typical immune response to an antigen exposure, the latency between the first (primary) exposure and development of the primary response is characterized by a lag phase, logarithmic phase, and plateau phase. The lag phase is characterized by the initial activation of B and T cells upon encounter with the antigen for which they are specific, and this triggers the cells’ differentiation into effector and memory cells. The lag phase between primary exposure to an antigen and the logarithmic phase is classically thought to be 4 to 7 days, but it varies depending on route of exposure and the antigen itself. For B cells, the logarithmic phase is characterized by an increase in serum antibody levels that classically is logarithmic. The plateau phase is characterized by the maintenance of peak antibody levels for a length of time that is followed by a decline in the serum antibody levels. For many antigens the latency (lag phase) between primary exposure and development of the primary antibody response is 7 to 10 days. Due to the development of memory B and T cells during the primary immune response, the latency between subsequent exposure to the antigen and development of the immune response will usually be shorter. The lag phase is generally 1 to 3 days; the logarithmic phase of the secondary antibody response occurs over the next 3 to 5 days. As mentioned for the primary immune response, these time periods will vary depending on the route of exposure, the timing of the subsequent exposure, the antigen itself, and the antigen dose. While this discussion is not specific to a particular antigen, it can be used as a reference point for the latency between antigen exposure and the initiation of some of the immune-mediated mechanisms described below.
Contributing to the activation of B and T cells and the initiation of the adaptive immune response are cells classically associated with the innate immune system (e.g., macrophages and dendritic cells). These cells play roles at each of the stages mentioned above and are usually the first cells of the immune system to be exposed to antigen. Upon antigen encounter, macrophages and dendritic cells engulf the antigen, a process that also activates these innate immune cells to become antigen-presenting cells. Antigen- presenting cells, as their name suggests, present the antigen to T cells (see “Effector Functions of T Cells” below) and release inflammatory mediators (e.g., cytokines and chemokines) that contribute to the recruitment, activation, and proliferation of B and T cells. Activated B and T cells in turn release inflammatory mediators leading to the recruitment and activation of additional immune cells that further amplify the immune response through the release of inflammatory mediators. Regulatory cells and soluble immunoregulatory mediators (not discussed in this report) play roles in suppressing the immune response. Chaplin (2010) provides a review of the immune response including discussion of the interplay between the innate and adaptive arms of the immune system, cells associated with the innate and adaptive immune systems, and inflammatory/immunoregulatory mediators.