Today, the global population is ageing, especially in developed countries. According to the Pew Research Center, the elderly represents a larger population, and this increased from eight percent in 1950 to ten percent in 2010 (Schmiedeberg et al., 2016) . According to recent statistics, this trend is expected to continue, and it is estimated that the elderly will represent twenty-one percent of the population by 2030 (Schmiedeberg et al., 2016) . From these statistics it can be concluded that today, people live longer than they used to love more than five decades ago and the longer they live, the longer the human body is exposed to factor in the environment that stimulated increased risks of age-associated diseases. In comparison with the younger population, the elderly suffers more frequently and severely from nosocomial and community-acquired infections (Schmiedeberg et al., 2016) . Besides, the elderly tends to experience poor outcomes from infections when compared to the younger generation.
Both infants and the elderly can be described as vulnerable populations as they face increased susceptibility to infections, inadequate responses to vaccinations and increased incidences of autoreactivity (Schmiedeberg et al., 2016) . The immune systems of infants are often described as naïve and immature because they are not able to control infections. Ageing of the immune system in the elderly affects the ability of their bodies to fight infections and hence exhibit decreased effectiveness to immunization (Schmiedeberg et al., 2016) . The immune systems of the elderly are also less responsive towards established and emerging pathogens. The most viable method of fighting infections in the elderly is through the use of vaccines; however, their bodies fail to generate a long-lasting memory in response to vaccinations. In the older populations, the response to vaccines such as hepatitis, influenza and tetanus is diminished (Moro-García, 2013) . The immune systems in infants are even more underdeveloped, and this is responsible for increasing the risks of severe diseases after infection. According to WHO (World Health Organization), infectious diseases contribute to more than one-third of infant deaths in the world.
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The primary purpose of immunizations is to defend human bodies by teaching immune systems to remember pathogens that are used in fighting diseases (Moro-García, 2013) . However, the immune systems in infants rapidly diminish thus prompting for booster shots after the initial vaccination. An ideal vaccine would be a single dose that is given to infants at birth that would generate long-lasting immunity. However, the immune systems of infants are different from that of adults, and there are immune functions in children that are not available in adults (Moro-García, 2013) . Through breastfeeding, antibodies are passed from the mother in milk and transplacentally before birth which stimulates protective immune responses in infants. Although infants have under-developed immune systems, research has found that children have an inbuilt defense mechanism to infections that work differently to adults (Schmiedeberg et al., 2016) . Regardless, they have been found to be effective in protecting infants. The study found that the T cells in infants are different to those in adults and they are not immunosuppressed, and they are responsible for the production of an anti-bacterial molecule called IL8 that activates neutrophils that attack foreign invaders in the body.
Due to longevity, innate defects occur in naïve T cells found in the elderly that leads to the impaired generation of memory. From an early age, vaccines continue to protect infants throughout adult life, but in the elderly, vaccines do not generate adequate memory that can protect the body from infections hence indicating the possibility of impaired naïve CD4 T cells in the elderly.
Although age-related factors contribute to how the body defends itself in infants and the elderly, infants have higher chances of survival to infections than the elderly because of acquired immunity from their mothers when they were in the womb and through breastfeeding. On the other hand, due to age-related factors and the depletions of the cells responsible for fighting infections, the cells are unable to regenerate or develop new ways of fighting infections in the elderly.
References
Moro-García, M. (2013). When aging reaches CD4+ T-cells: phenotypic and functional changes. Frontiers in Immunology , 4. https://doi.org/10.3389/fimmu.2013.00107
Schmiedeberg, K., Krause, H., Röhl, F., Hartig, R., Jorch, G., & Brunner-Weinzierl, M. (2016). T Cells of Infants Are Mature, but Hyporeactive Due to Limited Ca2+ Influx. PLOS ONE , 11(11), e0166633. https://doi.org/10.1371/journal.pone.0166633
