Tuberculosis
6.6 Understand how Mycobacterium tuberculosis (TB) and Human Immunodeficiency Virus (HIV)
infect human cells, causing a sequence of symptoms that may result in death.
Tuberculosis
• Tuberculosis (TB) is caused by the airborne bacteria Mycobacterium tuberculosis.
• It is a widespread disease that affects the respiratory system.
• TB can be treated using a course of antibiotics.
• However, antibiotic resistance strains of TB have emerged.
Infection
• When a person becomes infected with TB, an immune response is produced.
• A tissue mass called a tubercle forms around the infected site and after 3-8 weeks, the infected
region heals.
• The bacteria can survive inside macrophage for years until the immune system becomes weak.
Symptoms
• If a patient’s immune system is unable to control the infection, active tuberculosis may occur.
• The bacteria will multiply rapidly, destroying the lung tissue, which can be fatal.
• Common Symptoms: coughing, shortness of breath, loss of appetite, weight loss, fever, night sweats.
• Fevers and night sweats occur due to neutrophils and macrophages releasing fever-causing
substances as part of the inflammatory response.
• These chemicals (including interferons) affect the hypothalamus and increase core body temperature.
Dormancy & Activation
• The bacteria are taken up by phagocytes and replicate inside it.
• The infected phagocytes are sealed off in tubercles.
• While dormant, the infected person will experience no symptoms.
• Reactivated bacteria may be able to overcome the immune system, causing TB.
• Reactivation is more likely in people with weakened immune systems, e.g. people with AIDS.
Immune System Evasion Mechanisms
• When M. tuberculosis bacteria infect the lungs, they are engulfed by phagocytes.
• Here, they produce substances that prevent the lysosome fusing with the phagocytic vacuole.
• This means the bacteria are not broken down and can multiply undetected inside phagocytes.
• The bacterium also disrupts antigen presentation in infected cells.
This prevents immune system cells from recognising and killing the infected phagocytes.
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