Serology, another fascinating branch of forensic science analyzes body fluids to find factors that individualize that material. This means, in plain language, that the serologist is trying to isolate a combination of blood "factors" that could only belong to one person.

What do you mean, "factors", you say. Blood is a tissue made up of various cells that have individual functions, for example, white blood cells fight disease and red blood cells carry oxygen. All the blood cells work together to carry out the functions of circulation and immunity.

The "factors" are proteins located on the blood cells or in the liquid part of the blood called plasma. All our blood proteins are manufactured from genetic instructions or created as a result of a microbial invasion.

Everyone knows that all humans belong to one of the four major blood groups or Types, namely A, B, AB and O. That means if you are type A, your red blood cells possess a specific protein that types O and B do not have. Type B possesses a different protein not found in types A or O. O contains neither of those proteins, and AB has both of them. If we study a huge sampling of the human population, we find that each blood type represents a certain percentage of that population.

Here are some interesting facts.

• O is the most common blood type
• Since blood type depends on proteins, if a blood sample is kept in a dry environment which preserves the proteins, you can type that sample, even if it is thousands of years old, as is the case with Egyptian mummy's blood. Maybe even Egyptian daddy's blood.
• For you CSI fans, notice that bloody objects like clothing are never stored in plastic bags, because wet biological material kept in air tight containers are prone to mold growth. Molds degrade blood typing proteins. Keep tabs on those CSI techies in the movies. Watch what they put in those plastic bags.

The forensic scientist has to go further than just the ABO blood types. They must identify several other blood typing proteins which all occur in variations in the human population, just like ABO proteins. The most commonly known one is the Rh factor, named for the Rhesus monkey.

After they identify each protein in its specific variation, the serologist makes a profile including the statistical occurrence of each protein. By the time he is finished with his analysis, he has the blood owner's unique profile of blood proteins, and the percentage of the population that person represents. This is extremely important. If a suspect's blood profile matches blood at a crime scene but his blood represents 15% of the population, the state must link that blood evidence with another piece of evidence to make a strong case. The case against a suspect is much stronger if the blood profile represents only .01% of the population.

Scott Turow's new mystery, Identical, involves a crime with identical twins. Well what about the blood of identical twins? The blood proteins they inherit are, in theory, identical, but the proteins they have acquired due to infections may differ a great deal. Every microbial invasion causes an immune response in the body. This involves the production of arresting proteins called antibodies, which stay in the blood for a very long time. Allergic reaction cause the same chain of events. Therefore, identical twins can be differentiated by making a profile of their antibodies. Can we isolate those antibodies? think of the HIV test. It doesn't isolate the AIDS virus. It isolates the antibody the infected person produces. So, even identical twins are not identical. Ongoing research may find other differences.

A little known fact is that a certain percentage of humans secrete their blood typing protein into their saliva and other body fluids. They are called secretors, and if you collect their saliva, you can tell what their blood type is. Imagine the CSI unit processing a crime scene. Cigarette butts are collected by competent techies. Not only is the brand of possible importance, but even better, the smoker could've been a secretor, and even better, how about the presence of skin cells from the lips of the smoker?  It's amazing to think that a cigarette butt could convict a felon.

As discussed in previous articles, DNA analysis is nearly 100% effective in identifying biological material to its source. Forensic scientists, however, still conduct standard serology tests. This is because the rules of evidence accepted by the courts recognize their validity with greater certainty than DNA testing. Because it is so new and the courts are so cautious, DNA evidence is generally used to exonerate, not to convict.