Lymph Node in Inflammation

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Introduction

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 and present the antigen to the appropriate lymphocyte population. This is to B-cells in the follicular centres and to T-cells in the parafollicular areas.

When stimulated by antigen presentation, the respective areas undergo an increase in size by hyperplasia. 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, 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. The bacteria therefore set up an acute inflammatory response in the lymph node. If they are pyogenic, bacteria can produce abscessation in the lymph node, e. g. Strangles.

Strangles

An upper respiratory tract pathogen in the horse. Abscesses in the cervical lymph nodes burst to the exterior and 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 then generally cleared from the blood by other organs containing phagocytes, e.g. the liver, spleen and lungs. There are, however, 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. This is accompanied by severe signs of clinical illness, due to the liberation of bacterial toxins. These signs can include: high fever, rapid respiration, shock and can even result in the death of the animal.

Gross appearance

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

Histological appearance

The capillaries in tissues taken immediately after death show bacteria in their lumens.



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