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The Immune Response While many constituents of infectious microorganisms and their products (e.g., exotoxins) are or can be rendered immunogenic, only some stimulate protective immune responses that can prevent infection and/or clinical illness or (as in the case of rotavirus) can attenuate illness, providing protection against severe disease but not against infection or mild illness. The immune system is complex, and many factors—including antigen composition and presentation as well as host characteristics—are critical for stimulation of the desired immune responses (Chap. 308). The Primary Response The primary response to a vaccine antigen includes an apparent latent period of several days before. | Chapter 116. Immunization Principles and Vaccine Use Part 3 The Immune Response While many constituents of infectious microorganisms and their products e.g. exotoxins are or can be rendered immunogenic only some stimulate protective immune responses that can prevent infection and or clinical illness or as in the case of rotavirus can attenuate illness providing protection against severe disease but not against infection or mild illness. The immune system is complex and many factors including antigen composition and presentation as well as host characteristics are critical for stimulation of the desired immune responses Chap. 308 . The Primary Response The primary response to a vaccine antigen includes an apparent latent period of several days before immune responses can be detected. Although the immune system is rapidly activated it takes 7-10 days for activated B lymphocytes to produce enough antibody to be detected in the circulation. The primarily IgM antibodies seen initially are rapidly produced but have only a low affinity for the antigen. After the first week high-affinity IgG antibodies begin to be produced in quantity this switch from IgM to IgG production requires the participation of CD4 T-helper lymphocytes the middle men of the immune response. Because precursors for T cells mature within the thymus gland antigens that stimulate T cells are referred to as T or thymus-dependent antigens. Circulating antigen-specific T lymphocytes that implement cell-mediated immune responses are identified in the peripheral bloodstream only after several days but begin to increase in number immediately after antigenic stimulation. Activation of these responses typically requires co-recognition of the antigen by specific molecular species of HLA the major histocompatibility complex which is present on the surface of lymphocytes and macrophages. Some individuals cannot respond to one or more antigens even when repeatedly exposed because they do not have the genes for the
