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Tuberculosis, or TB, is a common disease; around one third of the world’s population has been exposed, but most people remain asymptomatic. People who do have an active tuberculosis infection tend to experience a classic pattern of symptoms and signs. Symptoms of tuberculosis typically include chronic cough which may produce bloody sputum, chest pain, weight loss, and fever.
In humans, tuberculosis is caused by Mycobacterium tuberculosis, a bacterium which infects the lungs. Diagnosis of tuberculosis is confirmed on the basis of chest x-rays, skin testing, and sputum testing, as a person with tuberculosis will have the bacteria in their sputum. Sputum is matter which has been coughed up from the lower respiratory tract and airways, and contains mucus, saliva, and any bacteria infecting the lower airways and lungs.
Preliminary diagnosis is typically made on the basis of symptoms of tuberculosis which may be present. The most common signs and symptoms of tuberculosis include a chronic cough, blood in the sputum, night sweats and fever, aching chest pain, and unexplained weight loss. A person with these symptoms will usually be referred for chest x-rays, sputum testing, and other tests to confirm a tuberculosis diagnosis.
The symptoms of tuberculosis are the result of infection by the M. tuberculosis bacterium. Infection starts when the bacteria begin to colonize the alveoli of the lungs. Once colonization begins, bacteria are ingested by immune cells called macrophages, a cell type responsible for ingesting and breaking down bacterial cells and cellular debris. Macrophages typically have no trouble digesting bacteria; however, they are unable to kill M. tuberculosis bacteria they have ingested.
As the infection advances other immune cells, including T cells and B cells, begin to congregate around the infected macrophages. The immune cells form clumps called granulomas, which are small regions of infected macrophages surrounded by T and B lymphocytes. The immune system forms granulomas in cases where immune cells cannot eliminate the infection; instead of killing the pathogen, the immune system walls off the infected area by surrounding it with a barrier of cells. Within the granulomas, lymphocytes secrete cytokines to help kill the invading bacteria.
The classic symptoms of tuberculosis, including bloody sputum, the characteristic cough, and fever, are all caused by this chronic infection. The pattern of infection may include multiple cycles of granuloma formation, tissue destruction, and healing, and can last years and even decades if the disease is not treated. As the infection advances, lung tissue becomes progressively scarred, reducing pulmonary function for affected individuals. People who are infected also become more contagious as the disease advances, because as their lung tissue becomes more necrotic they are more likely to cough up infected material.