Words have power, and sometimes that power misleads us.

We think of particles and antiparticles, we think of “annihilation”, perhaps even think that only “pure energy” remains.

But that is not how Nature works (at least to the best of our knowledge). Take a look at serious quantum field theory textbooks. You won’t read much there about “annihilation”. Rather, you will see a lot about propagators and vertex rules, i.e., mathematical terms that describe particles minding their own business vs. particles interacting.

So then, if it happens to be a textbook on the Standard Model of particle physics, you can easily look up what a neutrino can or cannot do. Looking up vertices with only an incoming and an outgoing neutrino (or, equivalently, an incoming neutrino-antineutrino pair) you will see that the interaction may produce either a Z-boson or a Higgs boson.

Now both the Z boson and the Higgs boson are very massive, which means that unless the neutrino and the antineutrino collide with a very large amount of excess kinetic energy in the center-of-mass reference frame, the resulting boson will be virtual, short-lived, and the interaction will be very unlikely to take place in the first place. But if it were to happen, the Z-boson or the Higgs boson would decay almost immediately. The decay products may include a new pair of fermions (e.g., an electron-positron pair), photons, or simply just a new neutrino-antineutrino pair!

The bottom line is that particles and antiparticles interact, their rest masses, kinetic energies and other conserved properties surviving in the form of new particles.