Major Histocompatibility Complex (MHC)

Definition

The major histocompatibility complex (MHC) is a group of genes that encode cell surface proteins responsible for the regulation of the immune system. These proteins play a crucial role in the recognition of self from non-self, allowing the body to distinguish its own cells from foreign or infected cells, and mount an appropriate immune response.

5 Must Know Facts For Your Next Test

1. The MHC is highly polymorphic, with hundreds of different alleles that can be expressed, allowing the immune system to recognize a wide range of foreign antigens.
2. MHC class I molecules present endogenous (self-derived) peptides to cytotoxic T cells, while MHC class II molecules present exogenous (foreign) peptides to helper T cells.
3. The expression of MHC molecules is crucial for the development and selection of T cells in the thymus, ensuring that T cells can recognize self-MHC but not react against it.
4. Differences in MHC molecules between individuals are a major barrier to successful organ transplantation, as the recipient's immune system will recognize the foreign MHC as non-self and mount a rejection response.
5. Certain MHC alleles have been associated with an increased susceptibility to autoimmune diseases, where the immune system mistakenly attacks the body's own tissues.

Review Questions

• Explain the role of the major histocompatibility complex (MHC) in the immune system's recognition of self and non-self.
  • The MHC plays a central role in the immune system's ability to distinguish between self and non-self. MHC molecules display peptide fragments on the cell surface, allowing T cells to recognize these peptides as either belonging to the host (self) or being foreign (non-self). This recognition is crucial for the development of T cells in the thymus, where they are selected to respond to foreign antigens while avoiding autoimmune reactions against self-peptides presented by the host's own MHC molecules. The high degree of MHC polymorphism ensures that the immune system can recognize a wide range of potential threats, while the expression of self-peptides by MHC helps maintain self-tolerance.
• Describe the differences between MHC class I and MHC class II molecules and their respective roles in immune responses.
  • MHC class I and class II molecules have distinct functions in the immune system. MHC class I molecules present endogenous (self-derived) peptides to cytotoxic CD8+ T cells, allowing them to recognize and destroy infected or cancerous cells. In contrast, MHC class II molecules present exogenous (foreign) peptides to helper CD4+ T cells, which then coordinate the broader immune response by activating other immune cells, such as B cells and macrophages. This division of labor is crucial for the immune system's ability to mount appropriate responses to different types of threats, with MHC class I-mediated recognition of intracellular pathogens and MHC class II-mediated recognition of extracellular pathogens.
• Explain the significance of MHC polymorphism and its implications for organ transplantation and autoimmune diseases.
  • The high degree of polymorphism in the MHC, with hundreds of possible alleles, is a key feature that enhances the immune system's ability to recognize a wide range of foreign antigens. However, this same diversity can also pose a challenge for successful organ transplantation, as the recipient's immune system is likely to recognize the foreign MHC molecules expressed on the transplanted tissue as non-self, triggering a rejection response. Conversely, certain MHC alleles have been associated with an increased susceptibility to autoimmune diseases, where the immune system mistakenly recognizes self-peptides presented by the host's own MHC molecules as foreign, leading to an inappropriate immune attack against the body's own tissues. Understanding the role of MHC polymorphism in these processes is crucial for developing strategies to improve transplant outcomes and manage autoimmune conditions.

Related terms

Antigen Presentation: The process by which MHC molecules display fragments of proteins (antigens) on the cell surface, allowing them to be recognized by T cells and triggering an immune response.

Self-Tolerance:

The ability of the immune system to recognize and tolerate the body's own cells and molecules, avoiding an autoimmune response, which is mediated in part by the expression of self-peptides by MHC molecules.

Transplant Rejection: The immune system's rejection of transplanted tissues or organs due to the recognition of foreign MHC molecules on the transplanted cells, leading to an immune attack and graft rejection.