other hosts. B. pertussis has the ability to inhibit the function of the host's immune system. The toxin, known as pertussis toxin (or PTx), inhibits G protein coupling that regulates an adenylate cyclase-mediated conversion of ATP to cyclic AMP. The end result is that phagocytes convert too much ATP to cAMP, causing disturbances in cellular signaling mechanisms, and preventing phagocytes from correctly responding to the infection. PTx, formerly known as lymphocytosis-promoting factor, causes a decrease in the entry of lymphocytes into lymph nodes, which can lead to a condition known as lymphocytosis, with a complete lymphocyte count of over 4000/μl in adults or over 8000/μl in children. Beside targeting lymphocytes, it limits neutrophil migration to the lungs. It also decreases the function of tissue-resident macrophages, which are responsible for some bacterial clearance. Another toxin that inhibits the immune response is the adenylate cyclase toxin. This toxin has an intrinsic adenylate cyclase activity, increasing intracellular cAMP almost immediately. It is also able to form cation-specific pores in the membrane of the target cell. The resulting deregulation of cell signaling leads to blocking of phagocytosis and reduction in the ability of neutrophils to kill bacteria. It also inhibits maturation of dendritic cells and their migration to the lymph nodes. The infection occurs mostly in children under the age of one when they are unimmunized, or children with faded immunity, normally around the ages 11 through 18. The signs and symptoms are similar to a common cold: runny nose, sneezing, mild cough, and low-grade fever. The patient becomes most contagious during the catarrhal stage of infection, normally two weeks after the coughing begins. It may become airborne when the person coughs, sneezes, or laughs. The paroxysmal cough precedes a crowing inspiratory sound characteristic of pertussis. After a spell, the patient might make a “whooping” sound when breathing in, or may vomit. Adults have milder symptoms, such as prolonged coughing without the “whoop”. Infants less than six months also may not have the typical whoop. A coughing spell may last a minute or more, producing cyanosis, apnoea, and seizures. However, when not in a coughing fit, the patient does not experience trouble breathing. This is because B. pertussis inhibits the immune response, so very little mucus is generated in the lungs. A prolonged cough may be irritating and sometimes a disabling cough may go undiagnosed in adults for many months. Prevention Pertussis vaccine has