A possible mechanism for intercellular invasion is that the strength of adhesion between host and invading cells is greater than the average of the strengths of homotypic adhesions. This hypothesis has been examined by a study of the kinetics of aggregation of dispersed populations of an invasive cell type (the rabbit peritoneal neutrophil granulocyte) and a host cell type (the chick embryo heart fibroblast) in shaken suspension culture. Since aggregation in mixed populations of the 2 cell types demonstrated tissue specificity, the hypothesis is not supported by these studies, heterotypic adhesions seem in fact to be weaker than homotypic adhesions
