The explanation on p. 203 is quite clear and to the point. The human HLA genes are a multi-locus gene system. The human lymphocyte antigen system is amazing in several ways, most notably for its tremendous number of alleles at each of several linked loci, for combinatorial assembly of the peptide products those loci to produce an almost unlimited range of antibodies, and for somatic mutation to produce still greater antibody diversity. The HLA system also provides examples of linkage disequilibrium, which probably have significance for disease resistance. Linkage disequilibrium can exist for several reasons.
Linkage can persist for some time while equilibrium is being approached.
Selection can act against particular haplotypes. Two-locus models of natural selection can be built. Don't get stuck on the models in this section. Concentrate on understanding the distinction between independent and epistatic fitness effects . For each type of interaction, Ridley offers hypothetical examples to illustrate how the effect might work.