The development of the ability of young rats to generate a prompt primary antibody response to polymerized flagellin, with IgM to IgG transition, is correlated in time with the development of structures in the cortex of lymph nodes that localize antigen to spherical areas which subsequently become primary lymphoid follicles.

Throughout development the increased magnitude of the antibody response parallels the increased ability of lymphoid structures to retain antigen.

During the first week of life primitive lymphoid tissue appears capable of undergoing the initial steps in differentiation toward antibody production in response to neonatal injections of polymerized flagellin. However, further maturation appears to be blocked resulting in a complex immunological state at the age of 2 weeks characterized by increased IgM and decreased IgG antibody response to antigenic challenge at this time.

The possible relationship between the block in cellular differentiation toward antibody formation and the ease of tolerance induction is discussed.

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