The Biological Basis For each of the genome's thousands of genes, multiple alleles exist. No two people have the same combination of alleles, so each individual's genotype is unique. DNA, found in each cell, is both a genetic fingerprint and a genealogical record. It determines the phenotype--sex, blood type, hair and eye color, susceptibility to disease, and other features--that contributes to an individual's sense of self.
Preventing Collateral Damage All cells contain molecules that identify the cell as native to the body and block immune system attacks. Major histocompatibility complex class I proteins on cell surfaces deter natural killer cells from inducing cell lysis. Siglecs, receptors that bind sialic acid residues on cell surface glycolipids and glycoproteins, protect cells by regulating leukocyte activity. CD47, a surface protein expressed on many cell types, binds to an inhibitory receptor on macrophages and prevents phagocytosis. Most lymphocytes that bind native cells and intracellular ...
