Normally, you can just say 'I've had a reaction', most audiences would know that you mean you've got an allergy and you are reacting to something.
When we say 'my allergies are going off', it means that the symptoms are starting to appear/happen/go off!
"I've had a reaction."（[アレルギー](https://eikaiwa.dmm.com/uknow/questions/33333/)）反応が出た
という言い方で、「あなたはアレルギーがあって、その[症状](https://eikaiwa.dmm.com/uknow/questions/38665/)が出た」ということが伝わります。
"my allergies are going off."と言った場合、「アレルギー反応が起きた（出だした）」、という意味で、appear/happenなどと同じ意味になります。
"I had an allergic reaction to..." You would have to finish saying this sentence with what you are allergic to.
"My allergies started acting up." If you started getting a runny nose, itchy eyes, or any kind of thing that happens when your allergies are bad you can say they were 'acting up'. 'Acting up' does not mean you are pretending, it means that you started to have a reaction to whatever you were allergic to, 'acting up' can mean to become worse (If a child starts to misbehave you can say "He is acting up").
"I had an allergic reaction to..."
（...に[アレルギー](https://eikaiwa.dmm.com/uknow/questions/33333/)（反応）が出た）
↑何にアレルギーがあるのかを言わないとこの文は完結しません。
アレルギーがひどい時（[鼻水が出る](https://eikaiwa.dmm.com/uknow/questions/39320/)、目がかゆい、など）は「acting up」が使えます。
「acting up」は「pretending（ふりをする）」という意味ではありません、アレルギー反応が出始めたことを表します。「acting up」には「悪くなる（become worse）」という意味もあります。（子どもの行儀が悪くなったら「He is acting up」と言えます）
If you have had a severe allergic reaction, you can say you had an allergy attack, literally meaning your allergies attacked you.
If you are having only mild reactions, you can explain to people that you are having allergies at the moment or say your allergies have been set off.
ひどいアレルギー反応が出たのなら、'I had an allergy attack'（アレルギーの発作が出ました）と言えます。文字通りにはアレルギーに襲われた（attack）という意味です。
軽い症状なら、'I have allergies right now'（アレルギーが出ています）あるいは 'My allergies have been set off'（アレルギーが出ています）と説明できます。
You could also describe an allergy as a 'hypersensitivity'
If you are prone to allergies, you could say that they are 'playing up' or 'triggered'
Prone to = likely to experience
'My allergies have been playing up since that meal we shared'
'I had an allergy attack triggered by breathing in dust'
Some allergies have specific names. An allergy to pollen is often known as 'hayfever' for instance.
'My hayfever is particularly bad today. It's because we had a picnic in that corn field.'
allergy'（アレルギー）は'hypersensitivity'と表すこともできます。
アレルギー反応が出やすいなら、'to play up'（悩ます）や'to trigger'（引き起こす）という表現が使えます。
Prone to = ～になりやすい
例文
'My allergies have been playing up since that meal we shared'
（食事をしてから、アレルギーが出ている）
'I had an allergy attack triggered by breathing in dust'
（ほこりを吸って、アレルギー反応が出た）
アレルギーの中には特定の名称があるものもあります。例えば「花粉アレルギー」は'hayfever'（花粉症）と言うことが多いです。
例文
'My hayfever is particularly bad today. It's because we had a picnic in that corn field.'
（今日は特に花粉症がひどい。トウモロコシ畑にピクニックに行ったからだよ）
When you have an allergy, it means your body is sensitive to a substance that most people are not sensitive to. The substance that causes an allergic reaction is called an "allergen".
Sometimes allergies can be mild (sneezing, itchiness) and sometimes they can be severe (trouble breathing, low blood pressure). If you have an allergy, it is important to tell your doctors and nurses what the allergen is, and what happens when you come into contact with it.
"I had an allergic reaction to penicillin. It made me itchy".
"I am allergic to pollen. That is why my eyes are red."
"I can't eat cheese because I have a dairy allergy."
アレルギーを持つとはあなたの体が多くの人が敏感でない物質に敏感であるという意味です。
アレルギー反応を起こす物質は"allergen”(アレルゲン)と呼ばれます。
時にはアレルギー症状は軽く(くしゃみ、かゆみ)、時には重い(呼吸困難、低血圧)です。あなたがアレルギーを持っているのであれば医者と看護師にアレルゲンとそれと接触すると何が起こるかを伝えることが大切です。
"I had an allergic reaction to penicillin. It made me itchy". (私はペニシリンにアレルギー反応を起こしました。かゆい思いをしました。)
"I am allergic to pollen. That is why my eyes are red." (私は花粉症です。だから目が赤いのです。)
"I can't eat cheese because I have a dairy allergy." (私は乳製品にアレルギーを持っているためチーズを食べることができません)
By telling someone you 'suffer from allergies' then they would know you can have an allergic reaction you would be best to tell them what you are allergic to so they would know if it happened and be able to help. By saying 'I had an allergic reaction' it means it happened but again describing what you had a reaction to is always best in case they might need to help you
I suffer from allergies'（アレルギーがあります）と言えば、アレルギー反応が出ることがあることが伝わります。何に対してアレルギーがあるのか言った方がいいと思います。そうすれば、相手はアレルギー反応が出たかどうかが分かり、助けることができます。
'I had an allergic reaction'は「アレルギー反応が出ました」という意味です。これについても、助けが必要な場合もありますから、何に対してアレルギーが出たのか伝えた方がいいです。
Sometimes we may talk about an allergy 'flaring up' when you get a bad reaction to something. If you have an allergy, most of the time you may not suffer at all - then something will cause it 'to flare up'.
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