The function of the immune system of the fetus and the newborn is suspect because of an increased incidence of infection, poor clinical response to infection, diminished lymphoid tissue, and sluggish immunologic responses to a variety of natural or test antigens.
Fetal immune mechanisms can be classified in a similar fashion as the human primary immunodeficiencies are classified that is, into antibody deficiencies, cellular immune deficiencies, phagocytic and macrophage deficiencies, and opsonin and complement deficiencies. Since most primary immunodeficiencies result from developmental arrest, the immunologic abnormalities in primary immunodeficiencies will occur at some stage of normal ontogenesis, and the methods used in the diagnosis of primary immunodeficiencies are valid in delineating defects or immaturity in fetal immune development.
This discussion will consider the following: (1) normal development of each of the major components of the human immune system, (2) factors that may activate the fetal immune system and the consequences
Stiehm ER. Fetal Defense Mechanisms. Am J Dis Child. 1975;129(4):438–443. doi:10.1001/archpedi.1975.02120410026011