Makindo Medical Notes

Introduction to Human Leukocyte Antigen (HLA)

The Human Leukocyte Antigen (HLA) system plays a crucial role in the immune system, specifically in the recognition of foreign molecules. It is a group of genes located on chromosome 6 that encode proteins responsible for the regulation of the immune system in humans.

HLA Classifications

• Class I HLA: Found on almost all nucleated cells, these molecules present peptides from inside the cell (such as viruses) to cytotoxic T cells.
• Class II HLA: These are primarily expressed on antigen-presenting cells like macrophages, B cells, and dendritic cells. They present extracellular peptides to helper T cells.
• Class III HLA: This class consists of a diverse set of genes that do not play a direct role in antigen presentation but are involved in immune system processes.

Role of HLA in the Immune System

• Antigen Presentation: HLA molecules present peptides (antigens) to T cells, which then trigger immune responses.
• Tissue Compatibility: HLA plays a crucial role in organ transplantation, as mismatched HLA can lead to graft rejection.
• Immune Regulation: HLA helps in recognizing "self" versus "non-self," ensuring that the immune system targets harmful invaders while ignoring the body's own cells.

Associations Between HLA and Diseases

• Autoimmune Diseases: Many autoimmune conditions are associated with specific HLA alleles. For example:
  • HLA-B27: Associated with ankylosing spondylitis and other spondyloarthropathies.
  • HLA-DR3 and HLA-DR4: Linked to Type 1 Diabetes.
  • HLA-DRB1: Strong association with rheumatoid arthritis.
• Infectious Diseases: HLA can affect susceptibility or resistance to infections. For example:
  • HLA-B53: Provides some protection against severe malaria.
  • HLA-B35: Linked to a worse prognosis in HIV infections.
• Allergies: Certain HLA alleles are implicated in hypersensitivity reactions, such as drug-induced hypersensitivity:
  • HLA-B*5701: Associated with hypersensitivity to the HIV drug abacavir.

Associations Between HLA and Disease

Conclusion

The Human Leukocyte Antigen system is central to immune regulation, and its various alleles are critical determinants in disease susceptibility and immune responses. Understanding HLA associations is vital for better insights into autoimmune diseases, infectious disease management, and improving transplantation outcomes.