CD40LG and infection: The low detection rate of RSV-IgM antibodies can be attributed to several factors: (1) the immature immune systems of infants and young children, which reduce their capacity to produce antibodies and may result in false-negative test results; (2) the required window period for IgM antibody production, during which antibodies may be undetectable in the early stages of infection, leading to a low positive rate; (3) the sensitivity limitations of detection methods; (4) potential viral mutations; and (5) the use of diagnostic reagents that may not be optimally suited to the local population.