ABSTRACT
In order to recognize and remove the multitude of invading microbes, the innate immune system possesses a number of cellular or soluble receptor molecules that not only act as a first line of defense, but also are important to the development of an adaptive immune response. Unlike the antigen-specific receptors expressed by lymphocytes (Section C1), which recognize specific epitopes (Section A4), these PRRs have evolved to recognize the pathogen-associated molecular patterns (PAMPs) of a wide variety of microbes and to facilitate their removal. The cellular PRRs include the Toll-like receptors, NOD-like receptors, mannose receptors, scavenger receptors, and CD14, all expressed by macrophages (Figure 1). Soluble PRRs include MBL (a collectin), ficolins, and galectins. A summary of key features of the PRRs is shown in Table 1. These receptors sense the “danger” signals given by invading microbes, but also are able to recognize some self components and are therefore important in recognition and clearance of dead (apoptotic) and dying cells.
