Co-evolution of pathogenic bacteria and hosts has led to the development of an array of virulence genes (the virulome) and a set of mechanisms of defense which constitute the immune system. Mechanisms of innate or non-specific immunity involve mucosal epithelial surface barriers, antibacterial peptides and enzymes, defensive molecules (collectins, complement), and cells (macrophages, dendritic cells, polymorphonuclear neutrophil leukocytes, natural killer cells). The presence of bacterial products in host tissues induces a complex series of non-specific responses which constitute the inflammatory reaction. Inflammation is critical to the resolution of infection, the key process being the ingestion and killing of bacteria by phagocytes, but inflammation also causes tissue damage and contributes to the pathology. The innate immunity is the initial step in the host defense against pathogens, but some pathogenic bacteria with appropriate virulence factors can overcome the innate immunity mechanisms. In these cases, the goal of innate immunity is to contain the infectious process until specific immunity is developed. The specific immune responses are performed by lymphocytes: B cells, T helper (TH) cells, and cytotoxic T lymphocytes (CTL). Specific antibodies are produced by B cells, usually with the cooperation of TH2 cells. In general terms, extracellular bacteria can be killed by antibodies and complement proteins, or cleared by phagocytosis after opsonization by antibodies, and bacterial exotoxins are neutralized by their specific antibodies. In contrast, resistance to intracellular bacteria requires the induction of TH1 and CTL responses. In this review, we will focus on current knowledge about the defensive mechanisms against bacterial infections. We will discuss the innate immunity mechanisms, connections between innate and specific immunity, the key role of T helper 1 (TH1) and TH2 cells, and the antibody and cell-mediated responses.
Keywords: Innate immunity, specific immunity, pathogen-associated molecular patterns, pattern recognition receptors, Tolllike receptors, complement, antibodies, cell-mediated immunity, Immunity, Bacterial Infections, pathogenic bacteria, immune system, antibacterial peptides, enzymes, collectins, macrophages, dendritic cells, polymorphonuclear neutrophil leukocytes, natural killer cells, Inflammation, phagocytes, lymphocytes, B cells, T helper (TH) cells, cytotoxic T lymphocytes, TH2 cells, bacterial exotoxins, eukaryotes, endosymbionts, metazoans, chromosomal DNA, bacterio-phage DNA, plasmids, transposons, neutropenia, Lysozyme, Gram-positive bacteria, Gram-negative bacteria, LL-37, cathelicidin, Ficolins, Pentraxins, N-acetyl-glucosamine, C-reactive protein, MBL-associated serine proteases, Legionella pneumophila, Typhimurium, immunoglobulin, Superantigens, N. gonorrhoeae, Chlamydia trachomatis
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