From a reading test on IELTS PTE Online titled "Allergy Testing,
A person does not become allergic to a particular substance until after the first exposure. However, in some cases, even trace amounts of a substance, such as peanuts or seafood in a mother's breast milk, can cause an allergic reaction in a subsequent exposure.
Does this "exposure" mean
[uncountable] exposure (to something) the state of being in a place or situation where there is no protection from something harmful or unpleasant
Source: Oxford Learner's Dictionaries
If it is, why before the second "exposure" has "a"? Because according to the Longman Dictionary, it is an uncountable noun.