B12 and Folate Deficiency
A deficiency of either B12 or folate stalls DNA synthesis in every dividing cell, so red cell precursors keep growing but cannot divide, producing large, immature megaloblasts, while B12 deficiency alone also strips myelin, adding neurological disease that folate cannot fix and may make worse.
First principles
Both vitamins are needed to make DNA, so their lack produces one shared blood picture
B12 and folate both act as cofactors in the pathway that makes thymidine for DNA synthesis. Without either, nuclear maturation in developing red cells lags behind cytoplasmic growth, so the cells keep enlarging without being able to divide on schedule: megaloblasts. This produces macrocytic anaemia with hypersegmented neutrophils on the film, and because every rapidly dividing lineage is affected, cytopenias in other cell lines can follow.
Educational content pending clinical review. Not medical advice.