1. BIOLOGICAL EVIDENCE
BLOOD

2. The Nature of Blood
Blood: a highly complex mixture of cells, enzymes, proteins and inorganic substances

3. The Components of Blood
Plasma
Fluid portion of blood
Composed mostly of water
Accounts for 55% of blood content
Contains suspended solid materials consisting chiefly of cells (accounts for 45% of blood content)
Red Blood Cells
erythrocytes
White Blood Cells
leukocytes
Platelets
Clotting agents

4. The Components of Blood
Serum
A pale yellowish liquid
Blood clots when a protein called fibrin traps the red blood cells
If you remove the clotted material, what is left is known as serum

5. Blood is very complex and any discussion of its function and chemistry would be quite extensive.
We will concentrate our discussion on those blood components that are directly pertinent to the forensic aspects of blood identification – the red blood cells and the blood serum.

6. Red Blood Cells
Transport O2 from the lungs to the body tissues and in turn remove CO2 from tissues by transporting it back to the lungs, where it is exhaled.
On the surface of each cell resides millions of characteristic chemical structures called antigens (a substance, usually a protein, that stimulates the body to produce antibodies against it)
These antigens impart blood-type characteristics to the red blood cells
Blood antigens are grouped into systems depending on their relationship to one another (more than 15 antigen systems identified to date)
The most important systems are the A-B-O and Rh systems

7. Four Main Blood Types
Type A: each red blood cell has A antigens on its surface
Type B: each red blood cell has B antigens on its surface
Type AB: each red blood cell has A & B antigens on its surface
Type O: each red blood cell has neither A nor B antigens on its surface
*It is the presence or
absence of the A and B
antigens on the red blood
cells that determines blood type.

8. Rh Factor or D Antigen Another important blood antigen
Those people having the D antigen are said to be Rh positive
Those people not having the D antigen are said to be Rh negative
In routine blood banking,
it is the presence or
absence of the three
antigens – A, B and D –
that must be determined
in testing for the compatibility
of the donor and recipient.

10. Antibodies A protein that destroys or inactivates a specific antigen
Found in blood serum
*A fundamental principle of blood typing is that for every antigen, there exists a specific antibody
Each antibody symbol contains the prefix anti- followed by the name of the antigen for which it is specific
anti-A is specific only for A antigen
anti-B is specific only for B antigen
anti-D is specific only for D antigen
The serum-containing antibody is referred to as the antiserum [a serum that reacts against something (antigens)]

11. Antibodies
An antibody will react only with its specific antigen and no other
If serum containing anti-B is added to red blood cells carrying the antigen B, the two will immediately combine, causing the antibody to attach itself to the cell
Antibodies are normally bivalent (they have two reactive sites – therefore, they can simultaneously be attached to antigens located on two different red blood cells)
This creates a vast network
of cross-linked cells usually
seen as clumping or
agglutination

12. Blood Type Antigens on Red Blood Cells Antibodies in Serum
In normal blood, antigens on red blood cells and antibodies can coexist without destroying each other because the antibodies present are not specific toward any of the antigens
However, if a foreign serum added to the blood introduces a new antibody, a specific antigen—antibody reaction will immediately cause the red blood cells to link together, or agglutinate
Nature has taken this into account:
Blood Type Antigens on Red Blood Cells Antibodies in Serum
A A anti-B
B B anti-A
AB AB Neither anti-A nor anti-B
O Neither A nor B Both anti-A and anti-B

13. Determining Blood Type
For every blood antigen there is an antibody
Serum contains anti-bodies
Blood with specific antigens clumps in the presence of antibodies
A blood will clump in A serum
B blood will clump in B serum
AB blood will clump in A and B serum
O blood will NOT clump in either A or B serum
Positive blood will clump in Rh serum
Negative blood will NOT clump in Rh serum

14. Blood group A individuals have red blood cells with the A antigen on their surface, and blood serum containing antibodies against the B antigen. Therefore, a group A person can only receive blood from people in groups A or O (with A being preferable).
Blood group B individuals have red blood cells with the B antigen on their surface, and blood serum containing antibodies against the A antigen. Therefore, a group B person can only receive blood from people in groups B or O (preferably B).
Blood group AB individuals have red blood cells with both A and B surface antigens, and their blood serum does not contain antibodies against either A or B antigen. Therefore, a person with type AB blood can receive blood from any group (preferably AB) but cannot donate blood except to another AB.
Blood group O individuals have red blood cells with neither A or B surface antigen, but their blood serum contains antibodies against both A and B antigens. Therefore, a group O person can only receive blood from group O.

15. Donors of blood type O can give to A, B & AB; donors of types A & B can give to AB.

16. Play the Blood Typing Game