Maternal Protection and Pathogen-Specific Humoral and Cellular Immunity Explain the Age-Specific Spectrum of Respiratory Infections in Children and Adults: A Mechanistic Synthesis

Respiratory pathogens infect children in a reproducible age order: respiratory syncytial virus (RSV) and pertussis dominate the first months of life, human bocavirus and adenovirus peak in toddlers, Mycoplasma pneumoniae pneumonia peaks at school age, rhinovirus and influenza infect all ages, and several of these pathogens resurge in older adults. Here we compile and organize the published evidence behind these patterns, treating each pathogen’s characteristic age window as a read-out of its immune mechanism within the classical age-structured tradition. Two interacting “clocks” organize the pediatric spectrum. The first is the maternal-antibody umbrella: transplacental IgG decays by infant catabolism, and the protective trough at 2–6 months is set by the distance between the starting titer and a protective threshold, measured in half-lives, rather than by any single half-life; maternal RSVpreF and Tdap vaccination and passive nirsevimab prophylaxis provide trial-level causal proof that this clock sets the infant window. The second clock is the cumulative acquisition of type-specific immunity, which closes the toddler windows of bocavirus and adenovirus but never closes for antigenically diverse rhinovirus or drifting influenza; since exposure accumulates on a similar schedule, this clock is only partially identifiable serologically. M. pneumoniae is the decisive boundary case: a single maternal clock under a constant force of infection cannot place a peak at school age—a known structural property of age-structured transmission models, no new simulations performed—so delayed first sensitization, waning partial immunity, and immunopathology on reinfection are required. For encapsulated bacteria, a “polysaccharide trap”—poorly transferred IgG2-dominated maternal antibody plus developmental T-independent unresponsiveness before age two—explains the earliest bacterial windows. We also synthesize breast-milk, innate-immune and microbiome modifiers of both clocks, and an adult “third clock” of immunosenescence. The framework yields falsifiable predictions for immunization timing and disease patterns in immunocompromised hosts.