Neural folds appear, which release cells that then form a neural crest.
Individuals with poor brain development, or with psychiatric and neurological disorders, also have poor immune responsivity, have a depressed antibody production, and impaired lymphocyte activity. Later we'll see how all this can be triggered by stress reactions. This sensitivity of the central nervous system is precisely why prenatal care is so important during the critical period of development. Unless the CNS is allowed to develop and grow properly, i.e. not be exposed to toxins such as alcohol, nicotine, drugs, and other agents, the immune system will not develop properly either. Babies are then born with underdeveloped spleens, thymuses, and lymph nodes, with a subsequent decrease in white blood cell production.