Forensic Serology

Chapter 12

Topics this Chapter:

• Blood typing– Class evidence (presumptive)

• DNA profiling (later chapter)– Individual evidence (confirmatory)

• Blood Spatter– Helps reconstruct crime scene

The Nature of Blood

• Blood – made up of cells, enzymes, proteins, inorganic substances– Plasma – fluid portion of unclotted blood

• Dissolved proteins, antibodies, hormones, clotting factors, nutrients (glucose, amino acids, salts, minerals)

• Comprises 55% of blood

– Solid portion – blood cells• Comprises 45% of blood

Types of Blood Cells

• Red blood cells – erythrocytes

• White blood cells – leukocytes

• Platelets – clotting factor

Erythrocytes

• Possess surface proteins called antigens

• Antigens impart blood type characteristics– A protein = Type A – B protein = Type B – AB proteins = Type AB– No proteins = Type O

Genetics• Chromosomes – threadlike bodies that appear in nucleus of

every cell; made up of DNA– Human body cells contain 46 chromosomes– Human reproductive cells contain only 23 chromosomes

• Genes – segments of DNA on chromosomes that are the basic unit of heredity– Control the development of specific characteristics

• Allele – alternative forms of genes located at the same point on a particular chromosome– A and B are possible alleles for the blood type gene

• Locus – physical location of a gene on a chromosome

Chromosome

Genotype vs Phenotype

• Homozygous – two identical alleles – AA for blood type

• Heterozygous – two different alleles– AB for blood type

• Genotype – combination of alleles present in the DNA (chromosomes)– AA, BB, AO, etc

• Phenotype – physical manifestation of a genetic trait– Shape, color, blood type

How do we inherit traits?

• Mother’s genotype vs. father’s genotype

• Punnet Square

• Dominant traits

• Recessive traits

Blood typing History

• 1901 – Karl Landsteiner– Noticed a huge number of

deaths in blood transfusions– Developed the A-B-O system

of classification – Still most important system for

matching a donor and recipient for transfusion

Blood Typing

• Three types (alleles) of blood type gene– A, B, O– Code for the antigen present on the surface of

red blood cells

• 6 possible combinations (genotypes)– AA, BB, OO, AB, AO, and BO– Genotype determines blood type

• Rh factor – D antigen– D antigen present: Rh positive– D antigen absent: Rh negative

Type O Blood

• Possessed by people whose genotype is OO– both parents passed on the O gene– Have no antigens

Type A

• Possessed by people with genotype– AA– AO

• A is dominant to O

• Have A antigens

Type B

• Possessed by people with genotype– BB– BO

• B is dominant to O

• Have B antigens

Type AB

• Possessed by people with genotype– AB– A & B are co-dominant

• Have both A and B antigens

Antigens and Antibodies

• Antigen - A substance on the surface of a cell that stimulates body to produce antibodies against it– Over 15 different types on red blood cells– A-B-O and Rh are most important

• Antibodies – a protein that destroys or inactivates a specific antigen– Antibodies develop about 2-8 months after birth– Found in blood serum– Responsible for ensuring that the only blood cells that

can survive in a person are cells of the correct blood type

Antigens and Antibodies

• Type A blood has A antigens– Produces B antibodies

(called Anti-B)• Type B blood has B antigens

– Produces A antibodies (called Anti-A)

• Type AB blood has A & B antigens– Produces no antibodies

• Type O blood has no antigens– Produces A & B

antibodies (Anti-A and Anti-B)

Antigen-Antibody Response

• Foreign invader is recognized by the immune system, and the immune system launches an attack against the invader– Immune system “recognizes” invader as

foreign by the foreign antigens on the surface of the cell

– Viruses, bacteria, red blood cells from different blood type have foreign antigens

Clotting - Agglutination

• An antigen and an antibody of the same type react to clump RBC (called agglutination)

• So, if antigen A is exposed to anti-A, agglutination will occur

Blood Typing

• Blood typing is done by reacting whole blood with antibody A and antibody B

• Antibody A will cause A and AB blood to clot

• Antibody B will cause B and AB blood to clot

• Type O blood contains no antigens so will not clot

Donors

Probability

• Typical Distribution in U.S.– O: 43%– A: 42%– B: 12%– AB: 3%

Additional Blood Proteins1. M and N proteins (MM,MN,NN)

2. Phosphoglucomutase (PGM)

3. Adenylate kinase (AK)

4. Adenosine deaminase (ADA)

5. Esterase D (EsD)

6. Glucose-6-phosphate dehydrogenase (G-6-PD)

7. Polymorphic Proteins: Group-specific Components (Gc) and haptoglobins (Hp)

Forensic Characterizations of Bloodstains

1. Is it blood?

2. From what species did the blood originate? (Usually human or animal)

3. If the blood is human, how closely can it be associated to a particular individual?

Is it blood?

• Benzidine color test– discontinued– Contains known carcinogen

• Kastle-Meyer color test – took place of benzidine color test– Bloodstain, phenolphthalein reagent and

hydrogen peroxide mixed: hemoglobin will form a deep pink color

Is it blood?

• Hemastix® - moistened with distilled water and placed in contact with bloodstain– Green color is positive

for blood

Is it blood?

• Luminol test – produces light rather than color– Large areas covered quickly– Objects must be in darkened area while being viewed

for luminescence– Extremely sensitive

• Can detect blood diluted up to 300,000 times

– Will not interfere with further DNA testing

• Takayama and Teichmann tests– Microcrystalline tests for blood– Specific chemicals added to form crystals with

hemoglobin

Luminol Test

Human or animal blood?

• Precipitin Test– determines whether blood

is human or not– Like immunoassay for

drugs except for human blood

– Uses antiserums of known original to react with blood sample

• Precipitin band is formed if a match

– Can be used on bloodstains 10 to 15 yrs old or older

Precipitin Test

Human or animal blood?

• Gel Diffusion – antigens and

antibodies will move toward one another on an agar-coated plate

• Electrophoretic method– Electrical potential is

applied to the gel

Can we narrow it down to a particular individual?

• Enzymes – another substance in blood used to try to individualize bloodstains

• This is more historical in use – before 1990’s

• DNA is primarily used now

Paternity Testing

• Encountered in civil courts, usually not criminal courts

• The more blood group systems tested, the better the chances of excluding an innocent male

• Routine testing involves other blood groups than A-B-O– HLA (human luekocyte antigen)

• White blood cells• Chances better than 90% suspect is father if he

cannot be excluded after this test