Prenatal Immunity
Without direct access to external antigens, it is no wonder that the immune system is not very capable (on its own) of a vigorous defense during its maturation process before birth (prenatal development). However, as part A of the figure shows, certain antibodies from the mother (maternal antibodies) can be actively transported across the maternal-fetal blood barrier (trophoblast). Only IgG antibodies in the mother’s blood can bind to the receptors. This binding then triggers endocytosis and transports each IgG antibody across to the fetal bloodstream. This mechanism provides passive natural immunity before and shortly after birth.
Part B of the figure shows that, at birth, the newborn has adult levels of IgG—but nearly all of it came from the mother (maternal IgG, blue line). Shortly after birth, the maternal IgG is broken down (blue line) and replaced with new IgG made by the newborn’s own immune system (red line).
Note also in part B of the figure that the concentration of IgM (broken blue line) is only about 20% of the adult level at birth but steadily increases after birth. IgM reaches adult levels in about 2 years. IgA, an important component of the mucosal immune system, also begins to rise at birth. IgA reaches adult levels in just a few months. All three types of antibody are also found in breast milk, providing another avenue of passive immunity after birth.
A, Transport of antibodies across the placenta. B, Antibody concentrations before and after birth.
There are three other types of immunoglobulins: IgA, IgE, and IgD. IgA is the major class of antibody in the mucous membranes of the body. It is also found in saliva and in tears. IgE, although present in very small amounts, can produce the major symptoms of allergies. IgD is also present in our blood, but in very small amounts. We don’t yet know its function. As Figure 19-19 shows, some immunoglobulin
Classes of antibodies. Antibodies are classified into five major groups: immunoglobulin M (IgM), immunoglobulin G (IgG), immunoglobulin A (IgA), immunoglobulin E (IgE), and immunoglobulin D (IgD). Notice that each IgM molecule has five Y-shaped basic antibody units, IgA has two basic antibody units, and the others have a single basic antibody unit.
molecules are created by the joining of several basic antibody units.
