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BIOLOGICAL EVIDENCE

BLOOD. BIOLOGICAL EVIDENCE. The Nature of Blood. Blood : a highly complex mixture of cells, enzymes, proteins and inorganic substances. The Components of Blood. Plasma Fluid portion of blood Composed mostly of water Accounts for 55% of blood content

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BIOLOGICAL EVIDENCE

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  1. BLOOD BIOLOGICAL EVIDENCE

  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.

  9. 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)]

  10. 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

  11. 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

  12. 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

  13. 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.

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

  15. Play the Blood Typing Game http://nobelprize.org/medicine/educational/landsteiner/index.html

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