analysis is a simple test which can be useful for many
cases involving a blood stained crime scene and in the
verification/identification of a unknown victim's identity.
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a stain is found at the scene of a crime, the
first thing that has to be determined is whether
the stain is blood or some other bodily fluid.
This is done using a simple test involving a solution
that changes colour when it comes into contact
in the blood. Another type of test commonly used
spray, which makes any residue containing
blood, glow in the dark as well as picking up
on traces of blood that may have been scrubbed
away. The next step is confirming whether the
bloodstain belongs to a human. Serologists, people
who study blood, place the sample and a testing
solution into small wells on a gel-coated glass
plate, and the two will defuse
towards each other. If the sample is human blood,
it will contain human antigens
and where the two solutions meet on the gel-coated
plate, a noticeable band forms.
Blood particles below a microscope show up like
the image above. Photo courtesy of www.imageafter.com.
|Determining which person
the bloodstain belongs to involves an investigation
of blood types. The human blood contains over 100 different
antigens, therefore it would be time consuming and unpractical
to test for every single one. Serologists instead use
a number different blood testing techniques, but by
far the most common and effective technique is the ABO
system. This system is also used to determine compatibility
for blood donors and recipients. The ABO blood type
system involves checking the surface of the red blood
cells for two antigens known as A and B, with blood
type being named after the type of antigens it contains
- A, B, AB and O. By noting that a blood clump forms
when the same type of antigen meets the same type of
an experiment can be done on the solution of blood to
determine the blood type.
|The test is done using
two solutions each containing antibodies to type A and
type B antigens. The first solution contains type A
antibodies and when mixed with type A blood, will cause
it to form clumps. The same concept is used to test
for B antigens, where a solution of type B antibodies
would cause all type B antigens in the blood to clump
together. If blood clumps under contact with both A
and B antibodies, then it is of the blood type AB, since
both antigens are present in the blood. O blood does
not clump with any other blood type and is therefore
identified because it is solitary.
|For finer results yet,
the blood groups can be assigned either a + or a - figure
after it to indicate the presence (+) or absence (-)
of a blood protein known as the Rh
factor (named after the Rhesus Monkey, in which
it was first recognized). Using an antibody solution
to the Rh protein, the same concept is used, where blood
clumping determines the absence/presence of this protein.
Thus, the finer blood groups include A+, A-, B+, B-,
AB+, AB-, O+ and O-.
|Blood is not the only
fluid that is excreted from the body and tested by serologists.
Substances like saliva, semen, urine and excrement contain
DNA, can be compared with a suspect. In cases concerning
rape, investigators need to be sure that the swab taken
or the stain found is semen and this is confirmed using
a test that changes colour on contact with SAP (seminal
acid phosphatase), spermine and choline.
Microscopes are also used to see individual sperm, but
this technique is not accurate, as a rapist who has
had a vasectomy
or is sterile will not show sperm under a microscope,
even if they committed the rape. However, blood, semen
and urine samples all contain DNA, which is slightly
more accurate (and expensive) in singling out the criminal.
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