Which statement about Bombay phenotype is true?

• A. They lack H antigen on red blood cells
• B. They have H antigen on red blood cells
• C. They express A and B antigens on red blood cells
• D. They cannot form anti-H antibodies

Answer: (C)

Bombay phenotype happens when red cells lack the H antigen because the hh genotype prevents the enzyme that creates H from building onto the cell surface. Since H is the precursor for A and B antigens, these cells cannot express A or B antigens either, regardless of any A or B genes they might carry. That’s why the true statement is that they lack H antigen on red blood cells. They do not have H, and they do not express A or B antigens. In fact, their serum typically contains anti-H antibodies, which react with any H-positive cells, making transfusion possible only with red cells that lack H altogether (Oh), which are extremely rare. The options claiming they have H antigen, or that they express A and B antigens, or that they cannot form anti-H antibodies, do not fit this pattern.