Lower Respiratory Tract Infections - Host Immunity

Host Immunity of Lower Respiratory Tract Infection (LRTI)

Human hosts have several non-specific defense mechanisms when potential infection in the lower respiratory tract (LRT) arises:

• anatomically- nasal hairs, convoluted passages, and the mucosal lining of nasal turbinates filter the potential pathogens

• secretory IgA and non-specific antibacterial substances (lysozyme) in respiratory secretions cause lysis of invading organism

• ciliary motion and mucosal lining of the trachea prevent attachment of infective agents onto the host cells

• reflexes such as coughing, sneezing, and swallowing can disrupt/dislodge pathogens from the respiratory tract

These mechanisms prevent foreign objects or micro-organisms from entering the bronchi and gaining access to the lungs, thus preventing Lower Respiratory Tract Infection (LRTI).

The aspiration of minor amounts of oropharyngeal material during sleep plays an important role in the pathogenesis of many types of pneumonia. Once particles escape mucociliary sweeping activity and enter the alveoli, alveolar macrophages ingest them and carry them to the lymphatics.

In addition, normal flora of the nasopharynx and oropharynx help prevent colonization by pathogenic micro-organisms by competing for space, and nutrients as well as the production of bacteriocins and metabolic products that are toxic to invading micro-organisms.

For unknown reasons, perhaps because of previous damage by a viral infection, loss of some immunity, or physical damage to the respiratory epithelium (eg. from smoking) normal flora from the human respiratory tract has been known to cause infections.

Colonization during Lower Respiratory Tract Infection (LRTI) does not always represent infection and is thus important to differentiate between them i.e. on the basis of specimen source, the number of micro-organisms observed, microbial colony cultured, and the presence of a quantity of WBC.