Immune complex is the key. Blood contaminated with partially digested food coming from the gastro-intestinal tract travels through the liver where most immune complexes are removed. If circulating immune complexes pass the liver filter, they may cause disturbances in many of the body's organs. The other path of malabsorption of food particles from the gastro-intestinal tract is through lymphatic drainage. The lymph channels in the gut wall converge to form the thoracic duct which drains its contents into the subclavian vein. The combination of antibody with complement in the blood stream is a circulating immune complex. Immune complexes attach to CR1 receptors of red and white blood cells. Individuals have varying numbers of CR1 receptors. In most cases, circulating immune complexes are simply removed from circulation by macrophages in the liver and spleen prior to triggering a cascade of events which may cause multiple symptoms and possible tissue damage.
Circulating immune complexes that are not removed can activate a complement cascade. This is a circulating system of 25 proteins which interact to produce a variety of molecular defensive weapons. There are two main functions of the complement cascade. The first is defense against bacteria, viruses and other pathogens. Antibodies covalently bind C3b which starts a chain reaction that triggers the complement cascade. The function of immune complex is to lyse cells by activation of this complex, assemble into pores on the cell membrane (membrane attack complex), and disrupt the cell membrane or cell walls. The net effect is that sodium and water flow into a cell causing the cell death. In addition, complement generates cytokine inflammatory mediation. For more information on this topic, read "Immunology" by Male, Brostoff Roth, and Roitt. It is found in most medical school libraries.
Clearly, activation of complement can have severe consequences causing tissue damage in any organ. Circulating immune complexes damage the integrity of capillaries which trigger inflammatory events. A classic model of immune complex induced pathology is the Arthus reaction to the small pox vaccine. These large insoluble complexes with complement excite inflammatory responses in target tissues, i.e. smallpox vaccinations that cause residual skin eruptions then scarring. Clinical examples of this are rheumatic heart disease and glomerulonephritis.