# 3.2 Immune responses to influenza

The immune system uses different types of immune defence against different types of pathogen. The responses against flu are typical of those which are mounted against an acute viral infection, but different from the responses against infection by bacteria, worms, fungi or protist parasites.

When confronted with an acute viral infection, the immune system has two major challenges:

1. The virus replicates very rapidly, killing the cells it infects. Since a specific immune response takes several days to develop, the body must limit the spread of the virus until the immune defences can come into play.
2. Viruses replicate inside cells of the body, but they spread throughout the host in the blood and tissue fluids. Therefore, the immune defences must recognise infected cells (intracellular virus) and destroy them. But the immune system must also recognise and eradicate free virus in the tissue fluids (extracellular virus) in order to prevent the virus from infecting new cells.

The kinds of immune defence that the body deploys against flu are briefly considered in the next section.

OpenLearn - Influenza: A case study
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