Spontaneous hemorrhage and thrombocytopenia in heterozygous progeny of immunized β₃^-/- mothers. (A) Thrombocytopenia in heterozygous pups delivered from immunized β₃^-/- mothers (transfused twice with WT platelets) crossed with WT males. Pups from breeding cages of WT × WT, β₃^-/- × β₃^-/-, and naive female β₃^-/- × WT were used as a control. Data are presented as means ± SEM; n = 7-25 for each group. (B) Spontaneous bleeding in heterozygous pups delivered from immunized β₃^-/- mothers. (i) A heterozygous pup delivered from a naive β₃^-/- mother crossed with a WT male as a healthy control. (ii) Bleeding in live pups. (iii) Dead pups with massive ICH or abdominal bleeding. Bleeding is indicated by arrows. (C) Massive ICH and/or abdominal bleeding was found in fetuses in β₃^-/- mice immunized with 4 weekly transfusions of WT platelets. (i) ICH and abdominal bleeding. (ii) ICH.