However, if the innate immune system exceeds its capacity or its defensive function becomes limited, it engages the adaptive immune system, activating specific T and B cells for pathogen clearance ( 65). Acute inflammation can be caused by bacterial or viral infection ( e.g., as in acute respiratory distress syndrome (ARDS), tissue necrosis ( e.g., as in acute myocardial infarction), trauma, radiation, burns, or by any foreign body present in tissue. These mediators, including histamine, platelet-activating factors (PAFs), bradykinin, and thrombin, increases vascular permeability followed by fluid accumulation (edema) and leukocyte extravasation. Most of the early vascular changes observed in acute inflammation are due to inflammatory mediators that are released by inflammatory cells at the site of injury ( 65).

Neutrophils are the first to adhere to endothelial cells, and they begin to migrate across the vascular wall at the site of infection to engulf the invading pathogens and also secrete vasoactive and pro-inflammatory mediators ( 222). Within a short period of activation of the innate immune system, the acute inflammatory response is started by immune cells enabling secretion of various cytokines and chemokines in order to recruit immune cells to the site of infection. PRRs are expressed by a variety of immune cells, including macrophages, monocytes, dendritic cells (DCs), and neutrophils, which enables early detection of pathogens ( 409). These receptors recognize conserved domains known as pathogen-associated molecular patterns, including flagellin, sugars, and the cell wall components of various microbes such as peptidoglycan and lipopolysaccharide (LPS) as well as danger-associated molecular patterns that are released by injured cells such as mammalian dsDNA and uric acid crystals. The family of PRRs includes both membrane-bound receptors such as Toll like receptors (TLRs) and c-type lectin receptors, as well as cytoplasmic nod-like receptors (NLR). The innate immune system recognizes a wide range of pathogens such as viruses, bacteria, and fungi, by germline-encoded receptors known as pattern-recognition receptors (PRRs) ( 409). The four cardinal signs of localized acute inflammation described almost 2000 years ago by the Roman physician Celsus are as follows: Calor heat, Rubor redness, tumor swelling, and Dolor pain, leading to Functiolaesa loss (or impairment) of function. While this is considered advantageous to the organism, it can if unchecked also lead to inflammation and disease. The innate immune system on encountering pathogens elicits the acute inflammatory response that is accompanied by systemic vasodilation, vascular leakage, and leukocyte emigration ( 65). There are two complementary immune systems in vertebrates that recognize and eliminate pathogens: innate immune system and adaptive immune system ( 65). The immune system of vertebrates has evolved this survival strategy property to facilitate tissue repair. I nflammation is a defensive immune response that is conferred by the host against foreign pathogens.

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