CHAPTER 16  NONSPECIFIC DEFENSES OF THE HOST

 

 

Since microbes are found in such large numbers in our environment and are equipped with various means of invading and causing disease, we must be able to defend ourselves against them. Our ability to fight disease is called resistance, and occurs in three main ways:

   1. Prevent microbes from entering our body in the first place

   2. Quickly remove microbes that do get in

   3. Combat the microbes that remain inside and keep them from causing serious harm

 

Lack of resistance is susceptibility.

 

Our means of resistance are of two main types:

   1. Nonspecific (innate) resistance—protect against pathogens, regardless of species –rapid response, always ready to go

   2. Specific (adaptive) resistance—works against a particular pathogen and is a function of the immune system—takes more time to get started

 

                                    SUMMARY OF DEFENSES

NONSPECIFIC

NONSPECIFIC

SPECIFIC (IMMUNE)

FIRST LINE OF DEFENSE

SECOND LINE OF DEFENSE

THIRD LINE OF DEFENSE

INTACT SKIN

MUCOUS MEMBRANES AND THEIR SECRETIONS

PHAGOCYTIC WHITE BLOOD CELLS

INFLAMMATION AND FEVER

ANTIMICROBIAL SUBSTANCES

SPECIALIZED LYMPHOCYTES (B AND T  CELLS)

 

ANTIBODIES

 

 

NONSPECIFIC RESISTANCE

Our first line of defense consists of the skin and mucous membranes, and works to keep pathogens out of the body by mechanical and chemical factors.

 

           

FIRST LINE OF DEFENSE: SKIN AND MUCOUS MEMBRANES

 

PHYSICAL FACTORS

1. Skin provides an effective barrier against the entry of microbes. It consists of two layers, the outer epidermis (epithelial tissue)  and the inner dermis (connective tissue). The epidermis is made up of layers of epithelial cells in continuous sheets, tightly connected to each other. The surface layers are dead cells filled with the protein keratin, which makes the layer tough and relatively waterproof. The surface of intact skin is rarely entered by microbes.

 

However, breaks in the skin or just excessive moistness in an area over time may allow microbes to penetrate into deeper tissues. Microbes most likely to take advantage of this are the staphylococci that inhabit the skin and associated glands. Fungus infections may also occur.

 

The skin also is involved in the immune response, by the action of Langerhans cells found in the epidermis. These cells process and present antigens to the helper T cells.

 

   2. Mucous membranes---these consist of an epithelial layer and underlying connective tissue, but in this case the epithelial layer contains living cells right up to the surface and is always kept wet by secretions, so it is more easily penetrated by microbes. Mucous membranes can still inhibit the entry of pathogens, but not as well as skin.

 

   3. Lacrimal apparatus---this consists of the structures that produce and drain tears. The lacrimal glands are located within the bony socket of the eye, above the outer corners. Tears flow from these glands across the surface of the eyes, and drain away through tiny ducts at the inner corners. This provides a washing action that removes microbes. In case of irritation, tear production increases, which also increases the washing action, and dilutes and carries away any associated microbes.

 

   4. Salivary glands---these produce saliva, which washes away large numbers of microbes from the teeth and mucous membranes of the mouth. Decrease in the normal amount of saliva can lead to severe tooth decay.

 

   5. Mucus---this is a secretion of the respiratory and digestive tracts, produced by goblet cells. It can mechanically trap microbes, as well as dust particles and pollutants. In the respiratory tract, cilia associated with cells lining the respiratory passages move the mucus and whatever is caught in it upward, somewhat like the action of an escalator.

 

   6. Vaginal secretions---microbes are also removed by the flow of vaginal secretions.

 

 

CHEMICAL FACTORS

 

 

   1. Sebum---sebaceous (oil) glands of the skin produce this secretion to help skin and hair retain moisture. Sebum also forms a protective film that inhibits growth of many pathogenic bacteria and fungi. By producing a fairly acid pH (3 - 5), sebum discourages growth of many microbes. Not all microbes are affected this way. Some bacteria are able to break down sebum for nutrients, and play a big role in producing body odor. These bacteria also are involved in acne.

 

   2. Lysozyme---this secretion also interferes with growth of bacteria. It contains enzymes capable of breaking down cell walls of some bacteria (this is most successful against gram-positive species but can also work against gram-negative). It is found in perspiration, tears, saliva, nasal mucus, and tissue fluids.

 

   3. Gastric juice---this secretion of the gastric glands of the stomach is highly acidic (pH 1.2-3). Most bacteria and bacterial toxins are destroyed by the acidity. Unfortunately, the toxins of Staphylococcus aureus and Clostridium botulinum survive the stomach acid. Pathogens may also pass through the stomach unharmed when they are protected within food particles. One species, Helicobacter pylori, actually lives and grows in the wall of the stomach, causing most ulcers. Vaginal secretions are also somewhat acidic.

 

   4. Transferrins---found in blood and inhibit microbial growth by reducing the amount of available iron.

NORMAL FLORA AND NONSPECIFIC RESISTANCE

 

Normal flora organisms may keep pathogen numbers under control by:

     1. Competing with them for nutrients

     2. Producing substances harmful to the pathogens

     3. Altering environmental conditions so that pathogens are inhibited


All of these could be considered nonspecific resistance.

 

 

SECOND LINE OF DEFENSE

 

PHAGOCYTOSIS

“Cell eating”----certain white blood cells have the ability to engulf microbes and other foreign particles. These cells are known as phagocytes, and include several types of white blood cells.

 

A review of blood----blood consists of a fluid called plasma, in which the formed elements are found. The formed elements include:

 

 

TYPE OF FORMED ELEMENT

NUMBER PER CUBIC MM OF BLOOD

FUNCTION

ERYTHROCYTES (RED BLOOD CELLS)

4.8 - 5.4 MILLION

TRANSPORT OF  02  AND  CO2

LEUKOCYTES (WHITE BLOOD CELLS)

5000 - 10,000

DEFENSE OF THE BODY

  1. NEUTROPHILS

60-70% OF TOTAL WBC

PHAGOCYTOSIS

  2. BASOPHILS

0.5-1%

PRODUCTION OF HISTAMINE

  3. EOSINIPHILS

2-4%

WORK AGAINST CERTAIN PARASITES AND SPECIAL PHAGOCYTOSIS

   4. MONOCYTES

3-8%

PHAGOCYTOSIS (AFTER BECOMING MACROPHAGES)

  5. LYMPHOCYTES

20-25%

B CELLS—ANTIBODY PRODUCTION

T CELLS—CELL-MEDIATED IMMUNITY

PLATELETS

250,000 - 400,000

BLOOD CLOTTING

 

During many kinds of infections the white blood cell count increases (leukocytosis). Other diseases cause a decrease (leukopenia). The total white count is the number of white cells per cubic mm, but a differential white count is also often performed, which determines the percentage of each specific type of white blood cell. In most bacterial infections, the neutrophils increase in number early in the infection. Later, monocytes increase in number.

 

The granular leukocytes are the neutrophils, basophils and eosinophils. When stained, their cytoplasm exhibits large prominent granules.

 

     1. Neutrophils---lilac (pinkish) granules with a lobed nucleus. These are often called polymorphonuclear leukocytes, polymorphs, or polys. They are highly phagocytic and can leave the blood, move to an infected tissue and destroy microbes and foreign particles. They are able to do this quite rapidly, and the number of neutrophils in the circulating blood can also be rapidly increased when needed.

 

     2. Basophils---dark blue-purple granules which may obscure the nucleus. They are not known to be phagocytic.

 

     3. Eosinophils---bright reddish-orange granules and usually a bilobed nucleus. They are able to produce proteins which are toxic to parasitic worms, and can actually attach to and destroy worms by means of peroxide ions. Their numbers also increase during allergic reactions. These cells carry out phagocytosis of antigen-antibody complexes.

 

     4. Monocytes---these cells leave circulating blood and change to a form called macrophages. This change may occur in lymph nodes, as well as other locations. The macrophages are of two types:

        a. Fixed macrophages (histiocytes)----located in certain tissues and organs, where they wait for the blood flowing through the area to bring microbes, debris, worn out red blood cells, etc. They remove these by phagocytosis.  These fixed macrophages are known as the reticuloendothelial system (mononuclear phagocytic system). These cells are located in many places in the body--liver, spleen, lungs, lymph nodes, intestines, etc.

        b. Wandering macrophages---these leave the blood and migrate to a trouble spot in tissue, where they are highly phagocytic and clean up dead tissue as well as microbes and debris.

 

     5. Lymphocytes—come in 3 main types. Two of these carry out the immune response and will be discussed in  chapter 17; natural killer (NK) cells are the third type. They attack and destroy any cell displaying abnormal proteins on its plasma membrane.

 

 

MECHANISM OF PHAGOCYTOSIS

 

There are four main phases in the process:

 

1. Chemotaxis---chemical attraction of phagocytes to microbes. The chemicals involved include:

·         Microbial products such as toxins

·         Chemicals released by white blood cells already in the area

·         Chemicals released by damaged tissue cells

·         Complement

 

2. Adherence---the plasma membrane of the phagocytic cell attaches to the surface of the microbe or  foreign particle. Attachment can be inhibited by the presence of a well-developed capsule on the microbe or by special proteins produced by some microbes which coat their surface (M protein of Streptococcus pyogenes, for example).  Organisms whose capsules interfere with adherence include Streptococcus pneumoniae and Hemophilus influenzae type b.

 

Opsonization is the coating of the microbe with certain plasma proteins which make the microbe more susceptible to phagocytosis. Proteins which cause this effect include some of the complement system and antibody molecules.

 

3. Ingestion---the plasma membrane of the phagocyte extends pseudopods that surround and engulf the microbe. The microbe is brought into the cell enclosed in a sac made of a bit of the cell’s plasma membrane---this is called a phagosome or phagocytic vesicle.

 

4. Digestion---at this stage, the microbe has not been harmed. The phagocytic cell must now take steps to destroy it. As the phagocytic vesicle moves into the cell, it comes in contact with lysosomes. The membranes enclosing the phagocytic vesicle and the lysosome fuse, forming one larger vesicle called a phagolysosome. The digestive enzymes of the lysosome are able to kill most bacteria within 10 - 30 minutes.

 

Lysosomal enzymes include lysozyme, which breaks down the peptidoglycan of the bacterial cell wall. Other enzymes, including lipases, proteases, ribonuclease, and deoxyribonuclease break down other bacterial components. These enzymes are designed to work best at a pH of about 4, so the phagocytic cell pumps H+ ions into the phagolysosome.  Another factor that works against the phagocytized bacteria is enzymes that produce superoxide radicals, hydrogen peroxide, singlet oxygen, and hydroxyl radicals. These are produced in the phagolysosome and are concentrated enough to kill even many bacteria which produce protective enzymes.

 

The indigestible material that remains in the phagolysosome is called a residual body. It moves toward the boundary of the cell and discharges its contents outside the cell.

 

Most microbes are killed by the process of phagocytosis. Unfortunately, some are able to evade phagocytosis or survive instead of being killed. Some microbes produce toxins which kill the phagocyte. Others produce enzymes that destroy the membrane of the phagolysosome. These may even live and reproduce inside the phagocyte, where they are protected against antibiotics. Some microbes deliberately invade phagocytes to hide from the immune system of the host. They are able to prevent the lysosomes of the phagocytic cell from fusing with the phagocytic vesicle. They may immediately reproduce inside the phagocytic cell or remain dormant for months or years. Bacteria which are usually not killed include:

I.                    Coxiella

II.                 Actinobacillus

III.               Listeria

IV.              Shigella

V.                 Staphylococcus

VI.              Mycobacterium

 

 

Phagocytosis also plays a role in immunity, which will be described in Chapter 17.

 

 

INFLAMMATION

 

Damage to tissue triggers a response call inflammation or the inflammatory response. Damage can be of any type, and may or may not include invasion by microbes. Signs and symptoms of inflammation include:

·         Heat

·         Pain

·         Redness

·         Swelling

·         Loss of function (if the damage is severe enough)

 

 

The inflammatory response is beneficial, although the body has a tendency to get “carried away” with it and cause it to progress beyond what is required. The purposes are:

   1. To destroy any infectious agents and remove agents and their by-products such as toxins from the

        body

   2. Limit the effects of infectious agents that cannot be destroyed or removed by confining them or

       walling them off

   3. To clean up the area and then repair or replace damaged tissue

 

During an inflammatory response, a group of proteins in the blood called acute-phase proteins, undergo activation and an increase in concentration. Included are complement, the cytokines, clotting proteins, and others.

 

 

STAGES OF INFLAMMATION

 

1. Vasodilation and increased permeability of blood vessels---blood vessels increase in diameter and bring more blood to the area, accounting for redness and heat. Increased permeability of blood vessels allows defensive substances and cells to move more easily from the blood to the damaged tissues. As fluid moves into the tissues, this accounts for the swelling (edema) associated with inflammation. Pain is due to nerve damage, toxins, or the pressure of edema, as well as chemicals released by damaged tissue. Vasodilation and increased permeability are caused by chemicals released by damaged cells:

·         Histamine, contained in mast cells, circulating basophils, and platelets, is released when these cells are injured

·         Kinins are a group of substances in plasma which become activated and attract neutrophils

·         Prostaglandins released by damaged cells intensify the effects of histamine and kinins and aid phagocytes in moving through capillary walls

·         Leukotrienes are produced by mast cells and basophils. They cause increased permeability of blood vessels and help in adherence of phagocytes

·         Complement system, a group of proteins present in plasma, become activated and stimulate the release of histamine, attract phagocytes, and promote phagocytosis

 

 

Vasodilation and increased permeability of blood vessels also help deliver clotting elements to the injured area. As blood clots form, they help prevent any microbes present from spreading to other areas of the body. A collection of dead cells and body fluids known as pus may collect in the area, producing an abscess.

 

2. Phagocyte migration and phagocytosis---phagocytes generally appear within one hour or less. Neutrophils are usually the first to arrive in large numbers. They move through the blood vessels to the damaged area, and then begin to stick to the endothelium of the blood vessels (margination). They begin to squeeze between the endothelial cells to reach  the damaged area. This is called emigration or diapedesis. The neutrophils mainly carry out phagocytosis of bacteria. Monocytes are also attracted to the damaged tissue, where they leave the blood and become wandering macrophages. They can phagocytize damaged tissue as well as microbes and dead neutrophils. The macrophages also eventually die and contribute to the formation of pus.

 

Phagocytosis is essential to body defense. Unfortunately, a number of things can suppress it:

·         Genetic defects

·         Old age

·         Anti-rejection drugs in transplant recipients

·         Radiation

·         AIDS

·         Cancer

 

3. Tissue repair---repair cannot proceed well until microbes as well as dead or damaged tissue have been removed by phagocytes. Tissues vary in their ability to replace dead cells with new cells of the same type, which is called regeneration. If scar tissue makes up most of the repair, some function will be lost.

 

 

FEVER

 

This is a systemic response which is most often caused by infection by bacteria, bacterial toxins, or viruses. The body’s “thermostat” is located in the hypothalamus of the brain. It is normally set at 98.60 F, but can be reset. Active phagocytes release interleukin-1, which causes the hypothalamus to release prostaglandins which reset the thermostat at a higher temperature. The body responds by constriction of blood vessels, increased rate of metabolism, and shivering, all of which tend to raise the temperature to the new setting.

 

Fever is a defense against disease, and is beneficial up to a point.

·         Interleukin-1 increases the production of T lymphocytes

·         High body temperatures intensify the effects of interferons

·         Higher temperature may not suit microbes as well, slowing their reproduction

·         Lowers the amount of iron available to microbes

·         May help body tissue repair themselves more quickly

 

 

 

ANTIMICROBIAL SUBSTANCES

 

 

THE COMPLEMENT SYSTEM

 

The complement system consists of about 30 different proteins found in an inactive form in the blood plasma. The system is made up of the following proteins:

·         C1 - C9

·         Factor B

·         Factor D

·         Factor P (properdin)

 

 

The proteins may be activated in one of three ways:

   1. Classical pathway---an immune reaction between antigens and antibodies. When antigens and antibodies bind together, this results several steps which activate the complement protein C3.

   2. Alternative pathway---direct interaction between polysaccharides found in the cell walls of certain bacteria and fungi, as well as the surface of some foreign mammalian red blood cells, with three of the complement proteins, Factors B, D, and P. This also leads to the activation of C3. This pathway is particularly important in responding to gram-negative bacteria that live in the intestines.

   3.  Lectin pathway—following phagocytosis of invaders, macrophages release chemicals which cause the liver to produce proteins called lectins. One of these, mannose-binding lectin (MBL), binds to the carbohydrate mannose, found on the outside of many bacteria and viruses. This results in:

       a. Opsonization

      b. Activation of C2 and C4, which results in:

       c. Activation of C3

 

Whichever of these pathways begins the reaction, the protein called C3 becomes activated and triggers events that result in the activation of the entire complement system. The complement system is nonspecific, but one of its major functions is assisting in immune responses.

 

 

The proteins are activated in an ordered sequence, or cascade, of reactions which mostly proceeds in numerical order. (The exception is that C4 jumps in after C2.)  Each protein activates the next in the series. Often the inactive protein is cleaved (split), and each piece has a new function. One piece of a cleaved protein might cause blood vessel dilation while another piece acts as part of an enzyme that activates the next protein in the series.

 

                   SEE FIGURES 16.12, 16.13, 16.14 P. 494 - 495

 

Activation of the complement protein C3 triggers several mechanisms that contribute to microbial destruction. C3 can be activated by either the classical or the alternative pathway. Activation splits C3 into two smaller pieces, C3a and C3b. This leads to three processes that help destroy microbes:

      a. Cytolysis---this involves damage to the plasma membrane of foreign cells. Cellular contents leak out and the cell dies. Steps (classical pathway):

         1) Once antibodies attach to the antigen (foreign cell), the complement protein C1 in turn binds to the antibodies and becomes activated.

         2) Activated C1 activates C2 and C4 by splitting them into fragments: C2a and C2b plus C4a and C4b. C2b and C4b combine to form an enzyme that activates C3, splitting it into C3a and C3b. . 

         3) C3b starts a series of reactions that activate C5 - C9. These form a membrane attack complex, which attacks the invading cell’s plasma membrane.

         4) Circular lesions called transmembrane channels (holes in the plasma membrane) are produced and allow extracellular fluid to leak in & results in cytolysis.  

 

Activation of the complement components is called complement fixation, and is used as a clinical laboratory test.

 

      b. Inflammation----C3a and C5a can contribute to the development of inflammation by binding to mast cells, basophils, and platelets to trigger the release of histamine, which causes increased permeability of blood vessels. C5a also acts as a chemotaxic chemical to attract phagocytes to the area.

 

      c. Opsonization---C3b can bind to the surface of a microbe and promote phagocytosis. It coats the microbe and promotes the attachment of a phagocyte.

 

Once activated complement has served its purpose, it is quickly inhibited or broken down by regulatory factors in the blood. This is to prevent excessive destruction of host cells.

 

Although rare, inherited lack of certain complement proteins does occur and interferes with defenses of those individuals affected. Capsules of some bacteria prevent complement activation. Others release an enzyme tht breaks down C5a.

 

 

INTERFERONS

 

Control of viruses in the body is especially difficult since viruses depend mainly on normal body processes for their reproduction. Anything that damages the virus also tends to damage normal host cells in the same way. One way the body tries to defend itself is by the production of interferons (IFNs). IFNs are a class of antiviral proteins. Although many species produce them, IFNs are specific to the species (human interferons work only in humans, for example). However, IFNs are not virus-specific—the same interferons may work against many different viruses.

 

Human IFNs are of three main types:

·         Alpha interferon (a-IFN)

·         Beta interferon (b-IFN)

·         Gamma interferon (g-IFN)

In humans, interferons are mainly produced by fibroblasts, lymphocytes, and other white blood cells. a-IFNs and b-IFNs are produced in very small quantities by host cells that have been invaded by a virus. The interferons diffuse to nearby uninfected cells and cause these healthy cells to start transcribing mRNA for the synthesis of antiviral proteins. These are enzymes that will disrupt viral multiplication.

 

g-IFN is produced by lymphocytes to enhance the killing of bacteria by phagocytes.

 

Natural interferons are produced in small amounts and are nontoxic to uninfected cells. Although they are a great help in fighting acute viral illnesses, there are some drawbacks:

·         Effective for only short periods

·         Cannot help cells already infected with the virus

·         Some viruses do not stimulate sufficient production

·         Some viruses resist the effects

 

Interferons are now produced by genetic engineering so that they may be used to supplement those naturally produced by the body. They even appear to have some ability to fight certain cancers as well as infections and other conditions:

·         Alpha-IFN (Intron A) is used in treating Kaposi’s sarcoma, genital herpes,  hepatitis B and C, and hairy cell leukemia

·         b-IFN (Betaseron) is used in slowing the development of multiple sclerosis

 

 

Injected interferons also have drawbacks:

·         High levels are toxic to vital organs

·         Malaise during treatment

 

TRANSFERRINS

Bind to iron and make it unavailable to microbes, which interferes with their reproduction.

 

ANTIMICROBIAL PEPTIDES

Bind to microbial plasma membranes and cause lysis. These are relatively newly discovered and are produced by mucous membrane cells and phagocytes.

 

                                 Table 16.3  P. 497--Summary