immunology

Hypersensitivity and Autoimmune Reactions

Introduction

This section introduces hypersensitivity reactions and their categorization, particularly focusing on autoimmune reactions against self-antigens.
The primary recollection tool for these reactions is the abbreviation A, B, and D, where each letter corresponds to categories within hypersensitivity events.

Autoimmunity

The text defines autoimmunity: an inappropriate immune response against self-antigens. It is emphasized that there are four specific types of hypersensitivity reactions, with Type I not being involved in autoimmune diseases.

Importance of Immunological Tolerance

  • Central Tolerance:

    • Mechanism by which auto-reactive immune cells are kept in check.
    • Prevents these cells from leaking into the peripheral circulation.

  • Peripheral Tolerance:

    • Functions to manage auto-reactive cells within the peripheral systems.
    • Is essential in limiting the activation of immune responses towards self-antigens.

Genetic Influences on Autoimmunity

The text discusses the association between certain human leukocyte antigen (HLA) subsets and the likelihood of developing autoimmune diseases. Thus, there exists a genetic predisposition to such conditions.
Additionally, chronic infections, notably Epstein-Barr virus infections, are highlighted as having links to the development of autoimmune disorders.

Mechanisms of Autoimmunity

B Cell and T Cell Activity

  • B cells differentiate and produce antibodies against self-cells when activated by specific antigens.
  • T cells, specifically CD4+ T helper cells, interact with antigen-presenting cells (APCs) (B cells, phagocytes) responsible for processing antigens into peptides.
  • Inflammatory cytokines mediate these reactions, leading to the destruction of auto-reactive cells.

Stimulation Signals

There is a discussion on the increased production of post-stimulatory signals which enhance immune reactions, particularly in individuals who show hyperactivity in cytokine and inflammatory molecule production.

Simplifying Mechanisms of Autoimmunity

  • In normal situations, B and T cells are designed to target pathogens without affecting host cells.
  • When viruses invade, antibodies ideally target these viruses specifically; however, issue arises when viral peptide fragments resemble those of the host.
  • This phenomenon, termed molecular mimicry, can result in antibodies targeting host cells alongside the virus.
  • Example: Virus fragments and self-cell fragments show similarities, leading to an unintended immune attack on the host's own cells.

Bystander Activation

  • This is a mechanism wherein immune cells, particularly T cells, attack host tissue due to the presence of similar surface receptors expressed by pathogens. Here too, unintentional destruction occurs.

Specific Autoimmune Disorders and Their Mechanisms

Graves' Disease

  • In autoimmune Graves' disease (thyrotoxicosis), the normal function of thyroid-stimulating hormone (TSH) is disrupted due to mimicking receptor activity.
  • TSH binds to receptors on thyroid cells, leading to inappropriate thyroid hormone production which can cause hyperthyroidism.

Multiple Sclerosis

  • This disorder involves auto-reactive T cells attacking the myelin sheath of nerves.
  • Lymphoid tissues produce antibodies directed against components of the myelin sheath.
  • Resulting effects include severe neurological dysfunction due to the dysfunction of signal transmission in neurons.
  • Blood-Brain Barrier Disruption: Antibodies deposited in the CNS lead to further damage, causing significant immune activation in the brain.

Diabetes Mellitus Type 1

  • The mechanism involves pancreatic β-cells that improperly express Major Histocompatibility Complex (MHC) Class II molecules.
  • These β-cells present self-fragments to CD4+ T cells, leading to the destruction of these cells, which are crucial for insulin production.
  • Results in impaired insulin secretion, with infection potentially exacerbating the autoimmune response.

Conclusion

The intricate details of autoimmune mechanisms underscore the negative repercussions of immune interactions meant for pathogen elimination, underscoring the need for targeted treatment approaches to regulate such conditions. Non-steroidal anti-inflammatory medications are often employed to reduce inflammation, aiding in the management of autoimmune diseases choking immune activity, and symptom alleviation.

Through the examples provided, the need for a comprehensive understanding of these processes is made evident to handle patient care effectively and develop therapeutic interventions.