While at the time of birth all the key components of a baby`s immune system are in place, a newborn`s ability to resist infection depends on the protective factors provided in breastmilk, exposure to certain bacteria and from here the rapid development of its own active (innate and adaptive) immune system processes.

This article reviews the early stages of immune system development and discusses how Healthcare Professionals can support parents toward the practices that will help promote optimal function not only in the early years but throughout life.

What is immunity?

The immune system is a collection of tissues, cells and molecules that work together to protect the body against invasion or infection by harmful antigens such as viruses, bacteria and parasites. Immunity is the ability to resist infection, enabling the body to distinguish self from non-self, preventing attack of its own cells.

Development in the first few weeks

Babies are born with some immunity called passive immunity, this is transferred via the placenta and then through colostrum and breastmilk.

The development of the infant`s own immune system is referred to as active Immunity. Although not considered fully developed until around 14 years of age, the most important stage of its development occurs in the first few weeks of life.

Passive immunity – immunity passed on from the mother`s breastmilk.
Active immunity – immunity acquired from birth as the baby comes into contact with numerous foreign micro-organisms, key nutrients and the immunological properties contained in breastmilk.

How the infant`s immune system develops

As the infant`s immune system develops, it will set up an incredibly complex network of organs, cells, chemicals and pathways that work together to provide defence. There are two arms – Innate and Adaptive^1,2.

Innate immune system is the more basic of the two arms of the system and it works at two levels.

i) Physical barriers (gut, skin, respiratory system) The first line of defence employed by the infant`s immune system is use of the body`s numerous physical barriers that include the respiratory organs and digestive tract. The gut barriers are particularly important and are lined with mucous membranes that have an enormous surface area of some 400 square metres.

Other physical barriers include the skin, acid in the stomach, tears, sweat, urine, lung membranes and the `friendly bacteria` that line the gut soon after birth.

ii) Cellular innate immunity

If harmful antigens do manage to get through these barriers, the next level of defences includes a collection of cells that work to co-ordinate a non-specific attack.

The cells involved include white cells phagocytes (neutrophils) and macrophages that will rush to a site of infection to engulf and try to destroy the bacteria, often causing inflammation or swelling. Other cells that are involved are natural killer cells. These are another type of white cell that can destroy bacteria, parasites, virus-infected cells and cancer cells, destroying them with proteolytic enzymes. This is referred to as cellular immunity. At birth the non-specific reactions of the innate immune system provides the first line of defence to a new born baby. Adaptive immune system is the more sophisticated arm of the immune system and unlike the innate immune system it has the ability to learn, adapt and remember past experiences.

Table 1: Development of the innate immune system

Key Components Development stage Key Components Gut, skin and respiratory tract Mucosal Barriers Gut barriers are more porous and therefore allow access of whole proteins and the immunoglobulin found in breastmilk. The gut mucosa has limited experience in discrimination between harmful invaders and those that it learns to tolerate such as the various food proteins. Breastmilk provides a range of substances that actively encourage the immune system to develop. It contains prebiotic oligosaccharides and IgA that help line the infant’s gut and protect against gastrointestinal infections. Gut bacteria or microflora The newborn’s gut is sterile at birth and inexperienced to deal with “common germs”. Introduction and colonisation of “friendly bacteria”. This occurs as the baby moves through the birth canal, comes in contact with the mother and receives its first feeds of colostrum. The build up of these friendly bacteria colonies is a key adaption that will help prevent harmful invaders from growing, multiplying and overrunning the infant’s gut. Cells of the innate system Cells (neutrophils, macrophages, natural killer cells) are lower in numbers and the ability of these cells to respond are well below that of an adult. Breastmilk (colostrum) supplies the infant with a range of Innate cellular system cells. These include neutrophils.

The adaptive immune system steps in when the innate is over run or unable to repel the more serious pathogens. The cells involved include: i) Lymphocytes (white blood cells) produced by bone-marrow – these cells divide into two main categories:

ii) The Thymus produces T-cells and

iii) bone marrow produces B-cells

The B-cells produce the antibodies (IgG, IgA, IgE, IgM and IgD). These antibodies are the most sophisticated army within the immune system, as they have a built in memory and can mount very specific and targeted attacks.

The antibodies include:

IgG – passes from the mother during the pregnancy and help provide immunity to the newborn for at least the first few months of life.

IgA – is transferred in colostrum and breastmilk and helps protect the infant from gastrointestinal infections.

IgM – is the main antibody created during infancy and accounts for around 10% of the body`s antibodies.

IgE – is the main antibody involved in the trigger of an allergic reaction.

IgD – has an undefined role in immunity.

Over time and through the process of experience, the infant`s adaptive immune system builds up a memory bank that can distinguish harmful bacteria and viruses from regular food proteins and the bacteria that are helpful to the body. This is a key adaptation of the developing immune system and is referred to as development of tolerance. The failure of the infant to learn tolerance and active key immune responses is believed to contribute to food-related allergy, autoimmune and inflammatory bowel disorders.

It is only at the age of around 6 months that an infant can produce its own antibodies and the passively acquired antibodies from the mother run out. This is why it is important for parents to take care when introducing new foods and the introduction of solids should be delayed until 6 months of age.

Table 2: Development of the adaptive immune system

Immune Component Development stage Key Adaptations in the First Few Weeks T-cells Infant has a limited ability to produce its own Tcells in the few weeks of life. T-cells are provided to the infant from its mother’s colostrum and breastmilk. B-cells Infant has a limited ability to produce IgA cells which help to catch foreign proteins and prevents them passing through the mucosal membrane3. Very few Ig-A producing B-cells circulate in the blood of newborns, although this number is approximately 75 times higher after the first month of life4. A form of IgA known as secretory IgA is provided in breastmilk. sIgA is resistant to digestive processes and is able to line the baby’s gut and help the infant resist the pathogens that enter the gut.

Building immunity in the infant

Natural Birth

First exposure to microbes and other foreign antigens will occur as the infant either moves through the birth canal or following a caesarean birth. Research suggests that the first exposures to bacteria in the birth canal are swallowed by the baby which could be a key part of the early stimulation of gut mucosal cells, providing the first opportunity for healthy bacteria to colonise and activate mucosal cells of the gut.

Immunoglobulin Properties of Breastmilk

It is known that breastfed infants are better able to ward off infection and illness. Breastmilk also helps protect against allergies and contains a number of key bioactive ingredients that help the immune system to develop. Breastmilk (particulary colostrum) may also have a role in programming of the immune system to help protect it against auto-immune illnesses including coeliac disease, Crohn`s disease and type 1 diabetes^5,6,7.

The Bioactive Components of Breastmilk

Breastmilk is a complex mixture of bioactive substances that support the early development of the immune system.

Key nutrients that support development of the immune system

Prebiotics

Prebiotics found in breastmilk are also referred to as oligosaccarides. Breastmilk is rich in oligosaccarides with around 1.5g of oligosaccarides per 100ml (2.2g per 100ml in colostrum)⁸. There have been over 130 different types of oligosaccarides identified in breastmilk and in addition to helping increase numbers of the `friendly bacteria`, they are known to act as a decoy to harmful bacteria. As they are a sugar the bacteria is attracted to them and these clever sugars bind with the bacteria and carry them out of the gut into the infant`s stool⁹. They protect the breastfed infant from parasitic infections such as giardia.

A prebiotic supplemented infant formula may support some improvements in immune system development when an infant is not breastfed¹¹.

Nucleotides

Nucleotides support development of the immune system by promoting lymphocyte (white blood cells) proliferation and other immune cell activities¹⁰.

Fatty acids and Long Chain Polyunsaturated fatty acids (LCPs)

Fatty acids including LCPs have an important role in the development of the immune system¹². They are vital for proper development and functioning of a child`s immune system, as they help to regulate the activity of disease-fighting lymphocytes and help protect the infant against allergies.

Best practices for the developing immune system

Natural Birth

The passage through the birth canal is the first exposure the immune system has to start to interact with pathogens. Ingestion of maternal microflora and fluids may help introduce the `friendly strains` of bacteria. The baby will therefore gain some immunity benefit if born naturally rather than by caesarean.

Promote breastfeeding

All Healthcare Professionals recognise the importance of encouraging mothers to breastfeed, particularly in the first few weeks when the greatest transfer of the immunological properties of breastmilk and colostrum occurs. However UK breastfeeding rates are lower than other industrialised countries¹³. As the population becomes more diverse, greater consideration is needed to support mothers who for cultural reasons choose not to breastfeed or do not give the baby colostrum.

NICE guidelines

The latest NICE clinical guidelines for post-natal care places significant emphasis on the importance of encouraging and supporting all new mothers to breastfeed their infant. The guidelines recommend that maternity care providers implement an externally evaluated, structured program that encourages breastfeeding, using the Baby Friendly Initiative (BFI) (www.babyfriendly.org.uk) as a minimum standard¹³. The Glasgow Experience is one example of how baby friendly programs can be implemented and evaluated effectively to help increase breastfeeding rates within a socially and culturally diverse population.

Vaccination

The aim of vaccination is to provide the infant with immunity from what were once common and life threatening childhood diseases. The vaccinations given at birth include hepatitis B, measles, mumps, whooping cough, polio and meningitis. Immunity is built up by injecting ready-made antibodies or innate versions of the microorganism that stimulate the infant`s body to launch an assault and thereby create the desired antibodies to the disease.

Good hygiene

Prevention of infection is a primary responsibility of both the Healthcare Professional and the parent. The most effective practice is good hand washing .techniques. Parents should also be advised on the practice of sterilisation, care of where the infant is placed (often they are on mats or bouncers close to the floor, pets and in reach of siblings) and in the first few weeks parents might want to be aware of passing the baby around to well-wishers with coughs and colds.

Summary

Parents need to know that there is much they can do to help promote optimal development of their baby`s immune system in the first few weeks of life. Good maternal nutrition (discussed in previous Nurture & Nutrition issues), a natural birth, breastfeeding (especially the colostrum), good hygiene practices and the early vaccinations are all recognised as key factors that will contribute how well the infant will be able to protect itself from harm, illness and infections not only in the early stages but for the many years that follow.



Click here to view our research papers relating to prebiotic oligosaccharides. Click here to view more information on Cow & Gate`s range of milks Click here for a useful leaflet on the benefits of breastfeeding.

References

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