how does the system of immunity work on the embryo?
Embryo gets passive immunity from maternal IgG antibodies through the placenta.
Fetus
Maternal IgG antibodies cross the placenta, giving the fetus passive immunity against those diseases for which the mother has antibodies. 
what is reactive attachment disorder? It is a severe and relatively uncommon disorder that can affect children. what causes it? It is based on a problematic history of care and social relationships. can it be diagnosed? It can olny be diagnosed by a psychiatrist and any assessment must include a comprehensive evaluation of the child's individual and family history. what are some of the symptoms? Infants up to about 18–24 months "may" present with non-organic failure to thrive and display abnormal responsiveness to stimuli. The core feature is severely inappropriate social relating by affected children. can this age group travel on plane? Many airlines refuse boarding for all babies aged under 7 days (for domestic flights) or 14 days for international flights. what kind of hair do they have? A fine, downy body hair called lanugo. can you find this hair in animals too? Yes, animals like seals and elephants too are born with this. how does this hair develops? It develops on fetuses as a normal part of gestation.
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Fetus [SEP] Immune system
The placenta functions as a maternal-fetal barrier against the transmission of microbes. When this is insufficient, mother-to-child transmission of infectious diseases can occur. Maternal IgG antibodies cross the placenta, giving the fetus passive immunity against those diseases for which the mother has antibodies. This transfer of antibodies in humans begins as early as the fifth month (gestational age) and certainly by the sixth month.