The Small Immune System Of The Newborn

A newborn mouse has only 1% of the immune system of an adult animal1. In man the information is less complete, but Brandtzaeg has shown how strikingly especially the IgA-producing B cells increase in number in the intestinal mucosa during the first several weeks - months of life2. The newborn can be compared with a gem-free, but not antigen-free, animal where one can see a 10-fold increase in the number of lymphocytes in the gut mucosa after colonization even with one single bacterial strain3,4These observations give a relatively good picture of the size of the whole immune system since about 2/3 ofthat is found in the intestinal mucosa5.

Not only has the newborn an immune system oflimited size, but it is also somewhat functionally deficient at birth. Switching through the immunoglobulin isotype genes is inefficient and early antibody responses are mainly composed of IgM antibodies]. Immunologic memory is reported to be sparse and several cytokines are only produced in small amounts like IFN-y (interferon-Y and IL-4 (interleukin-4). Phagocytes also have a decreased function. Even if newborns are sensitive to infections they are still apt at protective immune responses quite early. They are not like an immunodeficient individual who cannot respond. For instance newborns can mount a memory TH1 response to BCG-vaccine of a magnitude similar to that in adult6. On the other hand the newborn can only produce 10% of adult levels of IFN- y, known to be important in defense against Mycobacteria.

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