Forensics: Serology

Antigen

protein found on the surface of red blood cells

Blood type

determined by antigens

Antibody

a substance that reacts with an antigen

How to determine the direction blood travels

direction of the tail of the blood droplet

Most common blood type

O+

Universal donor

O-

Universal recipient

AB+

Size of blood droplets from a high velocity impact

gives a fine mist appearance; droplets are less than 1 mm in diameter

Size of blood droplets from a medium velocity impact

gives intermediate size blood spatter; droplets are 1-4 mm in diameter

Size of blood droplets from a low velocity impact

gives large size blood spatter; droplets are more than 4 mm in diameter

Passive drops

linear pattern of round droplets of blood due to gravity; indicates movement after injury occurs

Transfer pattern

blood stains caused by contact between a wet blood-bearing surface and a second surface that may or may not have blood on it

Shadowing/void pattern

area in middle of cast-off pattern that does not have any blood spatter; object was in front of the wall during injury but object has been moved

Castoff pattern

blood that is thrown from an object in motion; seen on walls or ceilings

Swipe

wet blood is transferred to a surface that did not have blood on it

-one edge of pattern is usually feathered

Area of convergence

the location of the blood source can be determined by drawing 2D lines from the various blood droplets to the point where they intersect

Area of origin

the 3-D location of the source of the blood spatter

How can blood evidence be used in a crime?

Position of victim, evidence of a struggle, confirm or refute statements made by principals in the case: are stain patterns consistent with reports?

to extract DNA

Presumptive drug test

indicates that a drug is present but not which drug

What are the 3 different alleles for human blood?

A, B, Rh

How is blood type determined?

antigens on the surface of red blood cells

If you are testing blood and the anti-A serum causes the blood to clump the anti-B does not and the anti-Rh does, what type of blood do you have?

A+

Interpreting bloodstains at a crime scene reveal what?

Activity at a crime scene, number of blows, position of victim and assailant, whether death was immediate or delayed, weapon characteristics

What type of evidence can blood typing provide?

class evidence

Agglutination

the clumping of molecules or cells due to or caused by an antigen-antibody reaction

Passive drops

circular with secondary satellites; fall due to gravity not force

Arterial gushes

spurt-line at varying heights

What does the size of blood spots tell you about the impact?

indicates the force of the impact

If you are testing blood and the anti-A serum causes the blood to clump the anti-B does and the anti-Rh does not, what type of blood do you have?

AB-

If you are testing blood and the anti-A serum, the anti-B and anti-Rh does not cause clumping, what type of blood do you have?

O-

Number of blood types

4 - each one can be positive or negative

Spine

spiky appearance around the edges of a blood droplet

Satellite

small secondary blood drops

As the angle of impact increases, the bloodstain gets

rounder

As the angle of impact decreases, the bloodstain gets

more elongated

Type B blood has

Type B antigen, anti-A antibodies

Type A blood has

Type A antigens, anti-B antibodies

Type AB blood has

A and B antigens, no antibodies

Type O blood has

no antigens, anti-A and anti-B antibodies

Confirmatory drug test

a test that identifies a specific drug