Pathophysiology of Pulmonary Tuberculosis
Pulmonary tuberculosis (TB) is a chronic infectious disease caused by the bacterium Mycobacterium tuberculosis. The disease primarily affects the lungs but can spread to other parts of the body. Understanding the pathophysiology of TB is crucial for effective diagnosis, treatment, and prevention.
1. Infection and Initial Response
- Inhalation: The infection begins with the inhalation of aerosolized droplets containing M. tuberculosis from an infected individual.
- Macrophage Infection: The bacteria enter the alveoli, where they are phagocytosed by alveolar macrophages. However, M. tuberculosis has evolved mechanisms to evade macrophage killing.
- Cell-Mediated Immunity: The body mounts an immune response, primarily involving T lymphocytes. These cells release cytokines like interferon-gamma (IFN-γ) that activate macrophages to kill the bacteria.
2. Primary Tuberculosis
- Granuloma Formation: The immune response forms granulomas, which are microscopic nodules containing macrophages, lymphocytes, and other immune cells. These granulomas try to wall off the infection.
- Caseation Necrosis: In some cases, the center of the granuloma undergoes caseous necrosis, a type of cell death that leaves a cheese-like substance.
- Latency: In most individuals, the immune system effectively controls the infection, preventing further spread and leading to a latent TB state.
3. Reactivation and Secondary Tuberculosis
- Weakened Immune System: Reactivation occurs when the immune system is weakened due to factors like HIV infection, malnutrition, or immunosuppressive drugs.
- Granuloma Breakdown: The immune response weakens, allowing the granulomas to break down. The bacteria can spread from the original site to other parts of the lungs and the body.
- Cavitary Lesions: The breakdown of granulomas can lead to cavities in the lungs, which are characteristic of active TB. These cavities can further spread the bacteria through coughing.
4. Clinical Manifestations
- Symptoms of Active TB: These include persistent cough, chest pain, fever, chills, night sweats, weight loss, and fatigue.
- Disseminated TB: The infection can spread to other organs, leading to complications such as meningitis, pericarditis, and skeletal TB.
5. Factors Influencing Disease Progression
- Bacterial Factors: The virulence of M. tuberculosis strain plays a role.
- Host Factors: Factors such as age, nutrition, immune status, and co-morbidities influence the progression of TB.
- Environmental Factors: Living in crowded conditions and exposure to air pollution can increase the risk of infection and reactivation.
6. Diagnosis and Treatment
- Diagnostic Tests: Diagnosis is based on clinical evaluation, chest X-ray, sputum smear, and culture.
- Treatment: Anti-tubercular drugs are used to treat active TB, typically for a period of six to nine months.
Understanding the pathophysiology of TB is crucial for guiding treatment strategies, controlling the spread of infection, and developing new diagnostic and therapeutic approaches.
