Immunity is the body's ability to defend itself against infectious organisms, foreign cells, and even body cells that have gone awry, such as cancer cells. Immunity includes nonspecific and specific defenses.
Nonspecific Defenses
Three nonspecific defenses are useful against all types of pathogens: barriers to entry, the inflammatory reaction, and protective proteins.
Barriers to Entry
The skin and the mucous membrane lining the respiratory and digestive tracts are mechanical barriers to entry by pathogens, which are disease-causing agents like bacteria and viruses. Secretions of the oil glands in the skin contain chemicals that weaken or kill bacteria. The respiratory tract is lined with ciliated cells that sweep mucus and trapped particles into the throat, where they can be expectorated or swallowed. In addition, the stomach has an acidic pH that inhibits the growth of many types of bacteria. A mix ofbacteria that normally reside in the intestine and other organs, such as the vagina, prevent pathogens from colonizing vulnerable tissues.
Inflammatory Reaction
When the skin is broken due to a minor injury, a series of events occurs that is known as the inflammatory reaction because of the reddening and swelling at the site of the injury.
Following an injury, capillaries and several tissue cells are apt to rupture and to release histamine, a molecule that causes a capillary to dilate and to become more permeable. The enlarged capillaries cause the skin to redden, and their increased permeability allows proteins and fluids to escape, resulting in swelling. The increased fluid pressure in the injured region stimulates free nerve endings, resulting in pain.
Any break in the skin allows pathogens to enter the body. Neutrophils and monocytes are amoeboid and can squeeze through capillary walls to enter tissue fluid, where they carry on phagocytosis (fag" o-si-to'sis), also called cell eating. When a neutrophil phagocytizes a bacterium, an intracellular vacuole is formed. The engulfed bacterium is destroyed by hydrolytic enzymes when the vacuole combines with one of the neutrophil's granules.
Monocytes differentiate into macro phages (mak'rofaj"ez)-large, phagocytic cells that can devour dozens of invaders and still survive. Some organs, like the liver, kidney, spleen, and brain, have resident macrophages that routinely act as scavengers, devouring old blood cells, bits of dead tissue, and other debris. Macrophages are also capable of bringing about an explosive increase in the number of leukocytes by liberating a growth factor that stimulates the production and release of white blood cells, usually neutrophils from bone marrow.
As an infection is being overcome, some neutrophils die. These neutrophils, along with dead tissue, cells, bacteria, and living white blood cells, form pus, a thick, yellowish fluid. The presence of pus indicates that the body is trying to overcome the infection.
The inflammatory reaction is a "call to arms"; It marshals phagocytic \Nhite blood cells to the site of bacterial invasion.
Protective Proteins
The complement system, often simply called complement, consists of a number of plasma proteins designated by the letter C and a number or letter. Once a complement protein is activated, it, in turn, activates another protein in a set series of reactions. Only a small amount of activated protein is needed because of a cascade response: Each protein in the series is capable of activating many proteins next in line.
Complement is activated when pathogens enter the body. One series of reactions is complete when complement proteins form holes in bacterial cell walls and membranes. These holes allow fluids and salt to enter the bacterial cell until it bursts.
Complement also activates chemicals that attract phagocytes to the site and induce inflammation. Complement completes certain immune responses, which accounts for its name. For example, some proteins bind to the surface of pathogens already coated with antibodies, which ensures that the pathogens will be phagocytized.
When viruses infect a tissue cell, the infected cell produces and secretes interferon. Interferon binds to receptors on the surface of noninfected cells, causing them to prepare for possible attack by producing substances that interfere with viral replication. Interferon is specific to the species; therefore, only human interferon can be used in humans. Formerly, collecting enough interferon for clinical and research purposes was difficult, but interferon is now a product of biotechnology and is produced by genetically engineered bacteria.
Nonspecific defenses against pathogens include barriers to entry, the inflammatory reaction, and protective proteins (the complement system).
