The dependence of immunological cross-reactivity upon sequence resemblance among lysozymes. II. Comparison of precipitin and micro-complement fixation results.

Rabbit antisera were prepared against seven different lysozymes purified from the egg whites of chicken, bobwhite quail, turkey, Japanese quail, ring-necked pheasant, and duck. Cross-reactivity tests by means of quantitative micro-complement fixation were conducted among all possible pairs of these lysozymes as well as with three other lysozymes. The amino acid sequences are known for five of the 10 proteins we examined. A correlation was observed between degree of immunological cross-reactivity and degree of amino acid sequence similarity, with the limitation that proteins differing from each other by 40 % or more in sequence exhibited no cross-reactivity in micro-complement fixation tests. This relationship between sequence resemblance and immunological resemblance is compatible not only with the fact that amino acid replacements have generally occurred on the outer surface of the lysozyme molecule during bird evolution but also with current ideas about the size and number of antigenic determinants on proteins and with the idea that the conformation of the polypeptide backbone is strongly conserved during evolution. Cross-reactivity measurements are affected by several parameters that were investigated, including the length of the immunization program, variability among rabbits, and the degree to which the results of reciprocal tests agree. Antiserum specificity broadened with time and reached a plateau after several months of immunization. Considerable variability among rabbits necessitated pooling several sera in inverse proportion to their degree of reaction with the homologous antigen in order to obtain a more representative measurement. The results of reciprocal tests advise measuring cross-reactivity in both directions to better evaluate the degree of antigenic difference between two lysozymes.