Pathology
Immune Responses
A patient suffering with immune thrombocytopenic purpura undergoes a splenectomy. She is advised to get the Pneumococcal vaccine booster every 5 - 10 years. Which of the following properties of Streptococcus pneumoniae make her prone to infection by this organism:
Answer:
Patients with hyposplenism are at lifelong increased risk of infection from a variety of organisms. This is seen particularly in children under 5 years and those with sickle cell anaemia. The most characteristic susceptibility is to the encapsulated bacteriae Streptococcus pneumoniae, Haemophilus influenzae type B and Neisseria meningitidis. Streptococcus pneumoniae is a particular concern and can cause a rapid and fulminant disease. Malaria and infection caused by animal bites also tend to be more severe.Function of Spleen
Pathology / Immune Responses
Last Updated: 24th April 2019
Functions
Control of Red Cell Integrity:
The spleen has an essential role in the 'quality control' of red cells. Excess DNA, nuclear remnants (Howell-Jolly bodies) and siderotic granules are removed in the spleen. In the relatively hypoxic environment of the red pulp of the spleen, the membrane flexibility of aged and abnormal red cells is impaired, and they are trapped within the sinus where they are ingested by macrophages.
Immune Function:
Macrophages and dendritic cells in the spleen initiate an immune response to antigens filtered from the blood and are involved in phagocytosis and antigen presentation to B and T-cells to start adaptive immune responses. This arrangement is particularly efficient at mounting an immune response to encapsulated bacteria and explains the susceptibility of hyposplenic patients to these organisms.
Extramedullary Haemopoiesis:
The spleen is a site of haemopoiesis in foetal life (around 3 – 7 months) but is not a site of erythropoiesis in the normal adult. However, haemopoiesis may be re-established in both the liver and the spleen as extramedullary haemopoiesis, in disorders such as myelofibrosis or in chronic severe haemolytic and megaloblastic anaemia. Extramedullary haemopoiesis may result from either reactivation of dormant stem cells within the spleen or homing of stem cells from the bone marrow to the spleen.
Splenomegaly
An enlarged spleen is often asymptomatic but if significant may cause abdominal discomfort. Splenomegaly is usually felt under the left costal margin, but massive splenomegaly may be felt as far as the right iliac fossa. The spleen moves with respiration and a medial splenic notch may be palpable in some cases.
In developed countries the most common causes of splenomegaly are infectious mononucleosis, haematological malignancy and portal hypertension, whereas malaria and schistosomiasis are more prevalent on a global scale. Chronic myeloid leukaemia, primary myelofibrosis, malaria and leishmaniasis are potential causes of massive splenomegaly.
Hypersplenism
Normally only approximately 5% of the total red cell mass is present in the spleen, although up to half of the total marginating neutrophil pool and 30% of the platelet mass may be located here. As the spleen enlarges, the proportion of haemopoietic cells within the organ increases such that up to 40% of the red cell mass, and 90% of platelets may be pooled in an enlarged spleen.
Hypersplenism is characterised by an enlarged spleen with reduction of at least one cell line in the blood in the presence of normal bone marrow function. Hypersplenism may be treated with splenectomy if symptomatic. It is followed by a rapid improvement in the peripheral blood count.
Hyposplenism
Functional hyposplenism is characterised by the blood film findings of Howell-Jolly bodies or siderotic granules on iron staining. The most frequent cause is surgical removal of the spleen e.g. after traumatic rupture, but hyposplenism can also occur in sickle cell anaemia, gluten-induced enteropathy, amyloidosis and other conditions.
Patients with hyposplenism are at lifelong increased risk of infection from a variety of organisms. This is seen particularly in children under 5 years and those with sickle cell anaemia. The most characteristic susceptibility is to the encapsulated bacteriae Streptococcus pneumoniae, Haemophilus influenzae type B and Neisseria meningitidis. Streptococcus pneumoniae is a particular concern and can cause a rapid and fulminant disease. Malaria and infection caused by animal bites also tend to be more severe.
Measures to reduce the risk of serious infection include:
- Patients should be informed about their increased susceptibility to infection and advised to carry a card about their condition
- Prophylactic oral penicillin is recommended usually for life
- A supply of appropriate antibiotics should also be given for patients to take in the event of onset of fever before medical care is available
- Vaccination against pneumococcus, haemophilus, meningococcus and influenza vaccination is recommended
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
