MHC Class II

Definition

MHC Class II molecules are proteins found on the surface of certain immune cells that present processed antigens to CD4+ T helper cells, playing a crucial role in initiating immune responses. These molecules are essential for the activation of helper T cells, which subsequently assist in orchestrating various aspects of the adaptive immune response, including the activation of B cells and cytotoxic T cells.

5 Must Know Facts For Your Next Test

1. MHC Class II molecules are primarily expressed on antigen-presenting cells such as dendritic cells, macrophages, and B cells.
2. The interaction between MHC Class II and CD4+ T cells is crucial for the activation of these T cells, which leads to a robust adaptive immune response.
3. MHC Class II molecules present extracellular antigens that have been engulfed and processed by antigen-presenting cells.
4. The genes encoding MHC Class II molecules are highly polymorphic, meaning there is a significant variation in the MHC molecules among individuals, which plays a role in organ transplantation and disease susceptibility.
5. Failure of MHC Class II function can lead to immunodeficiencies and is associated with autoimmune diseases where the immune system mistakenly targets self-antigens.

Review Questions

• How do MHC Class II molecules contribute to the activation of CD4+ T cells and the overall immune response?
  • MHC Class II molecules present processed extracellular antigens on the surface of antigen-presenting cells to CD4+ T cells. This interaction is essential for the activation of CD4+ T cells, which then release cytokines that help activate B cells and cytotoxic T cells. The coordinated efforts of these immune cells lead to a more effective adaptive immune response against pathogens.
• Discuss the significance of MHC Class II diversity in relation to individual immune responses and organ transplantation.
  • The high polymorphism of MHC Class II genes means that individuals have different versions of these molecules, which influences their immune responses. This diversity allows populations to have a broader range of responses to pathogens. However, it also complicates organ transplantation, as mismatched MHC Class II molecules between donors and recipients can lead to transplant rejection due to the recipient's immune system recognizing the transplanted tissue as foreign.
• Evaluate the implications of defective MHC Class II expression in relation to autoimmune diseases and immunodeficiencies.
  • Defective expression or function of MHC Class II can lead to significant health issues such as immunodeficiencies, where the body fails to mount an adequate immune response against infections. Additionally, improper functioning may contribute to autoimmune diseases, where the immune system erroneously targets self-antigens due to improper antigen presentation. Understanding these implications is vital for developing therapeutic strategies that address both types of conditions.

Related terms

Antigen Presentation: The process by which protein fragments (antigens) are displayed on the cell surface by MHC molecules to be recognized by T cells.

CD4+ T Cells: A subtype of T lymphocytes that express the CD4 glycoprotein and are primarily involved in helping other immune cells respond to infections.

Adaptive Immunity: A specific immune response that develops over time, characterized by the ability to recognize and remember specific pathogens.