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Fig. 2. | BMC Biology

Fig. 2.

From: Tissue macrophages: heterogeneity and functions

Fig. 2.

Selected plasma membrane receptors that mediate macrophage recognition of microbial and host ligands. Macrophages are able to express a large repertoire of membrane receptors implicated in the recognition and uptake of foreign and modified self ligands, some of which are illustrated here. These receptors incorporate a range of structural domains, illustrated schematically; they serve as useful marker antigens for immunocytochemistry and FACS analysis (e.g. F4/80, CD68, CSF1 receptor, Mer-TK, CD64). They function as opsonic (antibody and or complement coated particles to enhance uptake via Fc and complement receptors) or non-opsonic, carbohydrate-binding lectins and scavenger receptors. The phagocytic receptors mediate clearance of microbes (e.g. MARCO), apoptotic cells (for example CD36, SR-A, TIM4) and circulating ligands; for example, CCR2 and CX3CR1 are receptors for the monocyte/macrophage chemokines MCP-1 and fractalkine, respectively, for growth promoting and regulatory cytokines, for example, CSF-1 and angiopoietins, (Tie-2), and CD163 for clearance of injurious haptoglobin–haemoglobin complexes. Toll-like receptor-4 and CD14 react with bacterial membrane components such as lipopolysaccharide (LPS) to induce pro-inflammatory signalling; Dectin-1 recognises fungi through beta glucan in their wall, activating a range of innate immunological responses. Siglec-1 (CD169), a receptor for sialic acid terminal glycoconjugates, mediates adhesion of host cells and microbes, whereas CD206, a receptor for clearance of Mannosyl terminal glycoproteins, is a prototypical marker of M2 activation. The scavenger receptor SR-A internalises polyanionic ligands such as modified lipoproteins, as well as selected microbes, whereas CD36 mediates adhesion and M2-induced macrophage fusion and giant cell formation. TREM-2 mutations have been implicated in neurodegeneration and osteoclast dysfunction (see [25] and text for further details)

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