Innate immunity

• Microbes attempting to get into the body must first get past the skin and mucous membranes, which are rich in scavenger cells and IgA antibodies.
• The skin and mucosa are physical and chemical barriers, with the skin also being a biological barrier due to the presence of normal flora.
• If an organism is able to pass the first line of defense mechanisms, it encounters the 2nd line of defense.
• Elimination can occur by direct lymphocytic destruction of the antigens or by formation of specialized proteins called antibodies.
• Cells in the immune response include phagocytes, which are neutrophils and monocytes/macrophages, and natural killer (NK) cells.
• NK cells respond to IL-12 produced by macrophages and secrete IFN-γ, which activates the macrophages to kill phagocytosed microbes.
• The 2nd line of defense consists of immunologic mechanisms in which lymphocytes recognize the presence of foreign agents or substances: antigens and eliminate them.
• Either destroy the antigen or target it for destruction by other cells.
• Microorganisms and tumor cells can be internalized by phagocytosis and degraded by NK cells.
• NK cells kill host cells infected by intracellular microbes, thus eliminating reservoirs of infection.
• The complement system is a component of the immune response.
• The skin is the first line of immunological defense, made up of the epidermis, outer layer, and dermis.
• The epidermis is comprised of tightly packed cells rich in keratin, which impedes water from entering the skin and is slightly acidic which inhibits bacterial growth.
• The dermis contains the sebaceous glands from which hairs grow, and from which sebum is secreted, which inhibits the growth of some type of bacteria and fungi.
• Mucus secreted by the respiratory and gastrointestinal tract serves to protect the host by trapping many microorganisms.
• The flushing action of saliva, tears, and urine mechanically expels pathogens.
• Pathogens are mechanically expelled from the lungs through a process known as ciliary action.
• Saliva and tears contain antibacterial enzymes, such as lysozyme, which destroy bacterial cell walls.
• Vaginal secretions become slightly acidic following menarche, while semen contains spermine and zinc which repels some pathogens.
• Mother’s milk contains the powerful enzyme lactoperoxidase.
• Gastric acid, produced in the stomach, is a powerful defense against ingested pathogens.
• Epithelia at the portals of entry of microbes provide physical barriers, produce antimicrobial substances, and harbor intraepithelial lymphocytes that are believed to kill microbes and infected cells.
• Normal flora are the microbes, mostly bacteria, that live in and on the body with, usually, no harmful effects to us.
• We have about 10^13 cells in our bodies and 10^14 bacteria, most of which live in the large intestine.
• There are 10^3 –10^4 microbes per cm^2 on the skin (Staphylococcus aureus, Staphylococcus epidermidis, diphtheroids, streptococci, Candida, etc.).
• Various bacteria live in the nose and mouth.
• Phagocytes are immune cells that engulf, or eat, pathogens or particles.
• The cytoplasm of neutrophils is abundant with a few nonspecific granules and a full complement of rose-violet specific granules.
• Monocytes form from the stem cell in the bone marrow, circulate in the blood as monocytes, and are resident in all tissues of the body as macrophages.
• The lysosome contains enzymes and acids that kill and digest the particle or organism.
• Monocytes have the ability to do phagocytosis.
• Monocytes are large mononuclear phagocytes that are the immature stage of the macrophages.
• The total population of lymphocytes can recognize over a billion different antigens, in contrast all the receptors of innate immunity probably recognize less than a thousand microbial patterns.
• Pattern recognition receptors have evolved as a protective adaptation to potentially harmful microbes.
• White cell counts of 100000 X 10^6/L (normal WBC=4000 to 10000 X 10^6/L)
• Neutrophils are non-dividing short lived cells with a multilobed nucleus that migrate through tissues to destroy microbes and respond to inflammatory stimuli.
• Innate immune system receptors are nonclonally distributed, identical receptors are expressed on all the cells of a particular type, such as macrophages.
• The neutrophil accomplishes this by moving to the area of inflammation or infection, phagocytozing (ingesting) the foreign material, and killing and digesting the material.
• Polymorphonuclear Leukocytes (neutrophils) are the mature phagocytes.
• Once inside the cell, the invading pathogen is contained inside a vacuole called fagosome which merges with another type of vacuole called a lysosome.