The immunity of more complex organisms is distinguished by the development of a proper immune system consisting of specialized cells and tissues dedicated to controlling pathogens, toxins and malignancies. In the lower organisms these defenses are limited to innate immunity, which is established prior to the encounter with a pathogen and does not require education of the immune system to function. Innate immunity has been highly conserved throughout evolution, and entails specialized cells and molecules that recognize and target foreign structural components common to different classes of pathogens. For example, toll-like receptors are molecules expressed by cells to detect pathogen components, such as bacterial cell wall molecules and DNA, and are found in species from insects to vertebrates.
Specialized cells of the innate immune system include several types of phagocytic cells: neutrophils, eosinophils, monocytes, macrophages, and dendritic cells. These cells engulf and inactivate pathogens and substances that need to be cleared from the body. Toll-like receptors and other receptors on the surface of phagocytes may bind directly to substances to initiate phagocytosis, or the substances may first be coated with serum proteins called opsonins that are recognized by phagocytes and assist in phagocytosis. Opsonins include complement proteins of the innate immune system and antibodies, also called immunoglobulins, of the adaptive immune system. Other cells of the innate immune system, including basophils, mast cells and natural killer cells, are not phagocytic and use other mechanisms to kill pathogens. For example, natural killer cells recognize infected and malignant cells, and can kill these cells by signaling the cells to undergo apoptosis.
Innate immunity includes immediate (0-4 hours) and early induced responses (4-96 hours), which are activated by recognition of microbial-associated molecular patterns (MAMPs) and may involve inflammation, but do not generate immunological memory. Only if an infectious microorganism can evade these early lines of defense, does the adaptive immune system respond (> 96 hours) with generation of antigen-specific effector cells that target the specific pathogen, and memory cells that prevent re-infection with the same microorganism.
The two main routes of infection are mucosal surfaces (e.g airway, gastrointestinal, and genito-urinary) and skin. In spite of frequent exposures (inhaled, ingested, sexually transmitted, and traumatic respectively), infectious diseases are quite infrequent. Our epithelial surfaces provide effective mechanical, chemical, and microbiological barriers against most microorganisms. Tight junctions, longitudinal flow of air or fluid (breathing/coughing, peristalsis, urination), and bacterial trapping by mucus and movement of mucus by cilia provide a mechanical barrier. Fatty acids, pH, enzymes, and antimicrobial peptides provide an efficient chemical barrier. Normal microbiota (or nonpathogenic colonizing bacteria) that are associated with epithelial surfaces help compete with pathogenic microorganisms for nutrients and for attachment sites, and also produce antimicrobial substances (bacteriocins) providing microbiological barriers to infection. Furthermore, the microorganisms that do succeed in crossing an epithelial surface are efficiently removed by the innate mechanisms, such as phagocytic leukocytes that function in underlying tissue (e.g. macrophages in the lamina propria).
Innate immunity reference
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