During the
first months of life, infants are mainly protected by passive immunity in the form of IgG
antibodies passed from the mother during the third trimester of pregnancy. This protection
is only temporary and gradually decreases between the third and sixth month of life. At the
same time, the baby’s own immune system is just starting to produce antibodies, resulting
in a natural physiological “immunity gap.”
Understanding these processes makes it possible to consciously support the maturation of a
child’s immune system. Evidence-based preventive measures primarily include proper
nutrition, support of the gut microbiota, and timely completion of the vaccination schedule.
These elements form the foundation of health in the first year of life and ensure safe
progression through key stages of immune development.
Baby’s Immune System – Development in the First Year of Life
Development of the immune system during the first year of life involves a transition from an
initial non-specific response to a highly specialised, adaptive immune response. At birth, T
and B lymphocytes, responsible for recognising and neutralising specific pathogens, are
already present in the infant’s body, but they do not yet have the full ability to mount a
rapid and effective defence response. Their maturity and functional efficiency develop
gradually through exposure to antigens from the external environment. Each encounter
with microorganisms stimulates these cells to proliferate and form immune memory cells, in
a long-term, multi-stage process that continues throughout early childhood.
A key stage in this development is the so-called immunity gap, which most commonly occurs
between the third and sixth month of life. During this period, the level of maternal IgG
antibodies transferred during pregnancy declines significantly. At the same time, the infant’s
own antibody production is not yet sufficient to compensate for this loss. This leads to a
temporary weakening of immune defences, making infants more susceptible to respiratory
and gastrointestinal infections. Only towards the end of the first year of life does the child’s
antibody production reach levels that provide more effective independent protection
against common pathogens.
From an immunological development perspective, mild infections also play an important
role. Medically, exposure to viruses and bacteria is a necessary stimulus for the maturation
of adaptive immunity. The infant’s immune system learns to recognise threats correctly and
respond appropriately without triggering excessive inflammatory reactions. Overly sterile
environments and excessive isolation from natural microbial flora may delay this process
and, later in life, may increase the risk of allergic diseases.
Breastfeeding and Transfer of IgA Antibodies and Bioactive Factors
Breastfeeding plays a key role in supporting the development of the immune system
in newborns and infants. Of particular importance is colostrum, the early breast milk
produced during the first few days after birth. It has a unique composition in which
immune proteins are more concentrated than nutritional components. Colostrum, and later
mature breast milk, is rich in secretory IgA antibodies (sIgA). These immunoglobulins are
resistant to digestive enzymes in gastric juice, allowing them to reach the intestines intact.
There, they coat the mucosal surface, forming a protective mechanical and biological barrier
that prevents pathogens from attaching to epithelial cells and penetrating tissues.
In addition to immunoglobulins, breast milk contains a range of substances with proven
antimicrobial and immunomodulatory effects. One of the most important is lactoferrin,
which binds iron ions, reducing their availability to pathogenic bacteria and inhibiting their
growth. Another key component is lysozyme, an enzyme capable of destroying the cell walls
of Gram-positive bacteria. Human milk oligosaccharides (HMOs) also play a crucial role.
These complex carbohydrates are not digested by the infant but act as decoy receptors in
the intestinal lumen, binding bacteria and viruses and facilitating their safe elimination.
Natural feeding also directly supports the structural and functional maturation of the
infant’s gastrointestinal tract. Breast milk contains growth factors that promote the
development of intestinal epithelial cells and strengthen intercellular tight junctions. This is
essential, as the immature gut barrier in newborns is naturally more permeable, which in
formula-fed infants may increase the risk of antigen translocation and inflammatory
reactions. By optimising the intestinal environment, breastfeeding prepares the digestive
system for exposure to new proteins during dietary diversification.
Gut Microbiota as a Key Protective Barrier
The gastrointestinal tract is the primary site of immune system activity in humans. Gut-
associated lymphoid tissue (GALT) accounts for approximately 70–80% of an infant’s total
immune potential. Proper function and maturation of this system during the first year of life
depend directly on the process of bacterial colonisation of the gut. The dominance of
beneficial bacterial strains, particularly Bifidobacterium and Lactobacillus, is essential for
stimulating immune cells to produce immunoglobulins and cytokines that regulate
inflammatory responses. These bacteria form a biological barrier, competing with potential
pathogens for adhesion sites on the intestinal epithelium and for nutrients, thereby
preventing the growth of harmful microbiota.
The composition of an infant’s gut microbiota can be disrupted by perinatal and
environmental factors, which directly affect immune function. The most common cause of
dysbiosis is delivery by caesarean section, during which the newborn is not exposed to the
maternal vaginal microbiota and is instead colonised by bacteria from the hospital
environment and skin. Similar effects may result from antibiotic exposure in the mother
around delivery or directly in the infant during the first months of life. In such cases,
clinically appropriate probiotic supplementation may be considered, based on well-studied
strains such as Lactobacillus rhamnosus GG or Bifidobacterium animalis subsp. lactis. The
choice of preparation should always be made in consultation with a paediatrician to support
gut barrier recovery and proper immune programming.
Vaccination Schedule and Development of Adaptive Immunity
Vaccination is based on the controlled and safe introduction of antigens to the
infant’s immune system. Vaccines contain weakened or inactivated microorganisms,
or their purified components, which are incapable of causing the full disease but are
recognised as foreign by lymphocytes. This exposure stimulates the production of
specific antibodies and immune memory cells. As a result, when the body later encounters
the natural pathogen, it can respond rapidly, neutralising the threat before serious
symptoms or complications develop. Vaccination is the most effective form of preventive
medicine, actively shaping adaptive immunity without the risks associated with natural
infection.
The infant vaccination schedule is carefully structured to match periods of increased
susceptibility to specific infections. Mandatory vaccines protect against diseases such as
tuberculosis, hepatitis B, diphtheria, tetanus, pertussis, poliomyelitis, and Haemophilus
influenzae type b. Equally important are vaccines against pneumococcal, meningococcal,
and rotavirus infections, which are major causes of severe respiratory, gastrointestinal, and
invasive bacterial diseases in infants. Timely administration of vaccine doses ensures a
stable level of protection during the period when maternal passive immunity is no longer
effective.
Modern paediatrics widely uses combination vaccines (e.g. 5-in-1 or 6-in-1), which provide
protection against multiple diseases in a single injection. This significantly reduces the
number of injections required, minimising stress for the infant and lowering the risk of local
post-vaccination reactions. Clinical studies confirm that combination vaccines are safe and
do not overload the developing immune system, which is naturally capable of processing
multiple antigens simultaneously.
Dietary Diversification and Prevention of Nutritional Deficiencies
Introducing complementary foods, recommended by leading paediatric societies after 6
months of age (but not before 17 weeks and not later than 26 weeks), plays an important
role in the development of immune tolerance. Gradual and controlled exposure of the
gastrointestinal tract to new dietary antigens stimulates immune cells in the gut to respond
appropriately and helps prevent abnormal allergic reactions. Introducing a varied diet
during this optimal developmental window enables the body to distinguish between
harmless substances and true pathogens.
During this period, prevention of nutritional deficiencies is essential, as they can impair
immune cell division and function. Particular attention should be paid to iron intake, as
foetal iron stores are naturally depleted around 6 months of age. Iron deficiency leads to
anaemia and significantly reduces lymphocyte and macrophage activity. Zinc is also crucial
for thymus function and T-cell maturation, while omega-3 fatty acids have
immunomodulatory effects and support mucosal integrity.
Daily vitamin D3 supplementation, in line with current medical guidelines, is another key
element of immune health. Cholecalciferol acts as an immunomodulatory hormone
regulating both innate and adaptive immunity. It stimulates the production of antimicrobial
peptides such as cathelicidin, enhancing the body’s first line of defence against infections
and reducing the risk of respiratory illnesses.
Strengthening Resilience and Daily Habits That Reduce Infection Risk
Regular outdoor walks regardless of weather conditions help support cardiovascular
adaptation and thermoregulation in infants. Maintaining optimal indoor conditions is
equally important, with recommended temperatures of 18–20°C and humidity levels of
40–60%, which prevent drying of the respiratory mucosa. Well-hydrated mucous
membranes act as a natural barrier that traps pathogens and prevents their entry into the
body.
Sleep and recovery also play a crucial role in immune development. During healthy,
uninterrupted sleep, metabolic processes intensify and immune proteins are synthesised.
Deep sleep phases are associated with cytokine release, which coordinates immune
responses, regulates inflammation, and supports effective defence against pathogens.
Paediatric Care and Preventive Support in Our London Clinic
Our London clinic offers comprehensive paediatric care focusing on prevention and
healthy child development from birth. We provide ongoing medical supervision,
vaccination reviews, and tailored nutritional guidance. Our medical team supports
parents in monitoring infant health, selecting appropriate supplementation, and conducting
laboratory diagnostics, ensuring professional care.
