B cells are a specific type of white blood cell called a lymphocyte. Other types of lymphocytes include T cells and natural killer cells. B cells develop from stem cells in bone marrow. They remain in the bone marrow until they become mature.
Infantile behavior is something the masses are taught to exemplify since childhood. As adults they (mostly Betas) are encouraged to have sexual relations with multiple partners.
Infant is a more formal term for a baby, the youngest category of child. A child is no longer considered an infant when able to walk and talk, which, in most cases, is at about 1 year. The term child covers the age range from birth to puberty.