Any factor that interferes with these defenses, however, can promote serious infectious disease. The efficiency of a persons surface defenses, phagocytic defenses, and immune system plays a critical role in preventing transient colonization from developing into severe disease.
Host resistance is largely due to successive lines of defense that any potential invader must penetrate or evade before it can cause serious disease. These defenses are:
1. The innate immunity mechanisms also known as ‘nonspecific factors’, which include biological properties of the whole host organism that do not depend on the origin of the foreign invader (the mechanical, chemical, microbial defenses on the body's surfaces; immunobiological humoral factors and phagocytic cells that engulf and destroy organisms that penetrate the surface defenses).
2. Specific factors, that are specific immune mechanisms that aid in the destruction of foreign microorganisms and toxins. The immune system "remembers" an invader and prepares the body to resist subsequent encounters with the same pathogen or toxic substance.
The main factors of host defenses (=innate immunity mechanisms) are as follows.
· MECHANICAL DEFENSES
Surface Defenses. Most infectious diseases are initiated by the microbes contact with the surfaces of the body. The body is surrounded by its most essential line of defense—the skin and the mucous membranes that line the respiratory, alimentary, and genitourinary tracts and the conjunctiva. These barriers keep the vast majority of microorganisms out of the body's more vulnerable interior susceptible to infection than. Intactskin is rarely penetrated by bacteria, and viruses cannot infect the outer layer of dead epidermal cells. The living surface cells of mucous membranes are vulnerable to attack bythemicrobes. The respiratory tract is particularly vulnerable, because it is exposed to more of the environment than the digestive and genitourinary tracts.
NOTE: The average person inhales 50,000 liters of air each day, and the air in each breath may contain a million microorganisms. It is not surprising that 80 percent of all infectious diseases are acquired through the respiratory tract.
Cuts, puncture wounds, and other traumas breach surface barriers and may result in minor local infections or serious systemic disease. Weakened skin or mucous membranes are more susceptible to these traumatic injuries. Malnutrition may encourage the development of infectious disease by weakening the skin. Similarly, respiratory mucous membranes can be weakened by the noxious effects of tobacco smoke, air pollutants, and several gaseous anesthetics.
Dust and airborne particles often carry microbes that cause respiratory infections, The airways of the respiratory tract are protected by special mucous membranes called the ciliatedmucosa. These cells secrete a layer of mucus that provides an additional mechanical safeguard. The sticky mucus efficiently traps particles that get into the respiratory tract. The mucus-secreting cells possess cilia that move the mucus blanket about I inch every minute, sweeping trapped microbes harmlessly into the throat; when they are swallowed, they are killed by the acid and enzymes of the stomach. The mucus also helps prevent viral infections by blocking the attachment of viruses to their cellular absorption sites. Airborne particles are also moved out of the respiratory tract by two mechanical reflexes, the cough and the sneeze, that forcefully expel particles. Additional mechanical protection for the lungs is provided by the epiglottis, a flap of tissue that covers the opening leading to the lower respiratory tract (trachea, bronchi, and lungs) during swallowing.
Other mechanisms provide additional mechanical protection. Inhaled air is tumbled as it is drawn through a forest of nasal hairs. This turbulence increases the probability that particles will come into contact with and be trapped by the nasal mucus. Body hair also reduces the number of microbes settling onto the skin. The flow of urine and tears flushes microbes from the urinary tract and eyes.
The sloughing of the dead surface layer of our skin effectively discharges many microbes that have attached to these epithelial cells. A similar mechanism protects the gastrointestinal tract, which sloughs about a half a kilogram (about I pound) of its own cells every 2 days. (The entire intestinal epithelium is replaced every 36 hours.)
Microbes are usually prevented from passing to the brain and spinal cord by the bloodbrainbarrier, a system of thickened capillaries that lack pores, thereby physically protecting the central nervous system from microbes in the blood.
· CHEMICAL DEFENSES
Various body fluids contain substances that contribute to our resistance to potential pathogens. Mucous secretions, tears, sweat, and saliva all contain lysozyme, an enzyme that destroys the cell walls of many gram-positive bacteria. Fatty acids in sweat and earwax have bacteristatic and fungistatic properties. The stomach contains a high concentration of hydrochloricacid, which rapidly kills most microbes, and in the small intestines bile destroys many of the survivors. The high acidity of the adult vagina protects its membranous surfaces against colonization by many types of pathogens. The digestive system produces mucin, a glycoprotein that coats microbes and prevents their attachment to intestinal epithelium. Several antimicrobial peptides and proteins have been isolated from cells of the skin and gastrointestinal tract of humans or animals.
Another protective factor released into the blood is beta-lysin. This protein attacks bacterial membranes.
Transferrins are iron-binding proteins that reduce the level of free iron in the blood. Lactoferrins perform the same function in milk, respiratory, intestinal, and genital secretions. The presence of free iron is necessary for the growth of most pathogenic microbes. Iron is also required by human cells, but it can be extracted from the iron-binding proteins as it is needed, whereas most bacteria cannot obtain protein-bound iron. Reducing free iron levels in the body therefore helps protect against microbial proliferation, This strategy is enhanced by mononuclear phagocytes that remove even more free iron from the blood during infection.
· IMMUNOBIOLOGICAL DEFENSES (humoral and cellular factors)
The main nonspecific humoral factors are as follows.
Interferons are a group of soluble proteins produced by host cells during infection by viruses and some other microbes. Uninfected cells in the area that are exposed to virus-induced interferon become resistant to viral infection. Interferons also stimulate the activity of our phagocytic and immunologic defenses.
Interleukin-I is a fever-inducing protein safely isolated in vacuoles of phagocytic cells, where it has no effect on the host. When stimulated by certain foreign substances such as bacterial endotoxin, phagocytes discharge the contents of these vacuoles, which circulate to the hypothalamic region of the brain and trigger an elevated body temperature.
NOTE: Although fever can itself damage human cells, it has several protective effects: it raises temperatures above that which is optimal for growth of some pathogens; it accelerates the mobilization and protective efficiency of the body's defenses; it stimulates lymphocyte activity in the immune response; and it increases the rate of iron storage, reducing iron availability to pathogens.
Complement is a complex group of serum proteins that act in concert with the immune system or with other serum proteins to facilitate bacterial lysis and phagocytosis. In the classical pathway of complement activation, an antibody-pathogen complex activates the complement system. The alternative pathway occurs in the absence of an immune response but requires a blood protein (called properdin) and other factors to combine with surface molecules of the bacterial cell wall, launching the cascade of events that has the same outcome as the classical pathway— activation of phagocytes and damage to the bacterial membrane. Either pathway can eliminate the foreign cell that triggered the response. (The immune-mediated complement system is discussed in details in LESSON 13).
The main cellular factors of nonspecific host defense are phagocytic cells, natural killers (NK-cells).
Phagocytic Defenses. When infectious agents penetrate the surface defenses of the body, they are usually devoured by leukocytes (white blood cells) or other phagocytic cells. These cells engulf and destroy foreign particles by phagocytosis.
Lysosomes are characteristic of a group of white blood cells called granulocytes. Of the three types of granulocytes found in normal blood, neutrophils are the most abundant. These cells are also called polymorphonuclear leukocytes (PMNs) because of their irregular multilobed nuclei. Neutrophils are actively phagocytic and are usually the first protective cells to arrive at the site of trauma or infection. They do their work rapidly and are fairly short-lived. Two other types of granulocytes—eosinophils and basophils—are not phagocytic but contribute to defense by producing substances that help regulate the host response to injury or infection.
The other group of leukocytes is made up of agranulocytes, so called because they have fewer dark-staining lysosomes. Nonetheless, lysosomes are present in numbers great enough to effect the destruction of engulfed particles during phagocytosis. Agranulocytes include lymphocytes and monocytes. Monocytes have a single large oval or horseshoe-shaped nucleus. Circulating monocytes migrate into tissue, where they enlarge and differentiate into actively phagocytic macrophages. Most macrophages are attached to tissues of the liver, spleen, lymph nodes, and bone marrow and to the walls of blood and lymph vessels. They engulf foreign particles and debris from blood and lymph that flow through these regions. These fixed macrophages therefore function as filters to clean debris from the blood and trap potential pathogens. Wandering macrophages migrate to the lungs, spleen, and other sites where microbes are likely to be encountered. Macrophages also travel to areas of trauma or infection to participate in the body's overall protective response. In addition, macrophages play an essential role in helping the immune system recognize antigens and respond to these foreign substances.
The monocytes and macrophages constitute the body's mononuclear phagocytic system (part of the reticuloendothelial system, which consists of all phagocytic cells and tissues to which fixed macrophages attach).
Most phagocytic cells in the circulation contain large numbers of lysosomes in their cytoplasm. Lysosomes contain a number of digestiveenzymes — peptidoglycanases, lipases, and proteases —plus chemicals that oxidize and destroy engulfed cells. As a result of phagocytosis, an invading microbe is trapped within an intracytoplasmic vacuole called a phagosome. Lysosomes migrate to the vacuole membrane and discharge their contents into the phagosome. The phenomenon, called degranulation, occurs by fusion of lysosomal and vacuole membranes, and the phagocytic vacuole grows into a larger membranous sac called a phagolysosome. The chemicals in the phagolysosome kill and digest the engulfed microbe. Human tissue can also be damaged by the lysosomal contents. The vacuole membrane, however, usually prevents tissue injury by safely housing the discharged chemicals, preventing their sudden release into the cell’s cytoplasm or into the surrounding tissue. Phagocytes also kill microbes by converting oxygen to toxic products such as superoxide and hydrogen peroxide. Increased respiration, called the respiratory burst, initiated during the early stages of phagocytosis generates these toxic oxidants.
Outcome of phagocytosis. All types of phagocytic cells (granulocytes, macrophages in blood, and fixed macrophages of the reticuloendothelial system) may kill ingested microbes (complete phagocytosis), or may permit their prolonged survival or even their intracellularmultiplication (incompleted phagocytosis). The outcome of pagocytosis is determined by a complex factors, including the specific nature of the microbe as well as the genetic and functional makeup and the preconditioning of the phagocytic cells.
• INFLAMMATION
Phagocytosis is an essential component of inflammation —a concentrated protective response to infection or tissue injury in which the body attempts to localize and destroy infectious microorganisms and repair damaged tissues. The cells of injured tissues release their contents and increase acidity at the site of injury. This activates local enzymes, called kinins, that increase the permeability of capillaries in the immediate region, promoting the escape of plasma from the vessels into the local tissues. Kinins also promote the release of histamine, prostaglandins, and other substances from surrounding cells. These chemicals further increase vascular permeability, so even more plasma accumulates, bringing complement, antibodies, and other protective chemicals to the site of injury. They also stimulate chemotaxis, the migration of white blood cells toward the source of injury or infection. These responses characteristically elicit four symptoms at the site of injury: swelling, pain, redness, and heat. The pain associated with inflammation is also due to the release of irritant chemicals that bind to nerve endings.
Inflammation protects the host by promoting phagocytosis of microbes, by localizing infection (microbes are often walled off in capsules of clotted plasma), and by producing an exudate (pus), which in some cases allows direct drainage of microbes and dead tissue out of the host's body (as from a pimple, for example). The symptoms of inflammation also alert us to the presence of infection, increasing the likelihood of early diagnoses and treatment.
• MICROBIAL DEFENSES
The body's resident or normal bacterial flora provide another important line of defense. The normal flora continually compete with potentially pathogenic microbes for the limited nutrients and space on the body's epithelial surfaces). Because the normal flora is already established, these beneficial microorganisms usually win the competition and prevent either initial infection or the unrestricted growth of pathogenic bacteria and fungi. Resident bacteria also have the ability to alter their local environment so that it is generally unfavorable for the growth of most pathogens.
PRACTICAL WORK
1. Determination of Klebsiella pneumoniae virulence.
Bacterial virulence (the degree of pathogenicity) may be determined by biological examination. We usually use mice, rats, rabbits, guinea pigs, and other animals for this purpose. The experimental animals can be infected via different routes: intramuscularly, intravenously, pernasaly, intraperitonealy, etc.
To determine K.pneumoniae virulence the following experiment was performed.
Groups of experimental mice have been infected intraperitoneally with different doses of K.pneumoniae suspension. (The series of ten- fold bacterial dilutions have been used). Mice’ survival has been noted after an appropriate period of time. Then the internal organs of dead mice (spleen, liver, etc.) were examined microscopically and bacteriologically to determine the presence of K.pneumoniae. (The presence of microbe in the internal organs is a result of generalized infection, so the causative agent responsible for the mice’ death is detected).
The results are shown in Table 11-2.
TASK. Determine the LD50 for K.pneumoniae tested. To make calculations use the formula:
| lg LD50 = lg DN – δ (∑ Li – 0,5) |
DN - the largest dosage used for infecting animals
δ – Logarithm of ratio of the second dosage to the first one
Li – the ratio of the dead mice number to the total number of infected mice (in each group)
∑Li – the sum of Li for all groups
0,5 – coefficient
Table 11-2