Question

Encapsulated streptococcus bacteria evade the immune response by avoiding complement opsonization and by cleaving neutrophils extracellular traps.

So how then does the body clear pneumonia if the bacteria resists phagocytosis

Answer 1

S. pneumoniae invades its host by colonizing the nasopharynx asymptomatically. After colonization, if the bacterium is not cleared by the immune system it becomes pathogenic. A strong immune system and the balance between resident flora and invaders can help to clear S. pneumoniae before it becomes pathogenic. Host responses to the bacteria other than complement clearance and phagocytosis are:

1. mucociliary clearance: Once the pathogen is trapped in the mucus, the cilia move together to direct the trapped pathogen and the mucus to the mouth, for expelling the pathogen via coughing or swallowing.

2. The respiratory epithelial cells can recruit other cells by producing and releasing cytokines and chemokines. They also can directly kill pneumococcus by secreting antimicrobial peptides such as defensins, human apolactoferrin, and lysozyme.

3. the complement (C3) activates B cells. Following antigen stimulation, the naïve B cells differentiate into IgM+ memory B cells. Class switching produces other immunoglobulins needed for clearing the infection.

4. Cytotoxic T cells directly kill infected cells. CD4+ T cells have been found to provide protection to S. pneumoniae in an antibody-dependent manner.

5. Inflammasome mediated clearance of pneumococci.

6. Acute-phase serum proteins also helps in clearance.

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