<prompt> Diphtheria antibody titer considered optimally protective is0.1 IUml0.01 IUml0.05 IUml0.5 IUml  <response>: Diphtheria antibody titer considered as optimally protective is 0.1 IUml. Titers of IgG antibody against diphtheria toxoid vaccine reflect various levels of protection against the disease as follows: 0.01 IUml: No protection 0.01 IUml: Basic protection 0.1 IUml: Optimal protection. Ref: Textbook of microbiology; Ananthanarayan and panikers; 10th Edition; Pg; 245 