Lymph Node in Inflammation

• Antigenic material is transported in phagocytes via the tissue lymphatics to the local lymph node.
• In the subcapsular sinusoids of the lymph node, antigens are exposed to the reticular cell macrophages.
  • These macrophages form a meshwork around the subcapsular, cortical and medullary sinusoids.
  • These reticular cells present the antigen to the appropriate lymphocyte population.
    • To B-cells in the follicular centres
    • To T-cells in the parafollicular areas.
• When stimulated by antigen presentation, the respective areas undergo an increase in size by hperplasia.
  • The lymphocytes produced later leave the lymph node to seek out the source of the antigen in the damaged portion.
• Bacteria surviving the initial local inflammatory reaction also travel to the lymph node.
  • Travel either in macrophages or as a bacteraemia/septicaemia.
  • In the lymph node, many bacteria are trapped, degraded and killed, or exposed to antibody produced by plasma cells in the medullary area.
  • Bacteria set up an acute inflammatory response in the lymph node.
    • If they are pyogenic, baceria can produce abscessation in the lymph node.
      • E. g. Strangles.
        • An upper respiratory tract pathogen in the horse.
        • Abscesses in the cervical lymph nodes burst to the exterior.
        • Abscesses in the retropharyngeal node burst into the pharynx
        • Infection is usually localised to the head and neck, but can spread to cause abscessation elsewhere in the body.

Bacteraemia

• Bacteria may continue through the lymph nodes and circulate in the blood.
• These are generally cleared from the blood by other organs containing phagocytes.
  • E.g. the liver, spleen and lungs.
• There are species differences in the ability to remove bacteria.
  • In most species, the majority of clearance is carried out by the liver and spleen.
  • However, in the pig, the lung removes 60-80% of such bacteria.

Septicaemia

• Septicaemia is a condition where there are highly pathogenic bacteria circulating and rapidly multiplying in the blood.
  • Accompanied by severe signs of clinical illness due to the liberation of bacterial toxins.
    • High fever
    • Rapid respiration
    • Shock
  • Can result in the death of the animal.

Gross Appearance

• The lymph nodes, liver and spleen are very hyperaemic and swollen.

Histological Appearance

• Capillaries in tissues taken immediately after death show bacteria in their lumens.