Several different kinds of molecules that recognize microbes and promote innate responses exist in soluble form in the blood and extracellular fluids. These molecules provide early defense against pathogens that are present outside host cells at some part of their life cycle. The soluble effector molecules function in two major ways.
• By binding to microbes, they act as opsonins and enhance the ability of macrophages, neutrophils, and dendritic cells to phagocytose the microbes. This is because the phagocytic cells express membrane receptors specific for the opsonins, and these receptors can efficiently mediate the internalization of the complex of opsonin and bound microbe.
• After binding to microbes, soluble mediators of innate immunity promote inflammatory responses that bring more phagocytes to sites of infections, and they may also directly kill microbes.
The soluble effector molecules are sometimes called the humoral branch of innate immunity, analogous to the humoral branch of adaptive immunity mediated by antibodies. The major components of the humoral innate immune system are natural antibodies, the complement system, collectins, pentraxins, and ficolins. We will next describe the major features and functions of these components of innate immunity.
group antibodies, another example of natural antibodies, recognize certain glycolipids (blood group antigens) expressed on the surface of many cell types, including blood cells. Blood group antigens and antibodies are important for transplantation but not for host defense and are discussed in Chapter 16.
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