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Immune System Strengthening

The Immune System: How it Works

The immune system is an elegant and complex set of components that combine to fight disease, infection(s), and various pathogens. A healthy immune system distinguishes organisms in the body as “self” or “non-self.” An intact immune response identifies pathogens as “non-self” and rapidly destroys them. A depressed immune system, by contrast, will allow invading organisms to flourish.

When the immune system mistakenly recognizes a “self” cell as “non-self” and mounts an immune response, it can result in an autoimmune disorder (eg, rheumatoid arthritis).

In general, the body has two primary defense mechanisms: natural immunity and acquired immunity. Natural immunity is the “first responder” to attack. Natural immune response relies on various white blood cells and physical barriers to block or immediately attack any foreign invader and attempt to destroy it.

Acquired immunity, on the other hand, involves antibodies created in response to specific foreign antigens. This sort of response requires a few days for the body to recognize the invader and manufacture antibodies against it. Once the body has manufactured a particular antibody for a specific invader, the immune system response is faster and more effective the next time that invader appears (Janeway 1999; Beers 2004).

The natural immune system relies on a host of weapons to protect the body, including various kinds of white blood cells (see Table 1). These natural defenses include the following organs, chemicals, and processes:

Physical and chemical barriers. The body's first lines of defense are the skin and mucous membranes, which prevent the entrance of many pathogens. There are also many secondary barriers. For example, tears, sweat, and saliva combat some bacteria; also, the hydrochloric acid as well as protein-digesting enzymes secreted by the stomach are lethal to many, but not all pathogens (Janeway 1999; Beers 2004).

Inflammation and fever. Inflammation is a nonspecific response to infection or tissue injury. The four signs of inflammatory response are redness, swelling, heat, and pain. Inflammation begins when cells release certain cytokines, including interleukin (IL)-1, IL-6, and tumor necrosis factor-alpha (TNF-α) (Janeway 1999; Beers 2004).

Phagocytic cells. Phagocytic cells engulf and destroy foreign cells. Phagocytic cells are white blood cells and include neutrophils, eosinophils, and macrophages; they have short lives and must be continually replenished by the body. Neutrophils and macrophages are very important aspects of the innate defenses of the body (Janeway 1999; Beers 2004).

Natural killer cells. Natural killer cells destroy certain cancer cells and a variety of pathogens. Killer cells are active secretors of interferon, an important and potent protein. Natural killer cells attach directly to the surfaces of infected cells and cause them to burst. They can also kill a pathogen by making its outer membrane leak (Janeway 1999; Beers 2004).

Antimicrobial proteins. Infected immune cells produce interferon, which causes healthy cells to produce antiviral proteins. There are more than 30 distinct antiviral proteins. When an individual complement (immune system) protein is activated by infecting organisms, it triggers a cascade that activates other complement proteins. Activated proteins can destroy bacteria while sparing host cells or cause the infected cells to become engulfed by phagocytic cells (Janeway 1999; Beers 2004).

Cytokines. Cells use chemical messengers (ie, cytokines) to communicate and share information; each chemical sends a different message to other cells. Cytokines regulate immunity, inflammation, and the production of white blood cells. There are dozens of cytokines; each performs a specific set of activities against specific target cells. They can act in concert or in opposition. Cytokines are often produced in a cascade; in other words, a cytokine stimulates its target cells to make additional cytokines. TNF-α, IL-1, IL-6, and type I interferon are important cytokines in the regulation of natural immunity.

Acute-phase proteins. Acute-phase response is activated during critical illness. When phagocytic cells bind pathogens, they release pro-inflammatory cytokines. This response enables the body to recognize invaders before immune responses have been fully activated. Acute-phase proteins promote inflammation and stimulate phagocytes to move where they are needed.

Table 1. Major Cells of the Immune System (Janeway 1999; Nairm 2000)

Cell

Activity

Lymphocytes

Natural killer cells

Destroy a variety of tumor cells and antibody-coated target cells; not antigen specific.

Cytotoxic T (CD8+) cells

Secrete cytokines that attract macrophages and increase their phagocytic activity; destroy target cells that display the same antigen that activated their progenitor cell; lyse infected cells by releasing toxins. Cytotoxic T cells fight foreign invaders by destroying cells that display the antigen that activated its progenitor cells (immunological surveillance).

Helper T (T4+) cells

Stimulate cellular immunity and inflammation; secrete cytokines that stimulate proliferation of B cells and other T cells; amplify antibody production by plasma cells.

Suppressor T cells

Suppress activity of naïve (un-stimulated) and effector T cells.

Memory T cells

Recognize antigens that have invaded in the past, which allows for a larger and more rapid response when there is a second encounter with that antigen.

B lymphocytes (B cells)

Differentiate into antibody-producing plasma cells; process and present antigen to helper T cells; display immunoglobulin and class II MHC antigens.

Plasma cells

Main antibody-secreting cells.

Memory B cells

Descendants of B cells that remain after an immune response.

Phagocytes

Macrophages

Phagocytize antigens, then process and present them to T cells for destruction; attack dead and defective blood cells; secrete cytokines that induce proliferation of B and T cells.

Neutrophils

Major defense against bacteria; first on scene to fight infection.

Eosinophils

Active against parasites and commonly elevated in allergies.

Dendritic cells (interdigitating reticular cells)

Process and present antigens to T and B cells; most potent stimulators of T cell responses.

Antigen-presenting cells

Engulf antigens, process them internally, and then display fragments of them on their surface; surface markers alert other immune cells that there is an invader. Identified antigen-presenting cells: dendritic cells, macrophages, and B lymphocytes.

Inflammation, Free Radicals, and Cytokines

Although acute inflammation is an important immune system response, chronic inflammation has also been linked to many diseases, including heart disease. Besides being associated with pro-inflammatory cytokines, inflammation may be related to the overproduction of free radicals (Janeway 1999).

A free radical is an atom or group of atoms (ie, a molecule) with unpaired electrons. Free radicals are extremely unstable and react easily with other molecules, thereby changing their chemical composition. Oxygen is especially susceptible to free radical formation. Free radicals derived from oxygen are known as reactive oxygen species (ROS), or oxidants.

When the body has increased levels of ROS (ie, when it is experiencing oxidative stress), widespread damage may result. At high concentrations, free radicals can damage fats, proteins, and nucleic acids. They can also cause cell death, gene mutations, and cancer (Moslen 1994). Several diseases may be the result of cellular and genetic damage caused by free radicals, including several immune disorders (Moslen 1994).

In order to reduce the damage caused by elevated free radicals and cytokines (both part of the natural immune system), the body fights back by producing antioxidants and hormones (eg, cortisol) to suppress the immune system (Grimble 1996). Antioxidants are valuable because they pair with unstable free radicals, thereby limiting the damage free radicals can inflict on other cells.

What You Have Learned So Far...

  • The immune system declines as we age, making us more susceptible to various diseases and pathogens.
  • Immune system health is closely related to stress, frequency of exercise, and nutritional status. Poor intake of vital nutrients is closely associated with depressed immune response and increased rate of disease.
  • The immune system has two primary defense mechanisms: natural, which uses white blood cells and physical barriers to protect against disease, and acquired, in which specialized cells generate antibodies to defend against specific pathogens.
  • Inflammation is caused by multiple factors, including microorganisms, physical stress, tissue death, and inappropriate immune response. Chronic inflammation is linked to diseases such as heart disease. Inflammation is mediated by cytokines and free radicals. It is an important immune system response, but can also be dangerous because a chronic inflammatory state is linked to various diseases of aging.
  • Free radicals are unstable molecules that readily react with other molecules, especially oxygen, to change their chemical composition. Antioxidants are used by the body to scavenge for free radicals and limit the amount of damage they can cause.