Not a physicist but I can answer this. Antineutrinos have opposite lepton number compared to neutrinos. Lepton number is preserved by all particle reactions[1]

Not a physicist but I can answer this. Antineutrinos have opposite lepton number compared to neutrinos. Lepton number is preserved by all particle reactions[1] so for example beta decay must produce an antineutrino (L=-1) in addition to an electron (L=1). Antineutrons also exist. Neutrons are composed of three quarks, uud (u has charge 1/3 and d had charge -2/3), while antineutrons are composed of two up antiquarks (charge -1/3) and one down antiquark (charge 2/3). And as in the case of neutrinos, neutrons have baryonic number 1 like protons and antineutrons have baryonic number -1 like antiproton a. Hence, for example, a beta decay will always turn a neutron into a proton and an antineutron into an antiproton (though the latter is theoretical). [1] More precisely there is a lepton number for each generation (electron/muon/tau) and all three are separately preserved.