MHC Class II Pathway

Definition

The MHC Class II pathway is a cellular mechanism that presents extracellular antigens on the surface of antigen-presenting cells (APCs) to CD4+ T helper cells. This pathway is crucial for the activation of the adaptive immune response, especially against viruses, as it allows the immune system to recognize and respond to foreign proteins from pathogens that have been engulfed and processed by these cells.

5 Must Know Facts For Your Next Test

1. MHC Class II molecules are primarily expressed on APCs such as dendritic cells, macrophages, and B cells, enabling them to present antigens effectively.
2. The processing of exogenous antigens involves their uptake by APCs, followed by degradation into peptides that are loaded onto MHC Class II molecules within specialized compartments.
3. Once the MHC Class II-peptide complex is presented on the APC surface, it can be recognized by CD4+ T cells, leading to their activation and differentiation.
4. Activated CD4+ T cells play a pivotal role in helping other immune cells, including B cells for antibody production and cytotoxic T cells for killing infected host cells.
5. The MHC Class II pathway is essential for generating a robust immune memory response, enabling the body to respond more rapidly to subsequent infections by the same pathogen.

Review Questions

• How do antigen-presenting cells process exogenous antigens and present them through the MHC Class II pathway?
  • Antigen-presenting cells (APCs) process exogenous antigens by first engulfing them through phagocytosis or endocytosis. Once inside the APC, these antigens are degraded into smaller peptide fragments within endosomal compartments. These peptides then bind to MHC Class II molecules, which are transported to the cell surface. The resulting MHC Class II-peptide complex can then be recognized by CD4+ T cells, leading to their activation.
• Discuss the role of CD4+ T cells in coordinating the immune response after activation by the MHC Class II pathway.
  • Upon activation through recognition of the MHC Class II-peptide complex, CD4+ T cells undergo clonal expansion and differentiation into various subsets. These activated T helper cells secrete cytokines that enhance the activity of other immune cells such as B cells, promoting antibody production, and cytotoxic T cells, enhancing their ability to kill infected or malignant cells. This coordination is vital for mounting an effective immune response against viral infections.
• Evaluate how defects in the MHC Class II pathway might impact immune responses against viral infections.
  • Defects in the MHC Class II pathway can lead to impaired antigen presentation, resulting in a weakened activation of CD4+ T cells. This can severely affect the adaptive immune response, as these T helper cells are crucial for supporting both B cell antibody production and cytotoxic T cell activity. Consequently, individuals with such defects may have an increased susceptibility to viral infections and may struggle to mount effective immune responses, highlighting the importance of this pathway in maintaining immune system integrity.

Related terms

Antigen-Presenting Cells (APCs): Cells that process and present antigens on their surface using MHC molecules, playing a key role in initiating immune responses.

CD4+ T Cells: A type of T lymphocyte that interacts with MHC Class II molecules, helping to orchestrate the immune response by activating other immune cells.

Exogenous Antigens: Antigens that originate outside the body, which are taken up by APCs and presented via the MHC Class II pathway.