Major histocompatibility complex

Definition

The major histocompatibility complex (MHC) is a set of cell surface proteins essential for the immune system to recognize foreign molecules. These proteins are crucial for the presentation of peptide fragments derived from pathogens to T cells, facilitating the activation of immune responses. The MHC plays a pivotal role in both cellular and humoral immunity, influencing how the body identifies and responds to infections and foreign substances.

5 Must Know Facts For Your Next Test

1. MHC molecules are divided into two classes: MHC class I molecules present peptides to CD8+ cytotoxic T cells, while MHC class II molecules present peptides to CD4+ helper T cells.
2. The diversity of MHC alleles among individuals is critical for the effectiveness of the immune response, as it allows for a broad range of peptide presentation.
3. MHC molecules are encoded by a group of genes located on chromosome 6 in humans, known as the HLA (human leukocyte antigen) complex.
4. The binding of peptides to MHC molecules is highly specific, with each MHC molecule capable of presenting only a limited range of peptides.
5. Successful transplantation of tissues or organs often depends on matching donor and recipient MHC types to reduce the risk of rejection by the recipient's immune system.

Review Questions

• How do major histocompatibility complex molecules contribute to the activation of T cells?
  • MHC molecules are essential for T cell activation as they present peptide fragments from pathogens on the surface of antigen-presenting cells. When a T cell's receptor recognizes and binds to the peptide-MHC complex, it triggers a series of signaling events that activate the T cell. This activation is crucial for mounting an effective immune response, as it enables T cells to proliferate and perform their functions, such as killing infected cells or helping other immune cells.
• Discuss the differences between MHC class I and MHC class II molecules and their roles in immune responses.
  • MHC class I molecules are found on almost all nucleated cells and primarily present endogenous peptides from intracellular proteins to CD8+ cytotoxic T cells. In contrast, MHC class II molecules are expressed mainly on professional antigen-presenting cells, like dendritic cells and B cells, presenting exogenous peptides from extracellular sources to CD4+ helper T cells. This distinction is vital because CD8+ T cells primarily target infected or malignant cells, while CD4+ T cells help orchestrate a broader immune response by activating B cells and other immune effector cells.
• Evaluate the importance of MHC diversity in human populations and its implications for disease susceptibility and organ transplantation.
  • The diversity of MHC alleles in human populations is critical for effective immune responses against various pathogens. Different individuals have unique MHC profiles, which influences their susceptibility to diseases. For example, certain MHC variants may confer resistance or susceptibility to infections or autoimmune disorders. In organ transplantation, matching donor and recipient MHC types is crucial to minimize rejection; mismatches can lead to severe complications as the recipient's immune system may recognize the transplanted tissue as foreign. Thus, understanding MHC diversity has significant implications for both public health and clinical practices.

Related terms

Antigen: A substance that induces an immune response, typically by being recognized by antibodies or T cell receptors.

T Cells: A type of white blood cell that plays a central role in cell-mediated immunity, including helper T cells and cytotoxic T cells.

B Cells: A type of white blood cell responsible for producing antibodies in the humoral immune response.