If you have ever had a very bad or life-threatening reaction called angioedema.
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Please explain to me the use of "have had" structure here?
Is "had" a main verb and "have" helping verb?
Yes. We say eg I have a cold. eg I had a cold.
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JigneshbharatiIs "had" a main verb and "have" a helping verb?
Yes.
Have you ever had measles?
I have had a bad cold all week.
He has always had trouble with mathematics.
CJ
Does "had" mean "possess" here? Broadly speaking, yes. But really, you don't 'possess' angioedema in the same sense as you possess eg a car.
You undergo or suffer from angioedema.
Have they used the present perfect to cover the period from the past to present? Yes