But whenever antibody and antigen interlock, the antibody marks the antigen for destruction. Different types play different roles in the immune defense strategy. Immunoglobulin G, or IgG, works efficiently to coat microbes, speeding their uptake by other cells in the immune system. Immunoglobulin M is very effective in killing bacteria. Immunoglobulin E, whose natural job probably is to protect against parasite infections, is the villain responsible for the symptoms of allergy.
T cells contribute to the immune defenses in two major ways.
Some direct and regulate the immune responses. Others are killer cells that attack cells that are infected or cancerous. Less helpfully, killer T cells assail foreign cells transplanted as organ grafts. Binding to target cells, lymphokines mobilize many other cells and substances. They encourage the growth of cells, trigger cell activity, direct cell traffic, destroy target cells, and arouse phagocytes.