By Katherina Baranova MD and Matthew J. Cecchini MD PhD FRCPC, updated April 3, 2020
When you breathe air enters your body through your mouth and nose and travels down airways into your lungs. Inside the lungs, oxygen enters the blood and carbon dioxide is removed from the body.
The largest airway is called the trachea. When it enters the chest, the trachea splits into two branches call bronchi. The airways are hollow tubes lined by cells that have thin microscopic hair-like structures on their surface called cilia. The cilia help prevent infections by removing inhaled particles before they have a chance to enter your lungs. The airways also contain glands that produce thick mucus which traps particles in the air.
You have two lungs, one on the right side of your chest and one on the left. The lungs are divided into parts called lobes. The right lung has three lobes – upper, middle, and lower. The left lung is slightly smaller and has two lobes – upper and lower. As the bronchi enter the lungs they split into smaller and smaller airways known as bronchioles ultimately ending in air filled spaces called alveoli.
The alveoli are extremely small cup-shaped air-filled spaces lined by flat cells called pneumocytes. There are two types of pneumocytes. Type 1 pneumocytes are small and flat. Type 2 pneumocytes are thicker and shaped more like a cube. The alveoli are surrounded by a fine network of blood vessels called capillaries which carry blood in and out of the lungs.
Pneumonia is an infection of the lungs caused by viruses, bacteria, or fungus. Most pneumonias start when infectious particles, breathed in through the nose and mouth, travel down the airways into the lungs. Pneumonia can affect one or both lungs at the same time. The disease can involve only a part of the affected lung or it can involve the entire.
The most common symptoms of pneumonia include cough, fever, chills, mucus production, and difficultly breathing. Other less common symptoms include confusion, chest pain, blue lips and fingernails, and loss of appetite. Pneumonia, like other infections, is worse in older people and people whose immune system is impaired.
There are many types of pneumonia although most types are caused by bacteria or viruses. Less common types are caused by fungus.
This type of pneumonia starts when bacteria enter the lungs and alveoli, usually from the nose or mouth. Bacteria can also travel to the lungs from other parts of the body through the blood. Bacteria in the alveoli will grow within the air space until they are detected by the body’s immune system, which will attempt to remove the bacteria from the body. To aid in this process, small capillaries surrounding the alveoli will open to increase the amount of blood flowing to the lungs. This allows more inflammatory cells including neutrophils to reach the lungs. Pathologists describe this process as acute inflammation. The green or yellow sputum produced when a person with pneumonia coughs is made up of millions of neutrophils and debris from damaged tissue and dead bacteria. It is a sign that active inflammation is taking place in the lungs.
Although neutrophils are required to kill and remove the bacteria in the lungs, they can also damage the pneumocytes lining the alveoli. The combination of damaged pneumocytes with extra blood flow can cause fluid to fill the air spaces. This process is called edema. Because the fluid prevents air from getting into the alveoli, a person with pneumonia can have difficulty breathing. When an x-ray or CT scan is performed, areas of the lung with infection and edema are described as showing “consolidation”.
Once your immune system has the infection under control, specialized immune cells called macrophages will move in to clean up the dead bacteria and damaged tissue. At the same time, the edema fluid will organize in the air spaces and become more solid. These areas of the lung will look grey to white on an x-ray or CT scan.
A bacterial pneumonia can be separated into a lobar and bronchopneumonia. In a lobar pneumonia, the bacteria have spread to involve most of the lobe of one lung. In bronchopneumonia, the bacteria have spread through the small air spaces to involve parts of more than one lobe in a lung. Because of this, the areas of lung involved by bronchopneumonia are usually near an airway.
Types of bacteria that commonly cause pneumonia include:
This type of pneumonia starts when viral particles enter the body, usually through the nose or mouth. The virus travels to the lungs where they infect the pneumocytes or other cells in the alveoli and airways.
Viruses use specialized proteins found on their surface to attach to other proteins on the surface of cells in the lungs. For example, SARS-CoV-2, the virus that causes COVID-19 uses a protein on its surface to attach to the protein angiotensin-converting enzyme 2 (ACE2) on cells in the lung. After the virus attaches to the cell, it is brought inside the cell where it uses the machinery of the cell to make new copies of the virus.
Pneumocytes infected by a virus can become damaged and die. The body responds to this injury by replacing the thin type 1 pneumocytes with the thicker, stronger type 2 pneumocytes. Injured pneumocytes also release signals that tell specialized virus fighting immune cells called lymphocytes to come to lungs. As in bacterial pneumonia, fluid fills the air spaces which makes it difficult for a person with pneumonia to breathe.
Pathologists use the term hyaline membranes to describe thick pinks bands of tissue on the inner surface of the alveoli. These hyaline membranes are often seen in combination with type 2 pneumocytes and fluid filling the air spaces and alveolar walls. People with pneumonia have difficulty breathing because the thickened alveolar walls make it more difficult for oxygen to exchange between the lungs that the blood. The changes associated with viral pneumonia on x-ray or CT scan will also appear more grey or white because the alveoli have less air than normal in these areas.
Types of viruses that commonly cause pneumonia include:
A pathologist can make the diagnosis of pneumonia after examining a tissue sample from the lung under the microscope. A procedure that removes only a small sample of tissue is called a biopsy. The amount of tissue sent for microscopic examination will depend on the type of biopsy performed.
Common types of biopsies include:
Most of the changes associated with pneumonia can be seen when the tissue is stained with a combination of dyes called hematoxylin and eosin (H&E). However, in order to determine the type of pneumonia, your pathologist may order special stains such as a Gram, GMS, and PAS stains. Another type of test called immunohistochemistry may be ordered to look for specific types of viruses. However, the exact cause can only be determined after a tissue sample is sent to the microbiology laboratory which has special tools for identifying viruses, bacteria, and fungus.