W., and J. proportions of specific IgA1 ASC were 74%, 47%, 56%, and 80%, respectively. HR expression depended on the route of administration: expression of HRs was different after oral than after parenteral vaccination, while no difference was seen between BD-AcAc 2 HR expression of antigen-specific IgA1 and IgA2 ASC induced via the same route. The key factors determining IgA subclass distribution in a given secretion are the nature of the antigens encountered at a particular site and the site-specific homing instructions given to lymphocytes at that site. These two factors are BD-AcAc 2 reflected as differences in the homing profiles of the total populations of circulating IgA1 and IgA2 plasmablasts. Because humoral immunity is most frequently evaluated by titers of serum antibodies, which are dominated by IgG, the role of IgA as the major Ig isotype produced in humans has not been adequately appreciated: the daily production of IgA (66 mg/kg of body weight/day) exceeds that of all other immunoglobulin classes combined (10, 14, 40). Even if IgA levels in serum are lower than those of IgG, due to a shorter PRKCA half-life in the circulation, IgA is the predominant immunoglobulin in most external secretions (3, 14, 40). IgA, as the mucosal Ig isotype, plays a dominant role as the first immunological defense barrier in the body, since the mucosal sites act as a portal of entry to the majority of human pathogens (3, 40, 53). In humans, IgA comprises two subclasses, IgA1 and IgA2, which are unequally distributed in the body fluids (4, 13, 34, 36, 41). In serum, IgA1 is dominant, while in secretions, there is a significant contribution of IgA2. In external secretions, both IgA1 and IgA2 are present as secretory IgA (S-IgA), a polymeric form which is more resistant to proteolytic enzymes than any of the other isotypes (40). S-IgA is known to provide protection of mucosal membranes in several BD-AcAc 2 complementary ways: it can effectively neutralize viruses (46, 47, 49) or toxins (47) and displays antibacterial activity (47, 51). To evade the protective effect of S-IgA at the mucosal sites, some pathogenic bacteria (e.g., serovar Typhi Ty21a and had provided samples of external secretions (tears, parotid saliva, nasal wash fluid, and intestinal lavage fluid) after immunization. Samples for assessment of total concentrations of IgA1 and IgA2 were collected at a time point with no history of recent immunizations or recent infections, and for those providing samples for analysis of serovar Typhi-specific IgA1 and IgA2 responses, both total and specific IgA1 and IgA2 levels were assessed 28 days after vaccination. Total and specific IgA1 and IgA2 were assessed with an enzyme-linked immunosorbent assay (ELISA). Peripheral blood mononuclear cells (PBMC) were isolated from heparinized peripheral blood samples (18) and assessed for two kinds of cell populations by enzyme-linked immunospot (ELISPOT) assay: ISC were enumerated in BD-AcAc 2 a total of 48 volunteers, and antigen-specific ASC in 53 vaccinees. Peripheral blood samples for assessment of total ISC were collected either at a time point with no history of recent immunizations or recent infections or, for analysis of vaccine antigen-specific ASC, on day 7 after vaccination. Both ISC BD-AcAc 2 and ASC of IgA, IgG, and IgM isotype-secreting cells were enumerated individually. They were also assessed for the frequency of different homing-associated (ISC and ASC) and maturational markers (ISC) on their surface. This was performed by separating PBMC into marker-positive and -negative populations and assaying the resulting cell populations for.