Fetal RBCs can be detected in the maternal circulation after delivery, with anti-KEL RBC antibodies detectable in the serum of wild-type females after multiple pregnancies with KEL-positive males. (A) Unstained RBCs from a nulliparous wild-type female. (B) Unstained RBCs from a homozygous uGFP male. (C) Unstained RBCs from a representative wild-type female bred with a homozygous uGFP male, p.c. day 7, p.c. day 14, and hours after delivery. (D-E) Serum from nulliparous or multiparous animals was cross matched with TER119-positive RBCs. (F) Representative flow cross match against wild-type (shaded) or KEL (solid) RBCs, from a nulliparous and a multiparous female, with anti-mouse IgG as a secondary reagent. (G) Anti-KEL IgG in the serum of nulliparous or multiparous (3 pregnancies) females. (H) Anti-KEL IgG in the serum of females after 1, 2, or 3 deliveries; 42 total pregnancies are shown. *P < .05.