Nonspecific Defenses Against Infection

Defending Your Body

To defend against invaders—bacteria, viruses, and other pathogens—our bodies have two levels of defense.

1. Nonspecific defense mechanisms
2. Specific defense mechanisms (a.k.a. the immune system)

Nonspecific Defense Mechanisms

• A kind of defense that is not specialized for a particular kind of invader.
• It is a general defense against any kind of pathogen.
• There are two levels of nonspecific defense:
  1. First line of defense: barriers
  2. Second line of defense: internal defenses

1. Barriers

• Skin
  • A barrier on the outside of the body
  • Sweat glands produce oily and acidic liquids that inhibit pathogens.
• Mucous Membranes
  • Barriers on the inside of the body: they line the digestive, respiratory, and genitourinary tracts.
  • Secrete many kinds of fluids like: saliva, tears, nasal secretions, etc.
    • The secretions contain lysozyme, an enzyme that digests the cell walls of bacteria.
    • Mucus is a viscous fluid that can trap pathogens.
• Cilia
  • Small hairs lining the respiratory system that beat away from the lungs.
  • Move mucus and trapped particles out of the body.
• Gastric Juice
  • Concentrated hydrochloric acid (HCl) and proteases that destroy pathogens in the stomach.

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2. Internal Defenses

• If the pathogens penetrate the skin or the mucus membranes, they have moved across the first line of defense—the barrier.
• When this happens, the body uses its second line of defense, composed of three parts:
  1. Leukocytes
  2. Antimicrobial proteins
  3. The inflammatory response

2.1 Leukocytes

• If the pathogens penetrate the skin or the mucous membranes, they have moved across the first line of defense—the barrier.
• When this happens, the body uses its second line of defense: leukocytes.
  • Word derivation:
    • Leuko: white
    • cyte: cell
    • “White cells”—white blood cells
• There are many different kinds of leukocytes, which are classified into two broad categories:
  • Phagocytes
  • Lymphocytes
    • Most lymphocytes are part of specific defense mechanisms.
    • However, natural killer cells are part of the nonspecific internal defenses.

Phagocytes

• Word derivation:
  • Phago: eat
  • Cyte: cell
  • “Cell eaters”
• The most important part of the second line of defense
• There are at least six different phagocytes (do not memorize):
  • Basophils
  • Eosinophils
  • Neutrophils
  • Mast cells
  • Monocytes
  • Macrophages
    • Word derivation:
      • Macro: big
      • Phago: eat
      • “Big eaters”
    • The most famous phagocyte (although not the most abundant)
    • Moves around like an amoeba “eating” pathogens.

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Natural Killer Cells (NK cells)

• An example of a lymphocyte, a different kind of white blood cell from phagocytes
• Can identify virus infected cells and some tumor cells (cancer cells).
• Kills the cell by attacking the cell membrane, causing the cell to lyse (split open).
• The photograph below shows a cancer cell surrounded by a number of natural killer cells.

Part of an unlabeled figure, page 840, Campbell's Biology, 5th Edition

2.2 Antimicrobial Proteins

Two different antimicrobial protein systems exist:

• The complement system
  • Composed of twenty different proteins that “complement” other defenses.
  • Help activate the inflammatory response and attract phagocytes to the site of infection.
  • Attach to microbes, helping phagocytes destroy them.
  • Help lyse the cell membrane of microbes.
• Interferons
  • When cells are infected by a virus, they produce small amounts of proteins called interferons.
  • These proteins increase the resistance of neighboring cells to infection by the same or other viruses.
  • They seem to do this by inhibiting viral reproduction.

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2.3 The Inflammatory Response

• Word derivation: inflammo—to set on fire.
• Damage to tissue occurs (like a break in the skin).
• White blood cells (basophils and mast cells) in the tissue are stimulated to release histamine.
• Histamine causes the capillaries to dilate and become leaky.
  • Results in increased blood flow to the area.
  • Blood plasma and phagocytes leak out of the capillaries and into the infected tissue.
  • This causes the site of injury to swell, become red, warm, and tender.
• Complement proteins are released to attract phagocytes.
• Phagocytes engulf microbes and infected/damaged cells.

Figure 18.4, Purves's Life: The Science of Biology, 7th Edition

Cell Potency

• A cell's ability to differentiate into other cell types
• The more cell cell types a cell can differentiate into, the greater its potency
• Think of potency as on a continuum (from most potent to least potent)
  • Totipotent->Pluripotent->Multipotent

• Totipotent
  
• Word derivation:
  • Totus: total or all
  • Potis: powerful
  • “Totally or all powerful”
• Following fertilization (for up to 4 days), the cells are considered "totipotent" because they
• 1. Give rise to other totipotent cells
• 2. Are capable of producing an entire new organism (three germ layers- ectoderm, mesoderm, endoderm- and the extra-embryonic tissue like the placenta)
• In most animals, the only cells that are truly totipotent is the fertilized egg—the zygote—and its immediate descendants (for about 4 days).
• In many plants, cells remain totipotent throughout the plant's life—plants can be propagated from small pieces of tissue or even from single cells.
• Pluripotent
  • Word derivation:
    • Pluris: many, several
    • Potis: powerful
    • “Many or several powerful”
  • Cells that give rise to all three germ layers (ectoderm, mesoderm, and endoderm), and all the resulting differentiated fetal or adult cell types.
    • Not capable of producing:
      • Extra-embryonic tissues (like the placenta)
      • An intact fetal or adult organism (even though they can give rise to all the cells in that organism).
        
  • Embryonic Stem Cells are pluripotent (not totipotent)
    • They can become any cell type (~220 different cell types in humans)
    • Derived from the inner cell mass
    • Day 5-7 in human embryos
      
• Multiipotent
  
  • Can give rise to many cell types but not all cell types.
    • Some people refer to multipotent cells as pluripotent cells (be aware of this possible confusion)
  • Adult Stem Cells are Multipotent
    • Found in many different types of adult tissue and can become many types of cells but not all 220 cell types.
    • The Stem Cell for all blood cells is an example of an adult stem cell, and it is multipotent (can become many different kinds of blood cells)

Derivation of all Blood Cells: The Pluripotent Hematopoietic Cell

• The original cell from which all blood cells are made is called a pluripotent hematopoietic cell.
• From this original cell, all the different kinds of blood cells can be produced.
• This cell cannot make other kinds of cells or tissues, so it is not totipotent; it is just pluripotent.
• It is called hematopoietic because hemato means “blood” and poietic means “to make.”
  • Its name can thus be roughly translated as “a cell that is powerful because it makes many types of blood cells.”

Categories of Blood Cells

• Red blood cells (erythrocytes): transport oxygen and carbon dioxide
• Platelets (cell fragments without nuclei): initiate blood clotting
• White blood cells (leukocytes): defend against invaders
  • Many different kinds of white blood cells exist; they are broadly classified into two categories:
    • Phagocytes
    • Lymphocytes

Figure 18.2, Purves's Life: The Science of Biology, 7th Edition