Natural barriers and the immune system defend the body against organisms that can cause infection. (See also Lines of Defense.)
The immune system uses white blood cells and antibodies to identify and eliminate organisms that get through the body’s natural barriers.
(See also Overview of Infectious Disease.)
Natural Barriers Against Infection
Usually, the skin
Mucous membranes,
The airways
The digestive tract has a series of effective barriers, including stomach acid, pancreatic enzymes, bile
The urinary tract also has several effective barriers. The bladder is protected by the urethra, the tube that drains urine from the body. In males, the urethra is long enough that bacteria are seldom able to pass through it to reach the bladder, unless the bacteria are unintentionally placed there by catheters or surgical instruments. In females, the urethra is shorter, occasionally allowing external bacteria to pass into the bladder. In both sexes, when the bladder empties, it flushes out any bacteria that reach it.
The vagina is normally acidic. The acidity of the vagina prevents harmful bacteria from growing and helps maintain the number of protective bacteria.
The Blood
The body also defends against infection by increasing the number of certain types of white blood cells (neutrophils and monocytes
The number of eosinophils, another type of white blood cell, increases in allergic reactions and many parasitic infections, but usually not in bacterial infections.
However, certain infections, such as typhoid fever, viral infections, and bacterial infections that overwhelm the immune system, can lead to a decrease in the white blood cell count.
Inflammation
Wall off the area
Attack and kill any invaders
Dispose of dead and damaged tissue
Begin the process of repair
Many of the substances produced during inflammation stimulate the nerves, causing pain. Reactions to the substances released during inflammation include the chills, fever, and muscle aches that commonly accompany infection.
Immune Response
When an infection develops, the immune system see Acquired Immunity). Examples are
Killer T cells (a type of white blood cell) that can recognize and kill the invading microorganism
Antibodies that target the specific invading microorganism
How well the immune system defends the body against each microorganism depends partly on a person's genetic make-up.
Fever
Body temperature increases as a protective response to infection and injury. An elevated body temperature (fever) enhances the body’s defense mechanisms, although it can cause discomfort.
A part of the brain called the hypothalamus controls body temperature. Fever results from an actual resetting of the hypothalamus's thermostat. The body raises its temperature to a higher level by moving (shunting) blood from the skin surface to the interior of the body, thus reducing heat loss. Shivering (chills) may occur to increase heat production through muscle contraction. The body's efforts to conserve and produce heat continue until blood reaches the hypothalamus at the new, higher temperature. The new, higher temperature is then maintained. Later, when the thermostat is reset to its normal level, the body eliminates excess heat through sweating and shunting of blood to the skin.
Certain people (such as the very old, the very young, and people with an alcohol use disorder) are less able to generate a fever. These people may experience a drop in temperature in response to severe infection.
