Disorders of the Immune System


The purpose of the immune response is maintain normal health, however abnormal functioning of the immune system may lead to a disease state

The immune system may over-react (allergies), fail to distinguish between 'self' and 'non-self' (autoimmune) or cease working entirely (immunodeficiency)


Hypersensitivity Disorders

  • Hypersensitivity refers to an excessive or abnormal sensitivity to a substance that may not be inherently harmful
  • Such reactions require a pre-sensitized immune state of the host (i.e. prior exposure)
  • Upon re-exposure, the immune system responds excessively in a manner detrimental to the host
  • Allergic reactions are a common example of a (type I) hypersensitivity disorder


Allergic Reactions

  • An allergy is a local inflammatory response to an environmental substance that is not capable of causing disease (an allergen)
  • When a B cell first encounters the allergen (initial exposure) it differentiates into a plasma cell and makes large amounts of antigen-specific IgE
  • The IgE molecules attach to mast cells (effectively 'priming' them)
  • Upon re-exposure, the IgE-primed mast cells release large amounts of histamine and other inflammatory chemicals, leading to swelling and redness
  • These reactions tend to be localised to the region of exposure - often the airways and throat
  • A severe systemic allergic reaction is called anaphylaxis and can be fatal


Overview of an Allergic Reaction


Autoimmune Disorders

  • An autoimmune disorder occurs when the immune system fails to recognise its own body cells as 'self' and begins to attack its own cells and tissues
  • Some pathogens try to evade the immune system be producing antigens which closely resemble those of its host (antigenic mimicry)
  • If the pathogen is detected and destroyed, B-cells will be activated that produce antibodies specific to molecules similar to those on body tissues
  • This may lead to the destruction of tissues that contain those molecular fragments
  • Examples of autoimmune diseases include:
  • Type I diabetes mellitus (insulin dependent) - the insulin-secreting beta cells in the pancreas are destroyed by the immune system
  • Rheumatoid arthritis - chronic inflammation of the joints caused by a localised autoimmune response within the joint capsule
  • Multiple sclerosis - the myelin sheath is degraded as a result of an autoimmune response


Antigenic Mimicry Leading to an Autoimmune Response 


Immunodeficiency Disorders

Immunodeficiency is a state in which the immune system's ability to fight infectious diseases is compromised or absent entirely

These disorders may be inherited (e.g. SCIDs), of pathogenic origin (e.g. AIDS) or caused by drug treatments (e.g. cytotoxic drugs)


Acquired Immune Deficiency Syndrome (AIDS)

  • AIDS is caused by the human immunodeficiency virus (HIV) - a retrovirus that infects helper T lymphcytes (TH cells)
  • Reverse transciptase allows viral DNA to be produced from its RNA code, which is integrated into the host cells genome
  • After a number of years of inactivity (during which infected TH cells have continually reproduced), the virus becomes active and begins to spread, destroying the TH cells in the process (via the lysogenic cycle)
  • This results in lower immunity as antibody production is compromised - the individual is now susceptible to opportunistic infections


Timeline of HIV Infection