The neonatal immune system remains undeveloped causing a higher susceptibility to infections as a result of ‘Physical Immunodeficiency’ (Jaspan et al, 2006). An infant’s immune system begins to develop at the early stages of gestation, between the seventh and eighth week, beginning with the growth of the lymphocytes. By the end of the second trimester, the T-helper cells become functional, building up immune responses. During pregnancy an infant receives most of its immunoglobulin antibodies from the mother as they are passed through the placenta to the fetus. The only type of immunoglobulin that is able to pass the placenta during gestation (the third trimester) is Immunoglobulin G (IgG) (Bogart et al, 2009). IgG is an antibody that is composed of two antigen binding sites, which are composed of 4 peptide chains. The neonatal immune system becomes dependent on these maternal antibodies, providing protection to a foetus’s immune system, by allowing any forgin substances to be destroyed and killed (Jaspan et al, 2006). Thereby providing a host line defense. After about three to six months an infant’s immune system begins to develop by developing its own antigens, and not depending on maternal antibodies for host defense. Throughout childhood the acquired immune response build up as the child is exposed to more antigens.

Figure: Showing the way maternal antibodies are transported though the blood vessels to the placenta.

Immunglobulin G:Immunoglobulin G (IgG) is one of the most important antibodies in the body that are required for the host defense against any viral and bacterial infections (Bogart et al, 2009). After birth, the immune system of an infant consists of high levels of the mother’s antibodies, whereby the infant is still able to receive from the mother breast milk. Maternal breast milk consists of five types of antibodies:
1. Immunoglobulin G àIgG 2. Immunoglobulin A àIgA 3. Immunoglobulin E àIgE4. Immunoglobulin D àIgD5. Immunoglobulin M àIgM

Natural Acquired Immunity
The immunity present at birth, which is referred to as the immature immune system of an infant, which is exposed to different antigens in an environment (Jaspan et al, 2006). Allowing for the development of an infants immune responses from exposures to various pathogens and antigens.
Induced Active Immunity
The immunity gained from deliberate antigen exposure, providing protection against several diseases. This is the principle used for the enhancement of vaccinations and immunisations.
Passive Immunity
The maternal antibodies passed to the fetus during pregnancy through the placenta and from breast milk.

Adolescent Immune System: Adult Immune System Vaccine Development : Vaccine ImmunogenicityReferences Contributed by Rana Hammoud