The proportion of CD19+ B cells expressing BLV-gp51+ antigen increased with the progression of BLV infection, from 21.0% in the AL phase to 28.25% in the PL phase, and the proportion of CD19+ IgM+ B cells expressing BLV-gp51+ antigen increased from 11.4% in the AL group to 17.7% in the PL group. Here, CD19 is linked to deltaretrovirus infections.