  We note that in the limit where the NN 1S0 and 3S1 scattering lengths,
a^(1S0) and a^(3S1), go to infinity, the leading terms in the effective field
theory for strong NN interactions are invariant under Wigner's SU(4)
spin-isospin symmetry. This explains why the leading effects of radiation pions
on the S-wave NN scattering amplitudes vanish as a^(1S0) and a^(3S1) go to
infinity. The implications of Wigner symmetry for NN to NN axion and gamma d to
n p are also considered.
