The immune response functions to protect the body from disease. It is a highly interactive process between the cells of the immune system. The immune response involves the production of antibodies by B-cells, the activation and growth of CD4 cells and CD8 cells and the release of cytokines. Memory T cells are formed as part of the immune response. These cells remain in the blood long after infection has resolved and mount a very rapid response to any subsequent re-infections of the same ‘invading’ disease. This process is called immunity.