IB-202-17-06

The Immune System

Chpt 43 (all).

• Overview: Reconnaissance, Recognition, and Response

• An animal must defend itself from the many dangerous pathogens it may encounter in the environment (pathogenic microbes and parasites)

• Two major kinds of defense have evolved that counter these threats ---innate immunity and acquired immunity.

• New born that lack an immune system have a short life expectancy (Usually die soon after the maternally derived antibodies fade away).

Innate and Acquired Immunity

– Innate immunity is present before any exposure to pathogens and is effective from the time of birth. It involves nonspecific responses to pathogens. Here is an example of a white blood cell, a macrophage (big eater) ingesting a pathogenic yeast cell

Figure 43.1 3m

Yeast cell!

Figure 42.15

Cellular elements 45%

Cell type Numberper L (mm3) of blood

Functions

Erythrocytes(red blood cells) 5–6 million Transport oxygen

and help transportcarbon dioxide

Leukocytes(white blood cells)

5,000–10,000 Defense andimmunity

Eosinophil

Basophil

Platelets

NeutrophilMonocyte

Lymphocyte

250,000400,000

Blood clotting

• The cellular elements of mammalian blood

Separatedbloodelements

White cells can be seen as a buffy layer on the surface of the compacted red blood cells after centrifugation. 60% are neutrophil phagocytes.

Circulating monocytes turn into large macrophages that are about 50um in diameter. Reside in liver, spleen lymph nodes. Live for days!

Acquired immunity

• Also called adaptive immunity– Develops only after exposure to inducing

agents such as microbes, toxins, or other foreign substances (proteins, cell walls of bacteria etc).

– Involves a very specific response to pathogens

– (We take advantage of adaptive immunity when we have our children immunized)

Over view of body defenses!

INNATE IMMUNITY Rapid responses to a

broad range of microbes

ACQUIRED IMMUNITYSlower responses to

specific microbes

External defenses Internal defenses

Skin

Mucous membranes

Secretions

Phagocytic cells

Antimicrobial proteins

Inflammatory response

Natural killer cells

Humoral response(antibodies)

Cell-mediated response(cytotoxic lymphocytes)

Invadingmicrobes

(pathogens)

Figure 43.2

With cystic fibrosis the mucous from the bronchi is so thick that it is not easily expelled and hence becomes infected with bacteria.

External Defenses• Intact skin and mucous membranes

– Form physical barriers that bar the entry of microorganisms and viruses

• Certain cells of the mucous membranes produce mucus– A viscous fluid that traps microbes and other

particles

• In the trachea, ciliated epithelial cells– Sweep mucus and any entrapped microbes upward,

preventing the microbes from entering the lungs. We swallow the mucus and it along with the microbes is digested in the stomach.

Figure 43.3

10m

With cystic fibrosis the mucous from the bronchi is so thick that it is not easily expelled and hence becomes infected with bacteria.

• Secretions of the skin and mucous membranes– Provide an environment that is often hostile to

microbes

• Secretions from the skin– Give the skin a pH between 3 and 5, which is

acidic enough to prevent colonization of many microbes

– Also include proteins such as lysozyme, an enzyme that digests the cell walls of many bacteria. Your tears are full of lysozyme, saliva and some mucous secretions.

Innate immunity is first line of defense if skin is broken!

• Concept 43.1: Innate immunity provides broad defenses against infection

• Internal cellular defenses depend mainly on phagocytosis of microbes.

• Phagocytes are types of white blood cells that– Ingest invading microorganisms– Initiate the inflammatory response

Phagocytic Cells• Phagocytes attach to their prey via surface receptors

– And engulf them, forming a vacuole that fuses with a lysosome

Figure 43.4

Pseudopodiasurroundmicrobes.

1

Microbesare engulfedinto cell.

2

Vacuolecontainingmicrobesforms.

3

Vacuoleand lysosomefuse.

4

Toxiccompoundsand lysosomalenzymesdestroy microbes.

5

Microbialdebris isreleased byexocytosis.

6

Microbes

MACROPHAGE

Vacuole Lysosomecontainingenzymes

Toxic compounds may be reactive oxygen species such as hydrogen peroxide.

• Monocytes circulate in blood for only a few days and then settle in a tissue and develop into a macrophage. Spleen, liver and lymph nodes.

Adenoid

Tonsil

Lymphnodes

Spleen

Peyer’s patches(small intestine)

Appendix

Lymphaticvessels

Masses oflymphocytes andmacrophages

Tissuecells

Lymphaticvessel

Bloodcapillary

LymphaticcapillaryInterstitial

fluid

Lymphnode

• The lymphatic system– Plays an active role in defending the body

from pathogens Interstitial fluid bathing the tissues, along with the white blood cells in it, continually enters lymphatic capillaries.

1

Figure 43.5

Fluid inside thelymphatic capillaries,called lymph, flowsthrough lymphaticvessels throughoutthe body.

2

Within lymph nodes,microbes and foreignparticles present in the circulating lymphencounter macro-phages, dendritic cells, and lymphocytes, which carry out various defensive actions.

3

Lymphatic vesselsreturn lymph to theblood via two large

ducts that drain intoveins near the

shoulders.

4

Antimicrobial Proteins in the blood.• Numerous proteins circulate in the blood

whose function is to bind directly to microbes and punch holes in their cell membrane. About 30 proteins are involved and they work synergistically to destroy microbes.

• Circulating interferons provide innate defense against viruses and help activate macrophages

Inflammatory Response

• In local inflammation, histamine and other chemicals released from injured mast cells– dilate blood vessels making them leaky

causing more fluid, more phagocytes, and antimicrobial proteins to enter the interstitial spaces. This causes the injured area to feel hot and appear red and swollen.

Inflammation • Major events in the local inflammatory

response

Figure 43.6

Pathogen Pin

Macrophage

Chemical signals

CapillaryPhagocytic cells

Red blood cell

Bloodclottingelements

Blood clot

Phagocytosis

Fluid, antimicrobial proteins, and clotting elements move from the blood to the site.Clotting begins.

2Chemical signals released by activated macrophages and mast cells at the injury site cause nearby capillaries to widen and become more permeable.

1 Chemokines released by various kinds of cells attract more phagocytic cells from the bloodto the injury site.

3 Neutrophils and macrophagesphagocytose pathogens and cell debris at the site, and the tissue heals.

4

Natural Killer Cells

• Natural killer (NK) cells– Patrol the body and attack virus-infected

body cells and cancer cells– Trigger apoptosis in the cells they attack

Invertebrate Immune Mechanisms

• Many invertebrates defend themselves from infection– By many of the same mechanisms in the

vertebrate innate response

• Concept 43.2: In acquired immunity, lymphocytes provide specific defenses against infection

• Acquired immunity– Is the body’s second major kind of defense– Involves the activity of lymphocytes

Antigen-binding sitesAntibody A

Antigen

Antibody BAntibody C

Epitopes(antigenicdeterminants)

• An antigen is any foreign molecule– That is specifically recognized by lymphocytes

and elicits a response from them

• A lymphocyte actually recognizes and binds– To just a small, accessible portion of the

antigen called an epitope

Figure 43.7

Protein with epitopes being a few amino acids side chains