The body’s defense mechanisms and types of infection
How does the body defend itself?
After taking a look at the infection cycle, you may be asking yourself, why aren’t we sick all the time? There are so many ways for infectious agents to impact our health. Well, the human body has a number of ways to defend itself, and in the following sections, you will see how that works.
First line of defense
The first line of defense against pathogenic invasion is made up of the body’s physical and chemical barriers. An intact integumentary system, or skin, serves as a physical barrier to infection.
Other anatomic defense mechanisms include the following:
- Tears
- Cilia
- Mucous membranes
- pH of body fluids
- Defecation and vomiting
Second line of defense
The body’s second line of defense is nonspecific chemical and cellular responses. Phagocytic cells engulf and destroy the microbes. Inflammation occurs, bringing white blood cells to the area. The inflammatory response produces swelling, redness, heat, and pain.
The inflammatory response starts when the body experiences trauma or is exposed to pathogens. To defend itself, the body starts certain responses to destroy and remove pathogenic organisms and their by-products. If this is not possible, it will limit the damage caused by the invading pathogen.
This process results in the four classic symptoms of inflammation:
- Erythema (redness)
- Edema (swelling)
- Pain
- Heat
When the body is exposed to an infectious agent or a foreign substance, cellular damage occurs at the site. Inflammation mediators (i.e., histamine, prostaglandins, and kinins) are released, causing three different responses at the cellular level. All three actions are designed to increase the number of white blood cells (WBCs) at the injury site.
First, blood vessels at the site dilate, causing an increase in local blood flow, which results in redness (inflammation) and heat. Blood vessel walls become more permeable, which allows for the movement of WBCs through the vessel wall to the site. The WBCs begin to form a fibrous capsule around the site to protect surrounding cells from damage or infection.
Blood plasma also filters out of the more permeable vessel walls, resulting in edema (swelling), which puts pressure on the nerves and causes pain. Finally, chemotaxis, or the release of chemical agents, occurs, attracting even more WBCs to the site. The increased number of WBCs at the site results in phagocytosis.
Destroyed pathogens, cells, and WBCs collect in the area and form a thick, white substance called pus. If the pathogenic invasion is too great for localized control, the infection may collect in the body’s lymph nodes, where more WBCs are present to help fight the battle. This causes swollen glands, or lymphadenopathy.
If the body is too weak or the number of pathogens is too great, the infection may spread to the bloodstream. A systemic infection, called septicemia or blood poisoning, may occur, which ultimately could affect the entire body. Another term for septicemia is pyemia. Without appropriate medical intervention, death can occur from a systemic infection.
Third line of defense
The third line of defense involves specific immunity. The immune system responds by producing antibodies specifically designed to combat the presence of a foreign substance, or antigen. This process is called humoral immunity and is the responsibility of the body’s B cells.
Another immune system response involves T cells. This is called cell-mediated immunity and causes the destruction of pathogenic cells at the site of invasion or infection.
Types of infection
If an infectious agent does make it through and causes an infection, that infection can be classified in several different ways. The following sections discuss the different types of infections.
Acute infection
An acute infection has a rapid onset of symptoms but lasts a relatively short time. This type of infection will go through the following stages:
- Incubation: The period of time between exposure to the pathogen and the appearance of the first symptoms. It can be from a few days to a few months. The incubation period for mononucleosis is 4 to 6 weeks.
- Prodromal: The short period of time when the first symptoms appear. With mononucleosis, the patient may have 1 to 2 weeks of fatigue, malaise, myalgia, and a low-grade fever.
- Acute: The disease is at its peak, and symptoms are fully developed. With mononucleosis, pharyngitis, tonsillitis, and lymphadenopathy are common symptoms.
- Declining: The symptoms of the disease start to subside. If patients have been prescribed antibiotics, this is when they may decide to stop taking them. It is important to stress to patients that they must take all of the medication. With mononucleosis, the declining stage can take several weeks or even months.
- Convalescent: In this stage, patients will regain their strength and return to a state of good health.
The prodromal stage of an acute infection is the time when the patient first shows vague, nonspecific symptoms of disease. For example, the person is not vomiting and does not have a fever, but the individual just does not feel well.
In an acute viral infection, the host cell typically dies within hours or days. In most acute infections, such as the common cold, the body’s defense mechanisms eliminate the virus within 2 to 3 weeks.
Chronic infection
A chronic infection is one that persists for a long period of time, sometimes for life. In the case of chronic hepatitis B infection, patients are asymptomatic, or without symptoms. The virus is still detectable with blood tests and remains transmissible throughout the person’s life.
Hepatitis B infection is transmitted by blood or blood products and all body fluids. It is a serious health hazard to those who work in healthcare. All individuals employed in a healthcare setting should be immunized against hepatitis B. OSHA requires all employers to offer hepatitis B vaccination to their employees.
Latent infection
A latent infection is a persistent infection in which the symptoms cycle through periods of relapse and remission.
Cold sores and genital herpes are latent viral infections caused by the herpes simplex virus (HSV) types 1 and 2, respectively. The virus enters the body and causes the original lesion. It then lies dormant in nerve cells away from the surface until a certain trigger (illness with fever, sunburn, or stress) causes it to leave the nerve cell and seek the surface again.
Once the virus reaches the superficial tissues, it becomes detectable for a short time and causes a new outbreak at the site.
Another herpes virus, the varicella-zoster virus, causes chickenpox (varicella). This virus may lie dormant along a nerve pathway for years and later erupt as the painful disease shingles (herpes zoster).
Opportunistic infections
Opportunistic infections are caused by organisms that are not typically pathogenic but cause disease under certain circumstances.
A host with an impaired immune system response, such as individuals infected with HIV, is susceptible to opportunistic infections. Over time, the person’s immune system becomes weakened, and diseases result that are not typically seen in patients with a healthy immune system, such as Pneumocystis jiroveci pneumonia and oral candidiasis.
People without a compromised immune system can have an opportunistic infection.