The virus that causes avian (bird) flu can attach to the cells of the lower part of the respiratory system in humans, but not to the cells of the upper part of the respiratory system. The most likely reason for this is that these two groups of cells have different .

The correct answer is "receptors".

Explanation:

Virus infect and reproduce by recognizing specific receptors in their host cells. Virus-receptor binding is highly specific, this is the reason why some virus can infect only certain animals or plants at specific tissues. If the virus that causes avian flu attaches only to the cells of the lower part of the respiratory system, it is very likely that these cells have specific receptors that are not present in the upper part of the respiratory system, receptors that the viruses recognize.

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