The entry of bacteria, bacterial products or viruses into host cells may also occur via some of these receptors. Such binding evokes signaling systems and pathways involving, amongst others, mitogen-activated protein kinase (MAPK) (105), NF-κB (106), activator protein 1 (AP-1) (107), and various interferon regulatory factors (IRFs) [for a comprehensive review see (108)]. During infection, activation of these signaling pathways results in a cellular response that shares multiple cytoplasmic components, leading ultimately to the activation of a complex biomolecular network. Phosphorylation of relevant substrates (e.g., enzymes, microtubules, histones, and transcription factors) plays a crucial role in determining the host's cellular response (109). Viruses (110, 111), as well as bacteria (112), interact with and bind to HSPGs, using this proteoglycan as entry into the cell (see also Figure 1). LF acts as an important element in host defense mechanisms by binding to these receptors, but also binding to HSPG on cells, since these are locations where binding to bacteria and their cell wall products as well as viruses occur. The membrane-penetrating peptide HIV-tat, released from HIV-infected cells, also enters surrounding cells using HSPGs (86, 98). This binding capacity allows LF to compete with such molecules for receptor occupancy (113, 114), and therefore plays a vital role in host immunity (20). LF can also serve to prevent nephrotoxicity, e.g., of cisplatin (115).