Groups of infants and children with infections due to respiratory syncytial virus (RSV) were tested at the onset of illness and one, two, and five months later for the presence of RSV-specific cell-mediated immunity, as determined by in vitro lymphocyte transformation (LTF) activity, and for complement-fixing (CF) antibody. An LTF response specific for RSV was observed in 65 % of patients, and the response was largely associated with E-rosette-forming thymus-derived (T) lymphocytes. A significant cell-mediated immune response was observed in 78% of patients six months of age or younger, whereas only 46% of patients older than six months exhibited such responses. The difference between the mean stimulation index of the maximal LTF response in the two age groups was statistically significant (P less than 0.05). On the other hand, a fourfold or greater rise in titers of CF antibody was observed in only 22% of patients six months of age or younger, whereas significant rises in antibody activity were observed in 50% of subjects older than six months. In view of the fact that RSV infection is most severe in the young infant, these observations raise the possibility that cell-mediated hypersensitivity reactions may be related to the pathogenesis of RSV infection of infants.

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Neutralising inhibitors to respiratory syncytial (RS) virus have been demonstrated in the whey of most samples of human milk tested. Although high titres were secreted in colostra of some mothers (1/10-1/2,560; median 1/40) inhibitor levels in milk collected after the first week of lactation were uniformly low (median 1/10). High neutralising titres correlated with high colostral levels of specific antiviral IgA but, unlike neutralising activity, IgA antiviral antibody persisted in the milk of only four of 18 mothers. Similarly, antiviral IgG and IgM antibodies were not generally detected after the first post-partum week. Differences in antibody secretion among mothers did not correlate with differences in total protein or total immunoglobulin secretion, and appeared to reflect maternal immune status. In one mother a marked rise in specific antiviral IgA and IgG secretions during the second and third months of lactation suggested a response to virus infection. The relevance of maternal immunity and colostral and milk antiviral antibody to protection of breast-fed babies from RS-virus bronchiolitis is discussed.

Systemic cell-mediated responses to respiratory syncytial (RS) virus were detected, using a whole blood transformation assay, in 10 of 28 infants and children with RS virus infections during the period 1-14 days postadmission. Cell-mediated responses were unrelated to the age of the patient or the severity of illness. No correlation was found between cellular responses and fourfold or greater rises in antibody titre to RS virus, as determined by a membrane immunofluorescence technique. Patients under 6 months of age had significantly lower levels of IgA and IgG antibody to RS virus compared to older patients, although cell-mediated responses were similar in both groups. The presence of cell-mediated reactivity to RS virus was also demonstrated in 5 of 95 samples of cord blood examined, and cellular responses failed to correlate with the levels of IgG antibody to RS virus.