Dermis: the skin’s inner, thicker portion, is composed of connective tissue.
Epidermis: the outer, thinner portion, is in direct contact with the external environment.
The top layer of epidermal cells is dead and contains a protective protein called keratin.
Mucous membranes also consist of an epithelial layer and an underlying connective tissue layer. They are an important component of the first line of defense.
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The intact skin is the human body’s largest organ in terms of surface area and tracts
Mucus: a slightly viscous (thick) glycoprotein produced by goblet cells of a mucous membrane
Lacrimal apparatus: a group of structures that manufactures and drains tears
Microorganisms are also prevented from entering the lower respiratory tract by a small lid of cartilage called the epiglottis, which covers the larynx (voicebox) during swallowing
The external ear canal contains hairs and earwax (cerumen), which help prevent microbes, dust, insects, and water from entering the ear
The cleansing of the urethra by the flow of urine is another physical factor that prevents microbial colonization in the genitourinary tract.
Vaginal secretions likewise move microorganisms out of the female body.
Peristalsis, defecation, vomiting, and diarrhea also expel microbes.
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Sebaceous (oil) glands of the skin produce an oily substance called sebum that prevents hair from drying and becoming brittle.
The sweat glands of the skin produce perspiration, which helps maintain body temperature, eliminates certain wastes, and flushes microorganisms from the surface of the skin.
Earwax, besides serving as a physical barrier, also functions as a chemical protectant.
Saliva contains not only the enzyme salivary amylase that digests starch, but also a number of substances that inhibit microbial growth.
Gastric juice is produced by the glands of the stomach. It is a mixture of hydrochloric acid, enzymes, and mucus.
Vaginal secretions play a role in antibacterial activity in two ways.
Urine, in addition to containing lysozyme, has an acidic pH (average 6) that inhibits microbes.
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In commensalism, one organism uses the body of a larger organism as its physical environment and may make use of the body to obtain nutrients.
Probiotics are live microbial cultures applied to or ingested that are intended to exert a beneficial effect.
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==Blood consists of fluid, called plasma, and formed elements==—that is, cells and cell fragments suspended in plasma. The formed elements include erythrocytes, or red blood cells (RBCs); leukocytes, or white blood cells (WBCs); and platelets.
Formed elements are created in red bone marrow by stem cells in a process called hematopoiesis
Granulocytes owe their name to the presence of large granules in their cytoplasm that can be seen with a light microscope after staining.
Agranulocytes also have granules in their cytoplasm, but the granules are not visible under the light microscope after staining.
Monocytes are not actively phagocytic until they leave circulating blood, enter body tissues, and mature into macrophages.
Dendritic cells are also believed to be derived from the same precursor cells as monocytes.
Lymphocytes include natural killer cells, T cells, and B cells
Natural killer (NK) cells are found in blood and in the spleen, lymph nodes, and red bone marrow.
Some granules contain a protein called perforin, which inserts into the plasma membrane of the target cell and creates channels (perforations) in the membrane.
T cells and B cells are not usually phagocytic but play a key role in adaptive immunity
Leukocyte increase or decrease can be detected by a differential white blood cell count, which is a calculation of the percentage of each kind of white cell in a sample of 100 white blood cells.
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The lymphatic system consists of a fluid called lymph, vessels called lymphatic vessels, a number of structures and organs containing lymphoid tissue, and red bone marrow, where stem cells develop into blood cells, including lymphocytes
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Phagocytosis (from Greek words meaning eat and cell) is the ingestion of a microorganism or other substance by a cell.
Other macrophages are motile and are called free (wandering) macrophages, which roam the tissues and gather at sites of infection or inflammation.
The Mechanism of Phagocytosis
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Damage to the body’s tissues triggers a local defensive response called inflammation, another component of the second line of defense.
Dilation of blood vessels, called vasodilation, is responsible for the redness (erythema) and heat associated with inflammation.
Increased permeability permits defensive substances normally retained in the blood to pass through the walls of the blood vessels and enter the injured area.
Kinins are another group of substances that cause vasodilation and increased permeability of blood vessels.
Prostaglandins, substances released by damaged cells, intensify the effects of histamine and kinins and help phagocytes move through capillary walls.
==A tissue is repaired when its stroma or parenchyma produces new cells.==
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One of the most important is fever, an abnormally high body temperature, a third component of the second line of defense.
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The complement system consists of over 30 proteins produced by the liver that circulate in blood serum and within tissues throughout the body.
The cascade of complement proteins that occurs during an infection is called complement activation.
Classical Pathway
Alternate Pathway
The Lectin Pathway