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Absorptions of sera from groups of persons, both normal and after vaccination, resulted in complete removal of antibody to all strains of influenza virus within a type when a strain of antigenic configuration similar to that presumed to be the strain of first experience was employed. Absorption of these sera with strains encountered after the primary antigenic stimulus, removed antibody to strains recently experienced, but failed to absorb the primary antibody. By these methods, three age groups could be distinguished whose characteristic principal antibody was oriented to react with maximal efficiency either with FM1 or PR8 or Swine/1976 influenza viruses. Persons in these age groups had an initial experience with strains of the respective antigenic characteristics found in the variants of influenza virus mentioned. Basis for understanding these results was obtained with sera from successively infected ferrets, when serum absorptions demonstrated all type-specific antibody was oriented to react only with the strain of first infection. In view of this evidence there can now be little doubt as to the marked persistent influence an antigenic experience with influenza virus has upon the antibody-forming mechanisms of the virgin host.

Previous epidemiologic studies have shown that most primary atypical pneumonia illnesses in which cold agglutinins develop are associated with the agent first described by Eaton, Meiklejohn, and van Herick in 1944 (1-8). In addition the agent causes a spectrum of effects ranging from inapparent infection to febrile respiratory disease without pneumonia (5, 6).Recent studies have established that the organism, previously known as "primary atypical pneumonia virus" or "Eaton agent", is not a virus but a member of the genus Mycoplasma (pleuropneumonia-like organisms) (9-11). Thus, at least 30 strains have been grown in cell-free semisolid or liquid medium containing bovine heart infusion, yeast extract, and horse serum (10-13). Growth does not occur in the absence of serum or a suitable substitute such as egg yolk (10, 14). The colonies which grow on semisolid *agar medium exhibit a colonial morphology and fine structure characteristic of Mycoplasma (10). Certain microbial inhibitors such as thallium acetate, penicillin, and amphotericin B do not affect growth of the organism (10, 11). The agent is inhibited, however, by the tetracycline group of antibiotics (15).Until recently only four species of mycoplasma were known to infect man. These are M. hominis type 1, M. hominis type 2, M. salivarium, and M. fermentans. (16-18). When the atypical pneumonia organism was compared with these species by immunofluorescence or complement-fixation tests it was antigenically distinct (10, 19-21). It resembles M. fermentans in utilizing glucose and other sugars (22). The agent differs biologically from the four recognized human species of Mycoplasma by its ability to produce rapid and complete hemolysis of guinea-pig and horse red cells (23, 24). Under 662 R. M. CHANOCK

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