Measurement of splenic phagocytic and immunological function

Irreversible trapping of particulate matter is a function of the reticuloendothelial system. Colloid particles 1 |im in size or less will be taken up by the liver, larger particles by the spleen. There is a well-established relationship between splenic reticuloendo-thelial function and the rate at which heat-damaged red cells are removed from the circulation. The half-clearance time in normal subjects is about 8-16 min. Post splenectomy, the time is increased to 60-120 min or longer and significantly increased times also occur in conditions associated with splenic hypofunc-tion (see Table 21.2).

In these conditions, a slow clearance rate may identify splenic hypofunction before the blood film shows Howell-Jolly bodies and other morphological features. This test also provides a reliable functional assessment of residual splenic tissue after splenectomy for immune thrombocytopenic purpura or autoimmune haemolytic anaemia, thus indicating the extent to which the splenuncule can be incriminated for the occurrence of relapse. Faster than normal clearance times have been observed in patients with haemolytic anaemia and in some patients with splenomegaly.

The curve of the clearance time is complex, as it comprises three overlapping components, namely, intrasplenic cell transit, sequestration and phagocytosis or irreversible extraction. These may be interpolated from the curve which shows blood flow, cell transit time and extraction ratio (Figure 21.6). There is also a rapid uptake of some of the labelled material by the liver. When the spleen is functioning normally, transit time is 5-10% per

Blood Spleen

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Figure 21.6 Curve of disappearance from circulation of heat-damaged red cells. The components of the curve reflect sequential transit pooling, sequestration and irreversible trapping of the cells. 1, blood flow; 2, cell transit time; 3, extraction rate.

minute, with extraction of 20-50% of the labelled cells. Using red cells coated with IgG (anti Rh(D)) antibody, splenic Fc-receptor phagocytic function can be measured.

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