SOLDIERS OF THE IMMUNE SYSTEM:Less-Than-Optimal Defenses

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Health Psychology PSY408

Lecture 12

SOLDIERS OF THE IMMUNE SYSTEM

White blood cells play a key role in the immune system--they serve as soldiers in our counterattack against

invading substances in the body. There are two types of white blood cells. Lymphocytes, as we have seen,

are one type; phagocytes are the other.

Phagocytes are scavengers that patrol the body and engulf and ingest antigens. They are not choosy. They

will eat anything suspicious that they find in the blood stream, tissues, or lymphatic system. In the lungs, for

instance, they consume particles of dust and other pollutants that enter with each breath. They can cleanse

lungs that have been blackened with the contaminants of cigarette smoke, provided the smoking stops. Too

much cigarette smoking, over too long a time, destroys phagocytes faster than they can be replenished.

There are two types of phagocytes: Macrophages become attached to tissues and remain there, and

Monocytes circulate in the blood. The fact that phagocytes "are not choosy" means that they are involved

in nonspecific immunity--they respond to any kind of antigen.

Lymphocytes react in a more discriminating way, being tailored for attacks against specific antigens. The

Health Psychology PSY408

diagram shows that, in addition to the process of nonspecific immunity, there are two types of specific

immune processes: cell-mediated immunity and antibody-mediated "humoral" immunity. Let's examine

these two specific immune processes and how they interrelate.

Cell-mediated immunity operates at the level of the cell. The soldiers in this process are lymphocytes called

T cell--the name of these white blood cells reflects their having matured in the thymus. T cells are divided

into several groups, each with its own important function:

Killer T cells (also called (CD8 cells) directly attack and destroy three main targets: transplanted

tissue that is recognized as foreign, cancerous cells, and cells of the body that have already been

invaded by antigens, such as viruses.

Memory T cells "remember" previous invaders. At the time of an initial infection, such as with

mumps, some T cells are imprinted with information for recognizing that specific kind of

invader--the virus that causes mumps--in the future. Memory T cells and their offspring circulate

in the blood or lymph for long periods of time--sometimes for decades--and enable the body to

defend against subsequent invasions more quickly.

Delayed-hypersensitivity T cells have two functions. They are involved in delayed immune

reactions, particularly in allergies such as of poison ivy, in which tissue becomes inflamed. They also

produce protein substances called lymphokines that stimulate other T cells to grow, reproduce, and

attack an invader.

Helper T cells (also called CD4 cells) receive reports of invasions from other white blood cells that

patrol the body, rush to the spleen and lymph nodes, and stimulate lymphocytes to reproduce and

attack. The lymphocytes they stimulate are from both the cell-mediated and the antibody-mediated

immunity (also called "humoral" immunity) processes.

Suppressor T cells operate in slowing down or stopping cell-mediated and antibody-mediated

immunity processes as an infection diminishes or is conquered. Suppressor and helper T cells serve

to regulate cell-mediated and antibody-mediated immune processes.

What is antibody-mediated immunity, and how is it different from the cell-mediated process? Antibody-

mediated immunity attacks bacteria, fungi, protozoa, and viruses while they are still in body fluids and

before they have invaded body cells. Unlike the cell- mediated process of attacking infected cells of the

body, the antibody-mediated approach focuses on the antigens directly. The soldiers in this approach are

lymphocytes called B cells. The diagram shows that B cells give rise to plasma cells that produce antibodies.

This process is often induced by helper T cells or inhibited by suppressor T cells.

How are antibodies involved? Antibodies are protein molecules called immunoglobulins ("Ig") that attach

to the surface of invaders and accomplish three results. First, they slow down the invader, making it an

easier and more attractive target for phagocytes to destroy. Second, they recruit other protein substances

that puncture the membrane of an invading microorganism, causing it to burst. Third, they find new

invaders and form memory B cells that operate in the future like memory T cells. As you can see, antibodies

are like sophisticated weapons in immune system wars. Researchers have identified five classes of

antibodies--IgG, IgM, IgA, IgD, and IgE--each with its own special function and "territory" in the body.

For example, IgA guards the entrances of the body in fluids, such as saliva, tears, and secretions of the

respiratory tract.

DEFENDING THE BODY WITH AN IMMUNE RESPONSE

Now that we have seen the soldiers and weaponry of the immune system, let's see how all of this is

orchestrated in defending your body. Protection from disease actually involves a series of defenses. We will

start at the beginning, as the invader tries to enter the body.

Health Psychology PSY408

Your body's first line of defense is the skin and the mucous membranes that line the respiratory and

digestive tracts. The skin serves as a barrier to entry, and mucous membranes are coated with fluids that

contain antibodies and other antimicrobial substances. Even though these defenses are highly effective,

large numbers of antigens get through, either by eluding the antibodies or by entering a wound in the skin

or the mucous membrane.

Once an antigen penetrates this barrier, it encounters the second line of defense, which includes nonspecific

and specific immune processes. Phagocytes in your blood and tissues attack and consume invading

substances of all types.

They also have another important function: They present the antigen to B cells and helper T cells, as if to

say, "Here's the enemy, Go get `em!" The B cells respond to this message and to stimulation from helper T

cells by giving rise to plasma cells that produce the needed antibodies. The role of the phagocytes is

especially important if the antigen is new and the body has no memory B cells for this substance.

Antibodies in body fluids attach to microorganisms, thereby aiding the phagocytes and other protein

substances that can kill the invaders.

Antigens that manage to get through and invade body cells encounter the third line of defense in which

killer T cells destroy the invaded cells. Phagocytes often initiate this process by presenting antigens to T

cells, as we have seen. Once again, this is especially important if the antigen is new to the cell-mediated

system and the body has no memory T cells for the substance. As the invasion subsides, suppressor T cells

slow down the cell-mediated and antibody-mediated immune responses. Memory B and T cells are left in

the blood and lymph, ready to initiate the immune response if the same antigen invades the body again.

Less-Than-Optimal Defenses

If our immune systems always functioned optimally, we would become sick much less often. Why and in

what ways do our defenses function less than optimally?

The effectiveness of the immune system changes over the life span, becoming increasingly effective

throughout childhood and declining in old age. Newborns come into the world with relatively little immune

defense. They have only one type of antibody (IgG), for example, which they receive prior to birth from

their mothers through the placenta (the filter-like organ that permits the exchange of nutrients and certain

other substances between the bloodstreams of the mother and baby). Infants who are nursed receive

antibodies, particularly IgA, in their mothers' milk.

In early infancy, children in technological societies generally begin a regular schedule of immunization

through the use of vaccines. Most vaccines contain dead or disabled disease microorganisms that get the

body to initiate an immune response and produce memory lymphocytes but do not produce the full-blown

disease. The efficiency and complexity of the immune system develop very rapidly in childhood. As a result,

the incidence of illness serious enough to keep children home from school declines with age.

Throughout adolescence and much of adulthood, the immune system generally functions at a high level.

Then, as people approach old age, the effectiveness of the system tends to decline. Although the overall

numbers of T cells, B cells, and antibodies circulating in the blood do not decrease their potency diminishes

in old age. Compared with the T cells and B cells of younger adults, those of elderly people respond weakly

to antigens and are less likely to generate the needed supply of lymphocytes and antibodies to fight an

invasion.

Unhealthful lifestyles, such as smoking cigarettes and being sedentary, have been associated with impaired

immune function. Poor nutrition can also lead to less- than-optimal immune function. Diets deficient in

vitamins seem to diminish the production of lymphocytes and antibodies, for example.

When your immune system functions optimally, it attacks foreign matter and protects the body. Sometimes

this process goes awry, and the immune response is directed at parts of the body it was designed to protect.

Health Psychology PSY408

Several disorders result from this condition--they are called autoimmune diseases. One of these diseases is

rheumatoid arthritis, in which the immune response is directed against tissues and bones at the joints. This

causes swelling and pain and can leave the bones pitted. In rheumatic fever, the muscles of the heart are the

target. This disease can damage the heart valves permanently. Multiple sclerosis, a disease we considered

earlier, results when the immune system attacks the myelin sheath of neurons.

Many people believe stress and illness often are related--and they are right. Research has confirmed this

belief, showing, for instance, that the incidence of respiratory illnesses increases when people experience

high levels of stress. Why is this so? One likely answer is that stress suppresses immune functions in some

way, leaving the person open to infection. The phenomenon of stress is very important in the

understanding of health and wellness. We have devoted many upcoming lectures to cover this area.

GENETIC PROCESSES IN DEVELOPMENT AND HEALTH

Genetic Materials and Transmission

What did this search yield? By the early 20th century, researchers discovered threadlike structures called

chromosomes and proposed that these structures contained units called genes. Soon they determined the

basic substance common to all genetic material-- deoxyribonucleic acid, or DNA for short--and described

its structure. Today we know that DNA determines our growth patterns and physical structures, We also

know that genes are discrete particles of DNA and that strings of genes are organized into chromosomes.

The Impact of Genetics on Development and Health

Researchers have determined that every human cell contains 30.000--40,000 genes and have identified and

mapped almost all of the human system of genes. Genes control a vast number of traits, including more

than 3,000 diseases. For some diseases, researchers have even pinpointed the exact gene locations.

A biopsychosocial perspective in our examination of heredity is important. One way is to study the patients

of different diseases and find out if they have some clues in their lifestyles or behaviors. For example,

disease and genetic mutation can result from engaging in certain behaviors, such as smoking cigarettes and

eating diets that are high in fats and low in fiber.

Having this knowledge, we can also identify other similar behaviors that can lead to diseases, and find ways

to eliminate or modify them.

The biopsychosocial perspective of heredity is important in another way. Many researchers believe that we

often inherit a predisposition or susceptibility--rather than a certainty--for developing a disease. This

might account in part for the observation that not everyone who is exposed to harmful substances and

microorganisms in their environments become sick. People who inherit a high degree of susceptibility to a

form of cancer and have relatively little exposure to relevant antigens may be just as likely to develop the

illness as someone who has little genetic susceptibility but high antigen exposure. If physicians could

determine whether a patient has a genetic predisposition to a specific disease, they could provide the person

with instructions for taking early preventive action.

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