Optimal strategies in immunology III. The IgM-IgG switch

Summary During a primary immune response generally two classes of antibody are produced, immunoglobulin M (IgM) and immunoglobulin G (IgG). It is currently thought that some lymphocytes which initially produce IgM switch to the production of IgG with the same specificity for antigen. During a secondary immune response IgG is the predominant antibody made throughout the response. In this paper we address the question of why such apparently complicated modes of response should have been adapted by evolution.We construct mathematical models of the immune response to growing antigens which incorporate complement dependent cell lysis. By comparing the times required to eliminate antigen we show that under certain conditions it is advantageous for an animal to switch some of its lymphocytes from IgM to IgG production during a primary response, but yet to secrete only IgG during a secondary response. The sensitivity of such a conclusion to parameter variations is studied and the biological basis and implications of our models are fully discussed.Portions of this work were performed under the auspices of the U.S. Department of Energy. A.S.P. was also supported by the National Science Foundation under Grant No. ENG-7904852 and BRSG grant S07 RR05664-11 awarded by the Biomedical Research Support Grant Program, Division of Research Resources, National Institute of Health. A.S.P. is the recepient of an NIH Research Career Development Award 1K04 AI 00357-01. S.R. was a recipient of NIH Fellowship 5 F32 AI05107-02