Blood of every animal species contains different proteins, and blood from one species will not accept proteins
from a different species. Blood develops antibodies as a protective measure against disease and foreign
matter to render them harmless. Precipitin tests allow human blood to be distinguished from animal blood.
The investigator can recover these antibodies by bleeding the animal and isolating the blood serum. This
serum will contain antibodies that specifically react with human antigen (human antiserum).Serum for
the precipitin test is obtained from rabbits which have produced antibodies to destroy a small quantity of
human blood injected into them. A drop of this anti-human serum is added to suspect blood, which will
precipitate its protein if it is of human origin. Police laboratories hold anti-sera for most common animals, thus
allowing the crime investigator to confirm or disprove statements made by the suspects about the origin of
suspicious bloodstains. The precipitin test is sensitive, and will work on small traces of blood. The test is also
known as the Uhlenbuth test after the German scientist who developed it in 1901. The colour of dried
blood changes in time from red to brown, and the peroxidise test takes longer to develop with an old
stain. An experienced observer considering these factors might be able to give an opinion as to the age of a
particular stain, but it is now possible to measure colour-change scientifically. Spectrophotometric analysis of
bloodstains allows them to be aged within the range of one day to three weeks.