Pathology - Infections of the lung Flashcards
What are the 4 types of pulmonary infection?
- Nodular (tuberculosis granuloma)
- Diffuse parenchymal (airspace (lobar) pneumonia)
- Patchy parenchymal (bronchopneumonia)
- Interstitial (interstitial pneumonia)
What is the body’s first reaction to tuberculosis?
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
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.
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.
What is the result of the inflammatory response to MTB? Under the microscope, what do we see?
- Necrotic (dead) lung tissue
- Accumulation of epithelioid histiocytes (with characteristic elongated nuclei)
What are the 2 main defence mechanisms our body has against MTB?
- 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)
MTB often gain access to the blood. How?
- By entering the lymphatics and draining into the bloodstream
- Direct invasion of pulmonary blood vessels
What happens when MTB enters the blood and spreads throughout the body?
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).
After primary MTB infection, if the disease progresses, the disease is called…
progressive tuberculosis
What is a “Ghon focus”?
It is the initial site of tuberculosis disease in the lung
Where does the MTB usually spread during primary infection (3 common zones)
- Initial infection focus (“Ghon focus”)
- Regional lymph nodes
- Apex of the lung
Sometimes, latent TB becomes active after many years. Which zone of primary infection does this usually occur in? Why?
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.
What are the two main outcomes of reactivated pulmonary tuberculosis?
- Miliary tuberculosis
- Cavitary tuberculosis
Miliary tuberculosis
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.
Cavitary tuberculosis
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
What is the histologic appearance of reactivated tuberculosis?
The same as regular active tuberculosis:
* Caseous necrosis
* Epithelioid histiocytes
* Fibrosis
Before we developed drug therapy for tuberculosis, what would eventually happen to the lungs of TB-infected patients?
The infection would shrink and destroy entire lobes of the lung (usually upper lobe).
What type of pulmonary infection is tuberculosis?
Nodular pulmonary infection (granuloma)
Other than TB, what is another example of nodular pulmonary infection?
Fungal infection by histoplasmosis
*progression to active disease is extremely rare
What is airspace (lobar) pneumonia? Which of the 4 types of infection?
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.
Where does disease start in airspace (lobar) pneumonia
In the periphery of the lung
What type of bacterial infection usually leads to airspace (lobar) pneumonia (name the bacterium)?
Streptococcus pneumoniae
Describe the infection seen in airspace (lobar pneumonia). How do the organisms spread?
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.
What happens when the airspace (lobar) pneumonia spreads to the pleura? (3)
- 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.
- Pleural effusion: The inflamed pleura may produce excess fluid, leading to accumulation of fluid in the pleural space
- 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.
Anatomically, how is airspace (lobar) pneumonia distinguished from bronchopneumonia?
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).
