Lung - nontumor
Influenza pneumonia

Author: Elliot Weisenberg, M.D. (see Authors page)

Revised: 24 February 2017, last major update September 2011

Copyright: (c) 2003-2017,, Inc.

PubMed search: influenza pneumonia [title]

Cite this page: Influenza pneumonia. website. Accessed June 25th, 2018.
  • Most common cause of viral pneumonia
  • RNA virus, nucleoprotein determines type of virus: A, B or C (Virol J 2009 Mar 13;6:30)
  • Envelope contains the viral hemagglutinin and neuraminidase that establish the viral subtype, e.g., H1N1 (Wikipedia)
  • Preexisting antibodies to specific hemagglutinins and neuraminidases prevent or amerliorate future infections
  • Type A: causes most severe disease; major cause of epidemics and pandemics; also infects pigs, horses, birds; one subtype predominates at any given time
  • Antigenic drift leading to epidemics is caused by small mutations in hemagglutinins and neuraminidases, which allow the virus to evade most host antibodies
  • Antigenic shift leads to pandemics, and is caused by recombination of hemagglutinin and neuraminidase RNA with animal influenza RNA, leading to absence of human immunity
  • Antigenic drift and shift do NOT occur with influenza types B and C, which usually cause mild, self limited illness in children
  • References: World Health Organization Influenza update
Clinical features
  • Cause mild acute lung injury to necrotizing pneumonia to BOOP-like changes
  • May cause bronchiolitis or diffuse alveolar damage
  • Viral cytopathic changes do NOT occur
  • Airways show mucosal hyperemia with infiltrate of lymphocytes, histiocytes, plasma cells
  • Swelling and increased mucus production impair bronchociliary elevator, and may lead to secondary bacterial infection; small airways may become occluded causing focal atelectasis; in severe disease, this is more widespread with distal airways plugged by fibrinopurulent debris
  • If process is prolonged, obliterative bronchiolitis with irreversible lung damage may occur
  • Histologic findings are nonspecific; the diagnosis can be confirmed with molecular testing, viral culture, DFA, IHC, serology, antigen detection assays
Microscopic (histologic) images

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Heavy and firm lung

Gross images

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Hyaline membranes