(Chapter 21) • Immune System Overview • Innate Host Defenses

UNIT 6 Immunity (Chapter 21)

• Immune System Overview • Innate Host Defenses • Adaptive Defenses

(7th edition)

NOTE: The following information is based on the Immune System Interactive Physiology tutorials; I highly recommend that you view the interactive tutorials along with the lecture outline (at the same time); the following outline is a “blending” of the textbook and Interactive Physiology tutorials

(7th edition) Immune System Overview (view the Interactive Physiology tutorial “Immune System: Immune System Overview”)

• There are two major functions of the immune system:

destroy pathogens detect and kill abnormal cells, such as cancerous cells

• Pathogens are classified according to their size and where they are located in the body; there are 5 types of pathogens:

1. parasitic worms 2. fungi 3. protozoa 4. bacteria 5. viruses

*viruses are always intracellular (must reproduce inside cells), whereas the others are usually extracellular ; parasitic worms are macroscopic organisms, whereas the others are microorganisms , meaning that they can only be seen with a microscope

(7th edition)

Immune System Overview

Line of Defense (fig. 21.1)

• INNATE DEFENSES (nonspecific defenses)

innate external defenses - these are surface barriers, such as the skin and mucous membranes; if the innate external defenses are penetrated then the next line of defense is the innate internal defenses

innate internal defenses - these include cells and chemicals in body fluids (e.g. phagocytes and NK cells), fever , and inflammation ; internal defenses identify enemies by recognizing markers that are unique to the pathogens; when they are overwhelmed, they secrete chemical messengers to mobilize adaptive defenses

(7th edition) Immune System Overview

Line of Defense (fig. 21.1)

• ADAPTIVE DEFENSES (specific defenses)

differ from innate defenses:

they are specific (directed against an identifiable enemy)

they involve B and T lymphocytes

they have memory ; they will recognize an enemy if it attacks the body again in the future

they are systemic (can act anywhere in the body)

B and T lymphocytes recognize pathogens by binding to them; they recognize antigens of the pathogen by shape, also known as the antigenic determinant

specific B cells called plasma cells secrete antibodies , which bind to the antigens

(7th edition)

Immune System Overview

• Humoral (antibody-mediated) vs. Cellular (cell-mediated) Immunity

humoral, or antibody-mediated, immunity is directed against pathogens in extracellular fluid ; this immunity involves B lymphocytes and antibodies

cellular, or cell-mediated, immunity is directed against pathogens within the cells; this immunity involves T lymphocytes ; for example T cells would be activated if a cell has become cancerous or attacked by a virus, or if a cell has been transplanted from another individual

(7th edition) Innate Host Defenses (view the Interactive Physiology tutorial “Immune System: Innate Host Defenses”); innate defenses are present at birth and are genetically determined

• Innate External Defense System - first line of defense

surface barriers include the skin and mucous membranes of the respiratory, digestive, urinary, and reproductive tracts

characteristics of skin that help it to resist invasion:

water-resistant and tough keratin outer layer

intercellular junctions hold skin cells tightly together

skin secrections are acidic and have chemicals that make the skin inhospitable to pathogens; e.g. lysozyme destroys cell walls of certain bacteria

mucous membranes not only provide a barrier, but also produce a variety of protective chemicals (e.g. lysozyme ) and acidic secretions

the stomach secretes digestive enzymes and has a very low pH

the digestive and respiratory pathways are lined with sticky mucous that traps pathogens

(7th edition)

Innate Host Defenses

• Innate Internal Defense System - second line of defense; attempts to limit the spread of pathogens; this system is fast-acting and nonspecific

the internal defense system has 5 components:

phagocytic cells (e.g. neutrophils and monocytes/macrophages) NK cells (natural killer cells) antimicrobial proteins (complement and interferon ) inflammation fever

(7th edition) Innate Host Defenses

• Innate Internal Defense System - second line of defense; attempts to limit the spread of pathogens; this system is fast-acting and nonspecific phagocytes (fig. 21.2) neutrophils are the first cells to leave the blood and enter tissues at the sites of infection or trauma; these cells are short-lived monocytes follow the influx of neutrophils into the affected tissue; once in the tissue, they transform into macrophages ; they phagocytize many more pathogens than neutrophils phagocytes use special membrane receptors to recognize and bind molecules that are found on pathogens, but not on normal body cells when a phagocyte recognizes a pathogen it: - ingests the pathogen - releases chemical alarm signals that mobilize other cells of innate and adaptive immunity OPSONIZATION - some bacteria have capsules that make it difficult for phagocytes to grab them; the immune system makes molecules that “coat” the bacteria and enhance phagocytosis; this is called opsonization; both complement and antibodies can act as opsonins

(7th edition)

Innate Host Defenses

• Innate Internal Defense System - second line of defense; attempts to limit the spread of pathogens; this system is fast-acting and nonspecific

NK cells (natural killer cells)

type of lymphocyte involved in innate immunity

attack body cells that have been invaded by pathogens (e.g. viruses) or cancer; they will also attack the cells of transplanted tissues

NK cells are larger than B and T cells, and unlike B and T cells, do not have antigen receptors

both NK cells and T cells are involved in IMMUNE SURVEILLANCE (they continually scan our cells for abnormalities)

(7th edition) Innate Host Defenses

• Innate Internal Defense System - second line of defense; attempts to limit the spread of pathogens; this system is fast-acting and nonspecific

antimicrobial proteins

interferons (fig. 21.5)- interfere with viral replication and activate immune cells; cells that have been attacked by a virus release interferon to help protect neighboring cells that have not yet been affected

complement (complement system ) (fig. 21.6) - it “complements” or enhances other components of both innate and adaptive defenses; it can mark cells for phagocytosis, promote inflammation, and kill some bacteria

(7th edition)

Innate Host Defenses

• Innate Internal Defense System - second line of defense; attempts to limit the spread of pathogens; this system is fast-acting and nonspecific

inflammation

when the body is injured (e.g. a cut, abrasion, or bruise) a sequence of events called inflammation is initiated

tonsillitis, tendonitis, and laryngitis are examples of short-lived, or acute , inflammation; arthritis is an example of long-term, or chronic , inflammation

there are 4 cardinal signs of inflammation: pain, swelling, redness, and heat

the purpose of inflammation is to bring white blood cells and plasma proteins into an injured area; inflammatory mediators (e.g. histamine from basophils and mast cells) cause vasodilation (increasing blood flow to the area) and an increase in vascular permeability (allowing phagocytes and plasma proteins to enter the tissue)

plasma proteins and more fluid than usual leak into the injured area causing EDEMA (increased interstitial fluid); edema causes swelling, which can contribute to the sensation of pain

(7th edition) Innate Host Defenses

• Innate Internal Defense System - second line of defense; attempts to limit the spread of pathogens; this system is fast-acting and nonspecific

fever

generalized increase in body temperature

PYROGENS - chemicals secreted by leukocytes and macrophages that have been exposed to foreign substances in the body; they cause the body’s thermostat (located in the hypothalamus) to set its temperature higher

higher body temperatures enhance phagocytosis and cause the liver and spleen to sequester iron and zinc (making these essential elements less available to bacteria); pathogens also do not grow very well at higher temperatures

(7th edition)

Adaptive Defenses - the body’s third line of defense (view the Interactive Physiology tutorial “Immune System: Common Characteristics of B and T Lymphocytes”)

• Adaptive Defenses are Specific , Systemic , and have Memory ; they include Humoral Immunity (antibody-mediated) and Cellular Immunity (cell-meditated)

• B and T Lymphocytes are key players in adaptive immunity

• Antigens (fig. 21.7)

have multiple antigenic determinants (based on shapes)

self-antigens are the shapes that lymphocytes expect to find in the body (thus lymphocytes do not normally attack them)

antigen receptors are specific and diverse

(7th edition) Adaptive Defenses

• “Education” of Lymphocytes

immunocompetence - the lymphocyte is able to recognize its one specific antigen by binding to it

self-tolerance - the lymphocyte is unresponsive to self-antigens, so that it does not attack the body’s own cells

T cells become immunocompetent and self-tolerant in the thymus, whereas for B cells this occurs in the bone marrow

• Autoimmune Diseases - lymphocytes attack the body’s own cells; e.g. Type 1 Diabetes mellitus, Grave’s disease, and Multiple sclerosis

• Memory Cells - are created in large numbers during a primary immune response (exposed to antigen for first time); memory cells create a larger number of effector cells during a secondary immune response (exposed to antigen again); thus, the response to the second attack will be much greater

(7th edition)

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(7th edition)