In my protocol, balb/c female mices were divided into two groups :

1. PBS group

2. OVA group

which were immunized with PBS or OVA antigen respectively by introperitoneal injection on day 15, 25, and 35. (Day 0 is the begining of the experiment.) It's observed that mices immunized with OVA antigen started to die since day 39, but not observed in PBS group.

My OVA antigen used for injection are prepared as follows :

LPS free OVA protein in PBS solution (50μg/100μl) were mixed with equal volume of Imject® Alum (Thermo) adjuvant, and treat each mice 200μl mixture. (Prepared in biosafety cabinet.)

I'm wondering the reasons for the death of OVA-immunized balb/c female mices.