Mixed field agglutination encountered in ABO grouping with no history of transfusion would most likely be due to:

Mixed field agglutination observed during ABO grouping can occur for several reasons, but one common cause is the presence of A2 red cells in a sample that may also contain A1 red cells. Individuals with the A2 phenotype have a different expression of A antigens compared to those with the A1 phenotype, which may lead to a mixed field reaction.

In patients who are A2, the population of red cells may contain some cells that express the A antigen to a lesser extent or have altered antigen structure, causing a weaker agglutination reaction when mixed with anti-A serum. Since the serological testing involves recognizing these distinct A antigens, the presence of both A1 and A2 red cells can yield a mixed field pattern due to variations in antigen density and expression.

In contrast, the Bombay phenotype refers to individuals who lack the H antigen altogether, resulting in a lack of reactions with anti-A and anti-B antibodies, which does not align with the presence of mixed field agglutination. T activation is associated with a different type of agglutination in which there is an exposure of T-antigen, while a positive indirect antiglobulin test typically relates to the presence of antibodies against foreign blood group antigens, which