Lannes' \(T\) functor is the left adjoint to the functor \(M\mapsto H^*(B({\mathbb{Z}}/p{\mathbb{Z}}))\oplus M\) in the category of unstable modules over the Steenrod algebra. If \(A\) is a Hopf algebra over the Steenrod algebra as well as an unstable module then \(T(A)\) is a Hopf algebra. In this paper we derive a general form for \(T(A)\) when \(A\) is a graded commutative Hopf algebra with a conjugation. We use this to determine the action of \(T\) on the algebras \(M_ s\), studied by Campbell and Selick, and on the algebra of Carlsson modules. Using the algebra structure of \(T(M_ s)\) we determine the action of the \(T\) functor on a family of summands of \(H^*(B({\mathbb{Z}}/p{\mathbb{Z}})^ s)\) knownas the weight summands.