Blood and physiological fluid evidence
The laboratory work described in this chapter is most often called “forensic biology.” This chapter, then, discusses the identification aspect of blood and physiological fluids. A tube of blood drawn from a person’s vein and allowed to sit undisturbed for a few minutes will form a two-phase or two-fraction system. The blood will clot, and the lower, dark red fraction contains all the cells. Blood or body fluid evidence specimens are collected, subjected to identification testing, then to DNA typing. Blood or other physiological fluid stains or residues may be found on almost anything. Confirmatory tests for blood were traditionally crystal tests and immunological tests using antibodies specific for human hemoglobin. There are five categories of conventional genetic markers: blood groups; isoenzymes; plasma (serum) proteins; hemoglobin variants; and the HLA system. Before DNA typing, a bloodstain or semen stain could never be attributed to only one person just because all the types between the stain and a person matched.