**4. Mediators of inflammatory reactions in haemolytic transfusion reactions**

Receptors for complement activation products C3a and C5a are found on many cells: monocytes, macrophages, neutrophils, platelets, endothelium and smooth muscle. Their release causes an increase in the concentration of oxygen radicals, leukotrienes, nitric oxide and cytokines. The increase in cytokine release may also be due to the interaction of Fcγ R1 receptors with IgG molecules associated with red blood cells. Udani et al. [20] showed in vitro that in the case of ABO incompatibility, monocytes are directly involved in the formation of acute haemolytic transfusion reaction [15]. Incompatible red blood cells reduce CD14 expression and increase CD44 expression on monocytes in whole blood. After 24 incubations with incompatible red blood cells, monocytes show a significant increase in CD44 levels. The results of these studies indicate a critical role of monocyte activation in the development of intravascular haemolytic transfusion reaction [15].

In ABO incompatibility, in which anti-A, anti-B and anti-AB antibodies activate complement leading to intravascular haemolysis, a large amount of tumour necrosis factor-α (TNF) and interleukins CXCL8 (IL-8) and CCL2 are released into the plasma (MCP-1) [19–21]. TNF-α is released first, its elevated concentration is already detected within first 2 h. It carries a pro-inflammatory potential that is responsible for fever, leukocyte activation, stimulation of procoagulant activity, increased antibody production and vascular wall permeability [22]. TNF-α also stimulates endothelial cells to synthesise adhesion molecules and chemotactic cytokines [22]. CXCL8 and CCL2 produced in the blood during ABO incompatibility will appear later than TNF-α in very high concentrations. CXCL8 primarily activates neutrophils, which leads to the accumulation of leukocytes in the lung vessels of small diameter and damage to the endothelium of blood vessels and their higher permeability [1, 12]. CCL2 is mainly a chemotactic and activating factor for monocytes [1, 12].

In incompatibility, in which non-complement IgG antibodies cause extravascular haemolysis, cytokines belonging to two categories differing in response rates are produced: (1) synthesised at a concentration higher than 1 μg/ml within 24 h and (2) synthesised at a concentration of about 100 pg/ml. Low concentration cytokines include IL-1β, IL-6 and TNF-α. CXCL8 concentration is similar to that in intravascular haemolysis, whereas TNF-α is synthesised at low concentration, estimated at <100 pg/ml [1, 2]. IL-1ra (receptor antagonist) is produced in extravascular haemolysis, which is an IL-1 receptor antagonist. Its presence to some extent affects some clinical differences between extravascular and intravascular haemolysis [23]. IL-1β concentration and IL-6 produced by monocytes in response to red blood cells coated with IgG antibodies increase progressively within 24 h to a concentration of 100 pg/ml. Since IL-1β and IL-6 affect proliferation and differentiation

of β-lymphocytes, the synthesis of these two cytokines enhances the synthesis of allo- and autoantibodies, which are often involved in the formation of delayed haemolytic transfusion reaction [1, 24, 25].
