by Nicole Evans M.D.

Created on: May 30, 2010

When a full term infant is born many organs and tissues in their body are still in the process of becoming fully developed and functional.  The immune system is no exception.  Newborn infants are at an increased risk of infection due to their deficient immune system.

The deficient immunity of newborns or “neonates” is a natural state at birth and is medically known as physiologic immunodeficiency in the neonate.  The immune system actually requires several years before reaching a fully mature state in children.  The maturation process of the immune system occurs in stages starting in the fetus prior to birth and reaching completion between 10 to 14 years of age.

Before an infant is born they can produce a small amount of protective antibody, but it is mostly of the IgM type.  In the third trimester of pregnancy, the mother provides all the IgG immunoglobulins that the child will have at birth.  The IgG antibody crosses through the placenta and will last for approximately four to six months after the child is born.  Infants who are born prematurely often have low levels of immunoglobulins because they have not received the full amount of transplacental IgG. 

At four to six months of age, the amount of mother’s antibodies in the infant has declined to the lowest levels and the infant’s IgG begins to slowly increase over the next several years.  By 1 year of age, adult levels of IgM have been reached.  Adult levels of IgG are achieved by five to seven years of age and a child will have adult levels of IgA by 10 to 14 years of age.

Other aspects of the immune system that are immature at birth include T cells, cytotoxic activity, neutrophil production and function, and complement levels.  The white blood cells known as T cells are immature in the newborn and produce reduced quantities of cytokines, chemicals that stimulate and regulate the immune system.  The T cells are also less able to produce a T cell surface molecule that normally interacts with B cells to induce antibody synthesis by the B cells. 

The cytotoxic (foreign or infected cell destruction) responses are insufficient in the neonate as well.  The white blood cells known as neutrophils are produced at reduced levels in newborn infants and are less able to move to infected areas in response to triggering chemicals, a process known as chemotaxis.  In addition, neutrophil function is reliant upon activation by a specific cytokine, IFN-gamma, that is being insufficiently produced by immature neonatal T cells.

The complement system is a key part of a healthy immune system in children and adults.  In a newborn infant, the levels of key players in the complement system are reduced.  As a result, the newborn has less than 20% of the normal protective immune activity accomplished by the complement system in the adult.

Many different aspects of the immune system in newborn infants are immature and functioning suboptimally.  This physiologic immunodeficiency places the neonate at increased risk of infection, a risk that gradually declines as the immune system matures over several years.

Source: Essentials of Pediatrics

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