The Brian received a flu shot, while completely healthy to prepare beforehand to the environmental exposure of the flu virus. Vaccinations are a form of Adaptive Immunity classified as Artificially Induced Active Immunity. The vaccine presents a specific antigen into the body that triggers the primary immune response. T cells provide cellular immunity and B cells provide humoral immunity. Both work together to provide the immune response. First the weakened antigen fragment is inserted/injected into the body. A phagocyte will then engulf the abnormal antigen. Before destroying the bacterium a Class1 MHCP incorporates the bacterium and carries it to the plasma membrane for presentation. As a CD8 T Cell passes it first recognizes the antigen on the infected cell. Costimulation occurs to ensure that they are a match. Once confirmed the CD8 receptors bind to the antigen and it activates and undergoes clonal selection. It divides and differentiates into an Effector Cell which reacts to the antigen and a Memory Cell which remembers the antigen for the environmental exposure of the flu and its antigen. The Memory cell plays no role in the initial response, but waits for the second response. Cytotoxic cells destroy the cell by one of three ways and Helper T Cells coordinate the innate and adaptive immune response.
You can’t tell by reading this, but the whole process has a window period of about two weeks. The reasoning behind the vaccine. Which is the first response, is to prime the body for the Naturally Acquired Active Immunity, the environmental exposure of the flu and its antigen into the body. This is the second response that the Memory Cells have primed for. They are prepared for the antigen with plenty of antibodies which were cloned awaiting. This makes for a more effective and robust response. This second response now occurs in a matter of about a day. Pinky may not even realize that that one little sniffle was indeed the effect of his body just annihilating the same virus.-