Many links exist between the two major host defence mechanisms, haemostasis and immunity. Blood cells and endothelium are involved in these systems and their dysfunctions. The dual function of these cells is discussed. Once activated, cell-cell interactions enhance the biological response and contribute to development of pathology. Furthermore, autoimmunity presents a special clinical relevance, and could be involved in many haemostatic disorders. A hypothesis is presented on how some auto-antibodies are generated and can trigger pathology, by targeting the immune response onto blood or endothelial cells, or to body's organs.