ABSTRACT

Until the end of the 1960s, humoral immunity had been thought to be the primary mechanism of immune defense against bacterial infections by means of complement-dependent bacterial lysis and the opsonizing activity of antibodies. The alternative, nonhumoral type of acquired resistance mediated by cellular components was observed only in tuberculosis and brucellosis. However, due to technical difculties in animal experimentation using Mycobacterium tuberculosis and the susceptibility of virulent strains of Brucella to both opsonization and lysis by complement in the presence of specic antibodies, the precise mechanism of cell-mediated acquired immunity had not been characterized.