Reaction/reactions (plural?)

[Unregistered]

'There are different types of potential problems following tattooing; for example, allergic
________ to the ink, which may lead to a form of eczema or red spots on the
skin.'

I quoted this sentence from a gapped text I had to complete. It was a word formation exercise and the given root was 'react'. I wrote 'reaction', because I thought the whole clause referred to an example for 'potential problems following tattooing'. However, the right solution is 'reactions'. I think, if used in plural, 'for example' should refer to 'reactions' not to the first clause. So as the plural form would be necessary. But in this case I do not see why I should use the plural form. Are there more 'reactions' to the ink?

May someone explain this to me please?

Greetz,
a student who wants to get 1 more point on his exam

[Raymott]

'There are different types of potential problems following tattooing; for example, allergic
________ to the ink, which may lead to a form of eczema or red spots on the
skin.'

I quoted this sentence from a gapped text I had to complete. It was a word formation exercise and the given root was 'react'. I wrote 'reaction', because I thought the whole clause referred to an example for 'potential problems following tattooing'. However, the right solution is 'reactions'. I think, if used in plural, 'for example' should refer to 'reactions' not to the first clause. So as the plural form would be necessary. But in this case I do not see why I should use the plural form. Are there more 'reactions' to the ink?

May someone explain this to me please?

Greetz,
a student who wants to get 1 more point on his exam

In this type of sentence, 'reactions' is correct. If they wanted 'reaction', they'd have written "for example, an allergic ____ ".
Without the article (actually it's a zero article), you'd use the plural.
The plural form of 'a/an' is the zero article.
A reaction (sing.) -> reactions (pl.).