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> The fetus develops dendritic immune cells in the second trimester:
> 
> http://www.the-scientist.com/?articles.view/articleNo/49663/title/Fetal-Immune-System-Operational-By-Second-Trimester/&utm_campaign=NEWSLETTER_TS_The-Scientist-Daily_2016&utm_source=hs_email&utm_medium=email&utm_content=53166887&_hsenc=p2ANqtz-8nxPWBPzQ6N7Ym5HvRNLvTuQH7mW5fEKDa_BtWCiNXX942iBCI-1nV5tnae7DEyfXC-tNT_UoLvAIc-jbKRH0nYNQFHg&_hsmi=53166887
> 
> "These cells likely function to suppress an immune reaction to cells from the mother, the researchers found.
> 
> ***
> 
> "The researchers found antigen-presenting cells, called dendritic cells, by 13 weeks of gestational age in fetal skin, spleen, thymus, and lungs. They showed in vitro that fetal dendritic cells were capable of responding to antigens from bacteria or viruses by activating T cells, just as adult dendritic cells did.
> 
> "When the research team cultured the fetal dendritic cells with adult cells of a different genome, though, they induced the differentiation of regulatory T cells, which are involved in immune tolerance. This result indicated that the dendritic cells might be tamping down the fetus’s reactions to exposure to its mother’s cells.
> 
> "Adult dendritic cells in similar culture conditions induced the differentiation of fewer regulatory T cells and more killer T cells. When the researchers co-cultured the fetal and adult dendritic cells, the fetal dendritic cells suppressed the ability of the adult dendritic cells to induce killer T cells.
> 
> ***
> 
> "While the authors found globally similar gene expression profiles in fetal and adult dendritic cells, they also identified differentially expressed genes in pathways related to T cell education, immune suppression, and more. The researchers attributed the immune-dampening capabilities to arginase-2, which was highly abundant in fetal dendritic cells, but not in adult dendritic cells. They showed that arginase-2 interfered with the ability of both adult and fetal T cells to produce the proinflammatory cytokine TNF-α, which plays a role in T cell activation.
> 
> “'There has been a growing appreciation that the fetal immune system is not—as was previously thought—immature or non-functional,” says McCune. “Instead, it’s actually something that’s very functional, but quite different from that found in the adult.”
> 
> "While the authors found globally similar gene expression profiles in fetal and adult dendritic cells, they also identified differentially expressed genes in pathways related to T cell education, immune suppression, and more. The researchers attributed the immune-dampening capabilities to arginase-2, which was highly abundant in fetal dendritic cells, but not in adult dendritic cells. They showed that arginase-2 interfered with the ability of both adult and fetal T cells to produce the proinflammatory cytokine TNF-α, which plays a role in T cell activation."
> 
> Comment: the placenta is not a perfect barrier, and the fetus can react to the Mother's antigens which are not the same as in the fetus. A reaction to the mother's blood type can cause a very jaundiced baby, which is treated by doing an exchange transfusion replacing 90% of the baby's blood. This type of immune system must have been in place when the placental/uterine birth system began. Not by chance!