During the fourth week of pregnancy, a woman may just be finding out that she is going to have a baby. The fetus is only the size of a poppy seed. But even this early in development, some of the cells that are busy dividing and specializing will eventually become immune system cells. These early immune system cells, called hematopoietic progenitor cells, have proteins on their surface that allow scientists to identify them as precursors of immune system cells. Early in the pregnancy, these cells divide very rapidly, but as the fetus matures, they decrease in the speed with which they multiply and more of them become specialized cell types. Babies born prematurely tend to have greater quantities of these unspecialized progenitor cells than full-term babies.
The early progenitor cells travel through the blood into organs associated with the immune system, such as the liver, spleen and thymus. By the second or third month of pregnancy, some are already becoming T cells. Although these T cells are functional by the third or fourth month of pregnancy, the sterile environment of the womb does not require the fetal immune system to fend off potential pathogens. This is important because one of the most intriguing aspects of the immune system relates to pregnant women (See “Two immune systems, one body” section below).
Macrophages can be found in the fetal intestine by 11 or 12 weeks of gestation, and quantities increase rapidly during the fourth and fifth months of pregnancy. Likewise, B and T cells can be found in the intestine by about 16 weeks of gestation; and by about 19 weeks of gestation, they are organized into specialized lymph nodes in the intestine, called Peyer’s patches.
Reviewed on April 22, 2019