3. Macroorganism (Nonspesific factors of host defense)

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. Intact skin 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 by the microbes. 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 ciliated mucosa. 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 bloodbrain barrier, a system of thickened capillaries that lack pores, thereby physically protecting the central nervous system from microbes in the blood.