Pathogenesis of Pneumonia: Mechanisms and Implications

Pneumonia is fundamentally a problem of alveolar flooding. The alveoli are tiny air sacs where oxygen and CO2 exchange occurs through a thin membrane — in a healthy lung, this membrane is essentially dry and gas passes freely. When a pathogen (bacteria, virus, or fungus) enters the lung and overwhelms local defences, the immune system launches an inflammatory response: blood vessels dilate and become leaky, and inflammatory fluid (exudate) pours into the alveolar space. The alveolus fills with protein-rich fluid and immune cells. Once flooded, that alveolus can no longer participate in gas exchange — blood passing through is not oxygenated. This creates ventilation-perfusion (V/Q) mismatch: blood perfuses areas of the lung that are not ventilating, returning to the heart deoxygenated. As more alveoli fill, SpO2 falls. This is why pneumonia can cause hypoxia even though the rest of the lung is fine. The body responds by increasing respiratory rate (tachypnoea) to compensate — which is why fast breathing is one of the most sensitive early warning signs. In severe pneumonia, the flooding extends to multiple lobes; when the respiratory muscles fatigue from the increased work of breathing, respiratory failure can follow. This is the pathway from 'chest infection' to ICU admission that happens over hours to days — catching early signs (RR >24, SpO2 falling, confusion) is what enables intervention before the cascade completes.

The outcome of any pneumonia is shaped by the contest between the pathogen's virulence and the host's defences. On the pathogen side: Streptococcus pneumoniae (the most common cause of bacterial pneumonia) produces polysaccharide capsules that resist phagocytosis, pneumolysin toxin that disrupts alveolar membranes, and can rapidly multiply to overwhelming numbers. More aggressive organisms mean more damage before defences can mount. On the host side, several factors impair the respiratory defence system: (1) Mucociliary escalator: cilia lining the airways sweep pathogens upward; cigarette smoke paralyses cilia within minutes — even occasional smokers have compromised escalator function, explaining why smokers get pneumonia more often and more severely. (2) Alveolar macrophages: the resident immune cells in the alveoli are the first line of phagocytosis; alcohol impairs macrophage function significantly — heavy drinkers have 3–4× higher pneumonia risk. (3) Immunosuppression: steroid use, diabetes (which impairs neutrophil function), HIV, and cancer treatment all reduce the immune response. (4) Structural lung disease: COPD, bronchiectasis, or post-TB fibrosis leave areas of the lung with impaired drainage and mechanical clearance. (5) Age: infants (immune system immature) and the elderly (T-cell function declines with age) are at each extreme. In India, malnutrition compounds all of these — a protein-deficient child has impaired secretory IgA production, reduced complement activity, and weakened macrophage function. This is why pneumonia kills 400,000+ Indians annually, predominantly children under 5 and adults over 65.

Yes — the type of organism determines both the clinical picture and the antibiotic choice. Typical bacterial pneumonia (Streptococcus pneumoniae, Haemophilus influenzae): starts abruptly — sudden high fever (39–40°C), rigor (shaking chills), productive cough with rust-coloured or purulent sputum, pleuritic chest pain (sharp pain worsening with deep breath, from pleural involvement). CXR shows lobar or segmental consolidation. Responds well to amoxicillin or beta-lactam antibiotics. Atypical pneumonia (Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella): more gradual onset over several days, lower fever, dry or minimally productive cough (often described as a 'walking pneumonia' because patients may remain ambulatory), prominent extrapulmonary features — headache, myalgia, sore throat. CXR often shows interstitial or bilateral patchy infiltrates without clear lobar consolidation. Crucially: Mycoplasma has no cell wall, so penicillins and cephalosporins don't work. Treatment requires a macrolide (azithromycin, clarithromycin) or doxycycline. The clinical significance in India: Mycoplasma is particularly common in young adults aged 5–35, spreads in schools and colleges, and is a common cause of 'antibiotic not working' cases where amoxicillin was correctly prescribed but for the wrong organism. Viral pneumonia (influenza, SARS-CoV-2, RSV) can mimic atypicals but has distinct epidemiological patterns and responds to antivirals rather than antibiotics.

The pneumococcal vaccine protects against Streptococcus pneumoniae specifically — which is responsible for roughly 30–50% of community-acquired bacterial pneumonia cases requiring hospitalisation. It does not protect against Mycoplasma, Legionella, Klebsiella, viral pneumonia, or aspiration pneumonia from oral bacteria. So it is valuable but partial. Two types of pneumococcal vaccine are available in India: PCV13 (Prevenar 13) — covers 13 serotypes including the most virulent ones; recommended for all children in the Universal Immunisation Programme (given at 6 weeks, 14 weeks, and 9 months in states where UIP covers it). Pneumovax 23 (PPSV23) — covers 23 serotypes; recommended for adults. Who should get PPSV23: adults ≥65; adults of any age with COPD, diabetes, heart failure, asthma requiring hospitalisation, liver disease, or sickle cell disease; those without a spleen (asplenia — very high pneumococcal pneumonia risk); immunocompromised patients including those on long-term steroids. In India, adult pneumococcal vaccination is underutilised — most people in high-risk groups above are unvaccinated. The vaccine reduces hospitalisation for pneumococcal pneumonia by approximately 45–50% in the elderly. Annual influenza vaccine is a separate, important protection — influenza damages airway epithelium, creating the perfect entry point for secondary bacterial pneumonia; the 'flu then pneumonia' sequence was the main cause of death in the 1918 pandemic and remains important today. Both vaccines are available at private hospitals and many government centres; PPSV23 costs approximately ₹1,500–2,500 at private pharmacies.