Haemolytic anaemia II – Acquired disorders

Autoimmune haemolytic anaemia

Autoimmune haemolytic anaemia (AIHA) is an example of an acquired form of haemolysis with a defect arising outside the red cell. The bone marrow produces structurally normal red cells and premature destruction is caused by the production of an aberrant autoantibody targeted against one or more antigens on the cell membrane. Once an antibody has attached itself to the red cell, the exact nature of the haemolysis is determined by the class of antibody and the density and distribution of surface antigens. IgM autoantibodies cause destruction by agglutination or by direct activation of serum complement. IgG class antibodies generally mediate destruction by binding of the Fc portion of the cell-bound immunoglobulin molecule by macrophages in the spleen and liver. The disparate behaviour of different types of autoantibody provides the explanation for a number of different clinical syndromes.

Table 15.1 shows a simple approach to the classification of autoimmune haemolytic anaemia. The disease can be divided into ‘warm’ and ‘cold’ types depending on whether the antibody reacts better with red cells at 37°C or 0–5°C. For each of these two basic types of autoimmune haemolysis there are a number of possible causes and these can be incorporated into the classification. A diagnosis of autoimmune haemolysis may precede diagnosis of the causative underlying disease.

Table 15.1

Classification of the autoimmune haemolytic anaemias

Warm AIHA (usually IgG)
Primary (idiopathic)
Secondary	Lymphoproliferative disorders
	Other neoplasms
	Connective tissue disorders
	Drugs
	Infections
Cold AIHA (usually IgM)
Primary (cold haemagglutinin disease)
Secondary	Lymphoproliferative disorders
	Infections (e.g. mycoplasma)
	Paroxysmal cold haemoglobinuria¹

¹ Caused by a biphasic polyclonal IgG antibody (Donath–Landsteiner).

Clinical presentation and management

Warm autoimmune haemolytic anaemia

Warm AIHA (Figs 15.1 and 15.2) is the most common form of the disease. The red cells are coated with either IgG alone, IgG and complement, or complement alone. Premature destruction of these cells usually takes place in the reticuloendothelial system. Approximately half of all cases are idiopathic but in the other half there is an apparent underlying cause (Table 15.1

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