Major histocompatibility complex

Definition

The major histocompatibility complex (MHC) is a set of cell surface proteins essential for the adaptive immune system to recognize foreign molecules. These proteins present peptide fragments derived from pathogens to T cells, which play a critical role in orchestrating the immune response. MHC molecules are divided into two classes: MHC class I, which presents to CD8+ cytotoxic T cells, and MHC class II, which presents to CD4+ helper T cells, facilitating a coordinated immune response against infections.

5 Must Know Facts For Your Next Test

1. MHC molecules are highly polymorphic, meaning there are many different versions within the population, which enhances the ability of the immune system to recognize diverse pathogens.
2. The interaction between MHC molecules and T cell receptors is crucial for the activation of T cells, allowing them to respond effectively to infections.
3. MHC class I molecules present endogenous antigens, typically from proteins synthesized within the cell, while MHC class II presents exogenous antigens derived from extracellular sources.
4. MHC is not only important for immune responses but also plays a significant role in organ transplantation, as mismatches can lead to transplant rejection.
5. The process of MHC antigen presentation is essential for the development of immunological memory, which provides long-term protection against previously encountered pathogens.

Review Questions

• How does the major histocompatibility complex facilitate communication between the innate and adaptive immune systems?
  • The major histocompatibility complex (MHC) acts as a bridge between the innate and adaptive immune systems by presenting antigens derived from pathogens. When antigen-presenting cells (APCs) process these antigens and display them on MHC molecules, they can activate T cells, which are crucial for the adaptive immune response. This interaction ensures that T cells can recognize and respond specifically to pathogens, thus linking both immune responses.
• Discuss the differences between MHC class I and MHC class II molecules in terms of their structure and function.
  • MHC class I molecules are composed of a heavy chain and a beta-2 microglobulin chain and primarily present endogenous antigens to CD8+ cytotoxic T cells. In contrast, MHC class II molecules consist of two chains (alpha and beta) and present exogenous antigens to CD4+ helper T cells. This structural difference reflects their unique roles in the immune response; MHC class I is essential for signaling infected cells for destruction, while MHC class II facilitates help from T cells to activate other immune cells.
• Evaluate the implications of MHC polymorphism on population immunity and organ transplantation success rates.
  • MHC polymorphism significantly enhances population immunity by allowing diverse recognition of pathogens across different individuals. This genetic variation means that some individuals may effectively respond to specific infections while others may not. However, this same polymorphism complicates organ transplantation because mismatched MHC molecules between donor and recipient can lead to rejection of the transplant. Understanding this relationship helps in developing better matching strategies and improving outcomes in transplant procedures.

Related terms

Antigen: A substance that induces an immune response, often recognized by the immune system as foreign, such as proteins from pathogens.

T cells: A type of white blood cell that plays a central role in cell-mediated immunity; they include helper T cells and cytotoxic T cells.

HLA (Human Leukocyte Antigen): A specific set of MHC molecules in humans that are critical for immune system function and transplantation compatibility.