Bronchiolitis Patho

Respiratory syncytial virus (RSV) is an enveloped, single-stranded, negative-sense RNA virus of the family Pneumoviridae. The viral envelope contains two critical surface glycoproteins: the G protein (attachment protein) that binds to cell surface receptors (CX3CR1 on ciliated epithelial cells) mediating initial viral attachment, and the F protein (fusion protein) that facilitates fusion of the viral envelope with the host cell membrane, allowing viral RNA to enter the cytoplasm. Once inside the cell, the RSV RNA-dependent RNA polymerase (carried within the virion) transcribes the negative-sense RNA genome into positive-sense mRNA, which is then translated by host ribosomes into viral proteins. New viral RNA is replicated, assembled with structural proteins at the cell membrane, and released by budding — each infected cell produces thousands of new virions over a 20-24 hour replication cycle before the host cell undergoes necrosis and lyses. The innate immune response to RSV in infants is characterized by a Th2-predominant cytokine profile (IL-4, IL-5, IL-13) rather than the protective Th1 response (IFN-gamma, IL-12) seen in older children and adults. This Th2 skewing occurs because the infant immune system...