Final answer:
The antibody associated with a mixed-field reaction is the anti-Sda antibody. This reaction occurs when there is agglutination among a mixed population of cells.
Step-by-step explanation:
The antibody associated with a mixed-field reaction is the anti-Sda antibody. A mixed-field reaction is one where agglutination is seen among a mixed population of cells, some agglutinating and some not, which can happen, for instance, in the case of a patient who has recently been transfused with a different blood type than their own.
When anti-Sda antibodies in the patient's serum recognize and bind to Sda antigen on transfused cells, this could result in such mixed-field agglutination.