The Role of Genetics in the Immune Response

Abstract

Talk about the importance of genes actually applies only to cases where differences can be detected between alternate genes, or alleles, at a given locus of a chromosome. In higher animals, which normally carry two chromosomes of each kind, we can observe homologous genes (alleles) only in pairs, and refer to those as genotype. If the two genes carried at a given locus are identical, we say that the animal is homozygous for that locus, whereas if the alleles are different from one another the animal is heterozygous. Chickens have two sets of 38 chromosomes and a pair of sex chromosomes. We do not know how many pairs of genes are actually present in a chicken, and never will; but a very rough estimate would be that there is enough DNA for a million genes. Of that enormous number, geneticists have characterized only about 250 to 300 genes, of which some 170 are still carried in known stocks (30). From the point of view of the immunologist, however, fewer than 20 distinct genes are known to be important. Of those, 12 control red blood cell antigens (8), that is, structural differences in the surface of the cell; 5 genes have been described by Crittenden and co-workers (114,15) to control cell resistance of chickens to infection by Rous sarcoma or lymphoid leukosis viruses; and Benedict and co-workers (32) have demonstrated variability for genes controlling the structure of serum globulins (IgG and IgM, respectively). There is also good evidence that the response of leukocytes to phytohemagglutinins, plant extracts stimulating in vitro cell proliferation, is under control of a distinct gene locus (26). Obviously the mere enumeration of known genes means nothing. Most known genes in chickens have told us very little about the biology of the bird, nor have they contributed to practical solutions of problems. A case in point is blood-group systems based on differences in the surface structure of red blood cells. A given blood type is assessed by immunization of a recipient chicken with