The graph shows the response of the human immune system to the exposure of an antigen in two phases. The first phase is that of a primary response to the initial attack. The second response is very rapid, subsequent to attacks from the same antigen. The specificity of this system relies heavily on two groups of cells: B lymphocytes and T lymphocytes. In period I, the antigen is injected at zero days. This is where the antigen will attach itself to a macrophage. By forty days however, the B cells have made antibodies to protect the body ....