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💀anatomy and physiology i review

Class I HLAs

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

Class I HLAs, or Major Histocompatibility Complex (MHC) Class I molecules, are cell surface proteins that play a crucial role in the immune system's recognition and response to foreign antigens. These molecules are found on the surface of nearly all nucleated cells in the body and are responsible for presenting peptide fragments to cytotoxic T cells, initiating an immune response against infected or cancerous cells.

5 Must Know Facts For Your Next Test

  1. Class I HLAs are encoded by a highly polymorphic gene cluster known as the Major Histocompatibility Complex (MHC), which is located on chromosome 6 in humans.
  2. These molecules bind to and present a diverse array of self-peptides and foreign peptides derived from intracellular pathogens, allowing the immune system to monitor the cell's contents for signs of infection or transformation.
  3. Cytotoxic T cells (CD8+ T cells) recognize peptides presented by Class I HLAs and can initiate a targeted immune response to eliminate infected or cancerous cells.
  4. Differences in Class I HLA expression between the donor and recipient are a major factor in transplant rejection, as the recipient's immune system recognizes the donor's HLAs as foreign and mounts an attack.
  5. Downregulation or loss of Class I HLA expression is a common immune evasion strategy employed by cancer cells, allowing them to avoid detection and destruction by the immune system.

Review Questions

  • Explain the role of Class I HLAs in antigen presentation and the activation of cytotoxic T cells.
    • Class I HLAs are responsible for presenting peptide fragments, derived from intracellular proteins, to cytotoxic T cells (CD8+ T cells). This allows the immune system to monitor the contents of cells and detect the presence of foreign or abnormal peptides, such as those produced by viruses or cancer cells. When a cytotoxic T cell recognizes a cell displaying a foreign peptide on a Class I HLA, it becomes activated and can initiate a targeted immune response to eliminate the infected or transformed cell.
  • Describe the importance of Class I HLA matching in the context of organ transplantation and the risk of rejection.
    • Differences in Class I HLA expression between the donor and recipient are a major factor in transplant rejection. The recipient's immune system recognizes the donor's HLAs as foreign and mounts an attack, leading to the rejection and failure of the transplanted organ. Careful matching of Class I HLAs between the donor and recipient is crucial to minimize the risk of transplant rejection and improve the chances of long-term graft survival. Immunosuppressive therapies are often used to further suppress the recipient's immune response and prevent rejection.
  • Analyze the role of Class I HLA downregulation as an immune evasion strategy employed by cancer cells, and discuss the implications for cancer immunotherapy.
    • Downregulation or loss of Class I HLA expression is a common immune evasion strategy used by cancer cells. By reducing the presentation of tumor-derived peptides on their surface, cancer cells can avoid detection and destruction by cytotoxic T cells. This represents a significant challenge for cancer immunotherapy, as the immune system's ability to recognize and target cancer cells is impaired. Researchers are exploring various approaches to overcome this immune evasion strategy, such as the development of therapies that can restore or enhance Class I HLA expression on cancer cells, or the use of alternative immune effector cells that do not rely solely on Class I HLA recognition.