The high rates of K. kingae carriage and infection observed in the first two years of life followed by a sharp drop in older children, the scarcity of cases among adults, and the occurrence of disease in immunocompromised patients suggest that an acquired immune response is necessary to protect from colonization and infection. To study the immune response to acute infection, paired acute-phase and convalescent-phase sera were obtained from 19 children with culture-proven K. kingae septic arthritis, osteomyelitis or occult bacteremia and studied by an ELISA immunoassay using K. kingae outer-membrane proteins (OMP's) as the coating antigen (Slonim et al., 2003). Although the specific function of K. kingae OMP's has not been investigated yet, studies conducted with other respiratory organisms have shown that surface-exposed OMP's express adhesins and other virulence factors that interact with mucosal surfaces and the immune system and, as such, are frequent targets for antibodies that protect against mucosal and invasive infections. In most patients, a significant increase in the optic density values was measured in the convalescent sample implying that acute invasive infections elicit an immune response.
Using the same serological essay, the age-related incidence of invasive infections in the population was correlated with the serum antibody levels in a cohort of children followed prospectively for the first 2 years of life (Figure 15.4). Below the age of 6 months, the incidence of disease was extremely low coinciding with high levels of IgG antibodies. Low levels of IgA antibodies, which do not cross the placenta, found in this period suggested that the IgG-dependent immunity had a maternal origin. Decreasing antibody levels in children aged 6-18 months overlapped with
0 2 6 12 18 24 age in months the increasing incidence of invasive K. kingae disease in this age-group and indicated fading vertically-acquired immunity. Increasing antibody levels in older children and declining incidence of disease suggested cumulative experience with K. kingae antigens induced by carriage and infection (Slonim et al., 2003).
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