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Innate immunity: All humans are born with innate (natural) immunity. Innate immune responses are both immediate and nonspecific. In other words, immune cells involved in innate immunity are not specific to just one type of foreign substance. Instead, the immune cells engulf substances that are identified as foreign. This response primarily occurs on the external barriers of the body, including the skin, nose, throat, and gastrointestinal tract. It is the body's first line of defense to prevent disease–causing organisms from entering the body. Adaptive immunity: Adaptive immunity is a type of protection that develops over the course of an individual's life. Adaptive immunity involves the development of immunoglobulin antibodies that respond to specific foreign substances that enter the body. When individuals are exposed to certain foreign invaders, the body develops antibodies against the pathogens. Then, if the same substance enters the body in the future, the body is now prepared to respond quickly because the antibodies are already developed. Passive immunity: Passive immunity describes the immune system of babies who are less than six months old. Because a fetus' immune system is not fully developed, pregnant mothers pass immunoglobulin antibodies from their bloodstream, through the placenta, and to the fetus. These antibodies are an essential part of the immune system. They identify and bind to harmful substances that enter the body. When this happens, other immune cells are triggered to destroy the foreign substance. As a result, antibodies help prevent disease and infection. This provides the fetus with passive immunity. Immunoglobulin G (IgG) is the only antibody that crosses the placenta to the fetus during pregnancy. IgG antibodies are the smallest, but most abundant antibodies, making up for 75–80% of all the antibodies in the body. They are present in all body fluids, and they are considered to be the most important antibodies for fighting against bacterial and viral infections. These antibodies help protect the fetus from developing an infection inside the womb. Immediately after birth, the newborn has high levels of the mother's antibodies. Babies who are breastfed continue to receive antibodies via breast milk. Breast milk contains all five types of antibodies, including immunoglobulin A (IgA), immunoglobulin D (IgD), immunoglobulin E (IgE), IgG, and immunoglobulin M (IgM). This is called passive immunity because the mother is "passing" her antibodies to her child.
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