Final answer:
Helper T cells, including Th1, Th2, and Th17, play different roles in immune response: Th1 cells target intracellular pathogens, Th2 cells target extracellular pathogens, and Th17 cells defend against chronic mucocutaneous infections. The nature of the pathogen determines whether a Th1 or Th2 response is initiated.
Step-by-step explanation:
The immune system uses different types of helper T cells to respond to various pathogens. Th1 cells are involved in the body's response to intracellular pathogens such as viruses and some bacteria. They activate macrophages and cytotoxic T cells, stimulating the immune response to destroy infected cells. Th2 cells, on the other hand, respond primarily to extracellular pathogens, including parasites. They activate eosinophils, mast cells, and help B cells differentiate into plasma cells, which secrete antibodies. Furthermore, Th17 cells play a critical role in defending against chronic mucocutaneous infections by producing cytokines that promote inflammation and enhance the mucosal defense against invading pathogens.
Patients with reduced Th17 cells, such as those suffering from HIV, may experience increased susceptibility to bacterial and gastrointestinal infections. When it comes to the Th1-mediated response, we see macrophage activation that leads to the destruction of pathogens like Mycobacterium tuberculosis, which can survive within macrophages. In contrast, the Th2-mediated response involves B lymphocytes that differentiate and produce antibodies to target extracellular pathogens.
Overall, whether an immune response is Th1 or Th2, depends on the nature of the invading pathogen and the types of cytokines secreted by cells of the innate immune system.