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Naturally acquired opsonic antipneumococcal antibodies are commonly found in nonvaccinated adults and confer protection against infection and colonization. Despite this, only limited data exist regarding the adaptive immune response after pneumococcal exposure. To investigate the dynamics of naturally acquired antipneumococcal immunity in relation to an episode of infection, opsonic antibody activity was studied with paired acute-phase and convalescent-phase sera obtained from 54 patients with pneumococcal community-acquired pneumonia (CAP) using an opsonophagocytic assay (OPA). Results were compared with clinical characteristics and anticapsular immunoglobulin (Ig) concentrations. Interestingly, a nonfunctional opsonic antibody response (characterized by a decreased convalescent-phase serum OPA titer compared to that of the acute-phase serum or undetectable titers in both sera) was observed in 19 (35%) patients. A nonfunctional convalescent-phase response was significantly more common among patients with invasive pneumococcal disease (i.e., bacteremia) than in patients without invasive disease (53%; P =  0.019). Remaining individuals exhibited either an increased convalescent-phase OPA titer ( n  = 24 [44%]) or a detectable, but unchanged, titer at both time points ( n  = 11 [20%]). No correlation was found between anticapsular Ig concentrations and OPA titers. Our findings indicate that an episode of pneumococcal infection may act as an immunizing event, leading to an improved antipneumococcal adaptive immune status. However, in some cases, when patients with CAP also suffer from bacteremia, a nonfunctional opsonic antibody response may occur. Furthermore, the results suggest that factors other than anticapsular Ig concentrations are important for opsonic antibody activity in serum. IMPORTANCE Numerous reports on the dynamics of antipneumococcal immunity in relation to immunization with pneumococcal vaccines and on the prevalence of naturally acquired immunity in various populations have been published. In contrast, studies on the dynamics of the humoral immune response triggered by pneumococcal infection are scarce. This study provides valuable information that will contribute to fill this knowledge gap. Our main results indicate that a functional immune response frequently fails to occur after CAP, predominantly among patients with simultaneous bacteremia.

A Nonfunctional Opsonic Antibody Response Frequently Occurs after Pneumococcal Pneumonia and Is Associated with Invasive Disease