READ: Components of Blood

Components of Blood and Other Serological Evidence

Serology is the medical science dealing with serums found in body fluids such as blood, saliva, sweat, etc. A forensic serologist is a scientist who examines physical evidence in an effort to identify the source of the serum. For the purpose of this class we will focus on blood serum. Blood serum is the yellow fluid that separates from the solid components when whole blood is centrifuged. Blood serum is synonymous with plasma.

Structure and Function of Blood

The primary function of blood is to transport gases such as oxygen and carbon dioxide, distribute nutrients such as glucose, and provide defenses for the body such as platelets for blood clotting and anti-bodies to fight off bacteria, viruses and foreign substances introduced into the blood. Whole blood has a solid component and a liquid component. The solid portion of the blood is composed of red blood cells, white blood cells, and platelets.

Red blood cells (RBCs) are also called erythrocytes. The function of RBCs is to carry oxygen from the lungs to your body's tissues and take carbon dioxide back to your lungs to be exhaled. The surface of RBC's have chemical structures called antigens attached to them. Antigens are substances that stimulate the immune system to manufacture antibodies (immunoglobulins). Antibodies are any of various proteins produced in the blood in response to the presence of an antigen. The function of antibodies is to fight off intruder cells, such as bacteria or viruses, in the body. By becoming attached to antigens on infectious organisms, antibodies can render them harmless or cause them to be destroyed. Antigens stimulate the blood to fight other blood cells that have the wrong antigens. If a person with blood type A is given a transfusion with blood type B, the A antigens will fight the foreign blood cells as though they were an infection.

White blood cells (leukocytes), or WBCs, are one of the body's defenses against disease. Some white cells travel throughout the body and destroy bacteria, some produce antibodies against bacteria and viruses, and others help fight malignant diseases. DNA is gathered from the WBCs in blood samples.

Platelets, or thrombocytes, are small, colorless cell fragments in the blood whose main function is to interact with clotting proteins to stop or prevent bleeding. Interestingly, about 80% of the population are "secretors". This means that the majority of people secrete blood proteins from the blood into other bodily fluids. This is important because it means that the blood type of the person can be determined from a sample of a body fluid besides blood, such as saliva!

 The liquid portion of blood, or plasma, is a fluid, composed of about 92% water; 7% vital proteins such as albumin, gamma globulin, antihemophilic factor, and other clotting factors; and 1% mineral salts, sugars, fats, hormones and vitamins. The primary function of plasma is to serve as the medium that helps blood cells move effectively through the circulatory system. Some of the proteins floating in the plasma are antibodies.

Saliva

The presence of saliva, even if it has dried, can be detected by testing for an enzyme in saliva called amylase. Amylase is an enzyme that breaks down starch and helps the body begin to break down food. Amylase can be found in other body fluids such as blood, so detecting it in a suspected saliva sample is not considered confirmatory for saliva. Saliva has other forensic significance because it can be tested to determine the presence of trace amounts drugs in a body.

Georgia Virtual, Forensic Serology, CC BY-NC-SA 3.0