Immunization and immuno responses of an infant and factors affecting it

Neonates and infants are more vulnerable to infections in comparison to adults, which are associated with mortality and morbidity. In utero, the infant is protected from exposure to foreign pathogens but after delivery, the innate immune system boosts up to fight potential pathogens. The early immune system is mostly based on an anti-inflammatory state with a higher production of anti-inflammatory cytokines. The neonates are reported to have a higher count of natural killer cells, T-cells, B-cells, and, the circulation of neutrophils.

There is a multitude of factors affecting the development and function of the immune system amongst which maternal immunity and infections play a very important role. Neonatal exposure to HIV has visibly decreased from 30% to 3% globally. However, HIV-exposed but unaffected infants are seen to be more immunocompromised in comparison to unexposed infants. The reasons for such differences could be exposure to HIV Viral particles, HIV medications, lack of breastfeeding, etc. HIV exposed infants are reported to have a lower number of neutrophils, and monocytes with a reversal of an increased number of proinflammatory cytokines. Smaller thymic size with a declined output of T-cells has also been observed. However, the naïve T-cells are lower in comparison to Memory T-cells, portraying an experienced immune system.

Antenatal vaccinations are the second-factor affecting infant immune response. Maternal vaccination has been proven safe and effective against tetanus, pertussis,and influenza. Maternal vaccination is a milestone in the tetanus program resulting in a 96% reduction in neonatal tetanus. Women vaccinated during pregnancy have a higher level of IgG and the infants borne of them have higher antibody concentrations leading to a prolonged period of protection type. Though prolonged protection is a blessing, it is also associated with reduced response to the primary vaccination, dubbed a blunting effect. 

Preterm infants are more vulnerable than full-term neonates in terms of infection, and vaccine-preventable infections such as rotavirus, pertussis, etc. Variation in the innate and adaptive immune system as a result of prematurity has been visualized in a lower number of neutrophils, NK cells, and anti-inflammatory cytokines. Preterm infants are reported to have a lower concentration of mannose-binding lectin, impaired Toll-like receptor signaling, etc. Preterm neonates display a faster waning of antigen and not only have a lower vaccine response after primary immunization but also after booster doses.

Source: Zimmermann et al., The Pediatric Infectious Disease Journal: May 2021 - Volume 40 - Issue 5S - p S40-S46 DOI: 10.1097/INF.0000000000002773

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