamed pleura may produce excess fluid, leading to accumulation of fluid in the pleural space
3. Empyema: If the infection persists, the exudate may become purulent and result in empyema, i.e. the accumulation of greenish pus in the pleural space.

Question 24

Anatomically, how is airspace (lobar) pneumonia distinguished from bronchopneumonia?

Answer 24

In airspace (lobar) pneumonia, the infection involves consolidation of a large continuous area of a single lung lobe, rather than having a patchy distribution across multiple lobes (as in bronchopneumonia).

Question 25

What is bronchopneumonia?

Answer 25

Pneumonia characterized by the inflammation of the bronchi and lung parenchyma, resulting in patchy areas of consolidation throughout the lungs.

Question 26

By what process does bronchopneumonia lead to the formtion of patchy foci of disease and consolidation?

Answer 26

The inflammatory response is more brisk, resulting in rapid containment of the infection and formation of patches.

Question 27

What happens if bronchopneumonia progresses?

Answer 27

Enzymes, released by neutrophils and other immune cells during the inflammatory response to the bacteria, destroy the lung tissue and lead to the formation of necrotic areas. This results in the formation of a pulmonary abscess: the necrotic tissue breaks down and forms a pus-filled cavity.

Question 28

What is a pulmonary abscess?

Answer 28

Localized areas of pus-filled necrosis within the lung tissue, typically formed by an infection.

Question 29

What is empyema?

Answer 29

Condition in which greenish purulent exudate (pus) accumulates in the pleural space. It can result from the spread of airspace (lobar) pneumonia or bronchopneumonia.

Question 30

Interstitial lung disease is usually caused by…

Answer 30

viruses

Question 31

What is interstitial pneumonia? Which of the 4 types of infection?

Answer 31

Type of pneumonia characterized by inflammation and scarring (fibrosis) of the interstitium, i.e. the tissue surrounding the alveoli and the bronchi. It is an example of interstitial infection.

Question 32

What cell is most involved in the inflammatory reaction in interstitial pneumonia?

Answer 32

Lymphocytes (B and T cells) rather than polymorphonucleur leukocytes (neutrophils, eosinophils).

In other words, the adaptive immune system is more effective at responding to interstitial pneumonia

Question 33

Define tracheitis, bronchitis, bronchiolitis and pneumonitis

Answer 33

Tracheitis: inflammation of the trachea
Bronchitis: inflammation of the bronchi
Bronchiolitis: inflammation of the bronchioles
Pneumonitis: inflammation of the lung tissue

Question 34

A viral infection that starts in the upper respiratory tract (causing tracheitis) can…

Answer 34

progress downward, affecting the lungs and leading to pneumonitis (inflammation of the lungs) and interstitial pneumonia (infection of the lungs).

Question 35

What are the two possible outcomes of interstitial pneumonia

Answer 35

1. Infection is localized to interstitial tissue only
2. Epithelial cells lining the alveoli become extensively necrotic, causing exudate to fill airspaces (alveoli)

Question 36

Define diffuse alveolar damage (DAD)

Answer 36

Condition characterized by widespread injury to the alveolar epithelium and pulmonary capillary endothelium.

Question 37

The hallmarks of diffuse alveolar damage (DAD) (3)

Answer 37

1. Presence of hyaline membranes (eosinophilic, glassy layers in alveoli, composed of fibrin and necrotic debris)
2. Accumulation of proteinaceous material (eosinophilic bright pink appearance due to high protein content)
3. Alveolar edema

Question 38

What is a viral occlusion?

Answer 38

Blockage or obstruction of blood vessels or airways due to viral infection, leading to impaired circulation or respiration.