en primera línea

= in the front line, first-line, on the front line

Ex. Because they are in the 'front line,' reference librarians frequently determine whether a library is successful in serving its clientele.

Ex. Moisturising creams are first-line agents in the management of eczema.

Ex. The World Food Programme, the world's largest humanitarian organisation, is on the front line of the fight against climate change.

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= in the front line, first-line, on the front line

Ex: Because they are in the 'front line,' reference librarians frequently determine whether a library is successful in serving its clientele.

Ex: Moisturising creams are first-line agents in the management of eczema.

Ex: The World Food Programme, the world's largest humanitarian organisation, is on the front line of the fight against climate change.