Define about the Deficiency of cyanocobalamin?

Malabsorption of vitamin B₁₂ can occur at several points during digestion. By far, the most important condition resulting in vitamin B₁₂, malabsorption is the autoimmune disease called pernicious anaemia (PA). In most cases of PA, antibodies are produced against the parietal cells causing them to atrophy, and lose their ability to produce intrinsic factor and secrete hydrochloric acid. In some forms of PA, the parietal cells remain intact but auto antibodies are produced against the intrinsic [actor itself and attach to it, thus preventing it from binding vitamin B₁₂. In another less common form of PA, the antibodies allow vitamin B₁₂ to bind to the intrinsic factor but prevent the absorption of the intrinsic factor-vitamin B₁₂ complex by the ileal receptors.