What could cause high mch, high mcv, low rdw, low B12 and high iga??

Tapeworm. The High MCV and MCH would be expected with B12 deficiency as the RBC's are being discharge young from the bone marrow. Low RDW just tells you you have only one predominant population of RBCs (dark large ones) Elevated IgA can be caused by several conditions but a tapewor taking up residence in the terminal ilium would block the binding of intrinsic factor / B12 complex to its receptors.
More than one issue. Low B12 can explain the high mch, mcv and low rdw; these are indications of a macrocytic anemia. You may need B12 injections to address the issue. How high is the IgA? Need to know the actual value as there is considerable variation in normal values.