Chapter 21 - Section 21.4 - Cellular Immunity - Before You Go On - Page 829: 19

Cytotoxic T cells (CD8+ T cells) and helper T cells (CD4+ T cells) play distinct roles in the immune system and have different mechanisms of activation. One of the reasons cytotoxic T cells are activated by a broader range of host cells compared to helper T cells is related to their primary function and the nature of the antigens they recognize. **Cytotoxic T Cells (CD8+ T Cells):** Cytotoxic T cells are primarily responsible for recognizing and eliminating infected or abnormal cells. They recognize antigens that are presented on major histocompatibility complex class I (MHC-I) molecules. MHC-I molecules present antigens derived from intracellular proteins, which can include viral proteins produced within infected cells or abnormal proteins generated by cancerous cells. Since most cells in the body have MHC-I molecules on their surfaces, cytotoxic T cells are able to recognize a broad range of host cells. Cytotoxic T cells have a surveillance role to detect and eliminate cells that may be harboring intracellular pathogens or displaying abnormal proteins, thus helping to prevent the spread of infections and the development of cancers. **Helper T Cells (CD4+ T Cells):** Helper T cells primarily assist other immune cells in their functions. They play a critical role in orchestrating immune responses by releasing cytokines that activate B cells, cytotoxic T cells, macrophages, and other immune cells. Helper T cells recognize antigens presented on major histocompatibility complex class II (MHC-II) molecules, which are mainly found on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. Since MHC-II molecules are primarily expressed on APCs, helper T cells have a more specific mode of activation. They require direct interactions with these specialized cells, which present antigens from extracellular pathogens or antigens acquired from the environment. In summary, cytotoxic T cells have a broader range of host cell recognition because they target intracellular antigens presented on MHC-I molecules, which are found on most nucleated cells. Helper T cells, on the other hand, are activated by APCs presenting antigens on MHC-II molecules, resulting in a more specific activation mechanism. Both types of T cells work together to provide comprehensive immune responses.

Cytotoxic T cells (CD8+ T cells) and helper T cells (CD4+ T cells) play distinct roles in the immune system and have different mechanisms of activation. One of the reasons cytotoxic T cells are activated by a broader range of host cells compared to helper T cells is related to their primary function and the nature of the antigens they recognize. **Cytotoxic T Cells (CD8+ T Cells):** Cytotoxic T cells are primarily responsible for recognizing and eliminating infected or abnormal cells. They recognize antigens that are presented on major histocompatibility complex class I (MHC-I) molecules. MHC-I molecules present antigens derived from intracellular proteins, which can include viral proteins produced within infected cells or abnormal proteins generated by cancerous cells. Since most cells in the body have MHC-I molecules on their surfaces, cytotoxic T cells are able to recognize a broad range of host cells. Cytotoxic T cells have a surveillance role to detect and eliminate cells that may be harboring intracellular pathogens or displaying abnormal proteins, thus helping to prevent the spread of infections and the development of cancers. **Helper T Cells (CD4+ T Cells):** Helper T cells primarily assist other immune cells in their functions. They play a critical role in orchestrating immune responses by releasing cytokines that activate B cells, cytotoxic T cells, macrophages, and other immune cells. Helper T cells recognize antigens presented on major histocompatibility complex class II (MHC-II) molecules, which are mainly found on antigen-presenting cells (APCs) such as dendritic cells, macrophages, and B cells. Since MHC-II molecules are primarily expressed on APCs, helper T cells have a more specific mode of activation. They require direct interactions with these specialized cells, which present antigens from extracellular pathogens or antigens acquired from the environment. In summary, cytotoxic T cells have a broader range of host cell recognition because they target intracellular antigens presented on MHC-I molecules, which are found on most nucleated cells. Helper T cells, on the other hand, are activated by APCs presenting antigens on MHC-II molecules, resulting in a more specific activation mechanism. Both types of T cells work together to provide comprehensive immune responses.