defense against pathogens

1. ivyanatomy.com
Section 2, Chapter 16
Lymphatic System & Defense Against Infection
Pathogens that infiltrate the body may cause infection.
Pathogens Include:
• Bacteria, Protozoa, Fungi, Viruses, Ect.:
Defense against pathogens falls under two categories
1.
2.
Innate Defenses
•
General defenses against many type of pathogens
•
They function the same way regardless of the number of exposures.
Adaptive Defenses (Immunity)
•
More specific defense – target specific pathogens
•
Carried out by lymphocytes

2. Innate Defenses
Species Resistance – pathogens of one species often cannot infect
a different species.
• eg. Humans may transmit measles to other humans, but
not to other species.
Mechanical Barrier – prevent pathogens from penetrating the body
• Skin forms an impermeable barrier
• Mucus traps particles
• Tears, Saliva, and urine wash away microorganisms
• Coughing and sneezing forces microorganisms out of the body.

3. Innate Defenses
Chemical Barrier
• Lysozyme – enzyme within tears that destroys bacteria on eyes
• Gastric Juice – acidic environment that’s inhabitable to many bacteria
• Interferons – secreted by virus-infected cells
• Interferons bind to receptors on uninfected cells, and block the
replication of a virus into uninfected cells.
Cellular Defense
• Natural Killer (NK) Cells - Target viral-infected cells
• NK cells secretes perforins that lyse infected cells.
• Neutrophils & Monocytes – phagocytize various pathogens and debris.
• Basophils – secrete histamines and heparin.

4. Innate Defenses
Inflammation – produces local redness, swelling, heat, and pain.
• Redness – results from increased blood flow to affected area.
• Swelling – Results from an increased permeability of capillaries
• Fluid leaks into interstitial space.
• Leaky vessels help WBCs arrive to infection.
• Heat – Blood from deeper body warms the surface.
• Increased heat impedes the growth of pathogens.
• Pain – results from stimulation of pain receptors.

5. Innate Defenses
Fever –Elevation in body temperature.
• Pathogens stimulate the secretion of interleukin-1 (IL-1) from lymphocytes.
• IL-1 raises the thermoregulatory set point in the hypothalamus.

6. Adaptive Defenses
• Also known as immunity
•An immune response is based on the ability to distinguish
molecules that are part of the body (“self” from “non-self”)
•Antigens are molecules that can elicit an immune response
They may be:
• Proteins
• Polysaccharides
• Glycoproteins
• Glycolipids

7. Origin of Lymphocytes
Lymphocytes originate from stem cells within red bone marrow.
T-Cells migrate to the thymus where they complete development.
B-Cells continue to develop within the red bone marrow.
Ratio of Circulating Lymphocytes
• T-cells constitute 70% of circulating lymphocytes
• B-cells constitute 30% of circulating lymphocytes
Monoclonal Lymphocytes

8. Clonal Selection of Lymphocytes
The surface of T-Cells and B-Cells are covered with thousands of
identical copies of a receptor that binds for an antigen.
The surface of T-Cells and B-Cells are covered with thousands of
identical copies of a receptor that binds for an antigen.

9. Origin of Lymphocytes

10. Types of Lymphocytes
T-Cells contribute to cell-mediated defense.
• Helper T-cells – activates other T-cells & B-cells in response to a threat
• Cytotoxic T-cells (Killer) – directly attacks pathogens
• Memory T-cells – remember a previous threat.
B-Cells contribute to antibody-mediated defense (humeral response).
• Plasma cells – activated B-cells that secrete antibodies
• Memory B-cells – remember a previous threat.

11. Lymphocytes Clones
antigen
receptors
lymphocyte
lymphocyte
lymphocyte
T-Cells and B-Cells are covered with identical
copies of a receptor that binds for an antigen.
lymphocyte
lymphocyte
lymphocyte
Cell proliferates
lymphocyte
When the lymphocyte's receptor matches
an antigen precisely, the lymphocyte makes
many identical copies, forming clones.
lymphocyte
Cloned cells can now mount an assault on
any cell possessing that antigen.

12. T-Cells and the Cellular Response
A Lymphocyte must be activated before it can respond to an antigen.
•T cell activation requires antigen-presenting cells
• Antigen-presenting cells may phagocytize a pathogen,
then display its antigen on their cell surface.
Figure 16.18. An antigen-presenting cell engulfs the pathogen,
then displays the antigen on its surface to T-cells.

13. T-Cells and Cell-Mediated Immunity
Activated T-cells
Helper T-Cells – Stimulates antibody production by B-cells.
• They also activate cytotoxic T-cells.
Cytotoxic T-Cells – Destroy tumor cells and viral-infected cells
• They secrete perforins, which cut large pores into the membrane of target cells.
Memory T-Cells – Provide future immune protection.
• Does not respond to initial exposure of the pathogen, but is activated after a second
exposure.
Activated T-cells also secrete cytokines – chemicals that enhance
cellular responses to pathogens.

14. B-Cells and Antibody-Mediated Immunity
• B-cells can be activated when an antigen fits the shape of its receptor
• However, most of the time B cell activation requires T cells
• T-cells release cytokines that stimulate B-cells
• Some B cells may become memory B cells while others differentiate
into plasma cells and produce and secrete large globular proteins called
antibodies or immunoglobulins

15. B-Cells and Antibody-Mediated Immunity
Figure 16.19. An activated B-cell proliferates after stimulation by cytokines released
by Helper T-cells, and by receptor-antigen interaction. Some of the B-cells give rise
to antibody-secreting plasma cells, and others to dormant memory B-cells.

17. Antibodies
Antibodies, or immunoglobulins (Ig) are globular plasma proteins.
An antibody consists of:
• Two identical heavy chains and two identical light chains
• Disulfide bonds – holds the two chains together
• A constant region – identical among most antibodies.
• A variable region – specialized to fit the antigens
• Antigen Binding Site

18. Types of Immunoglobulins
IgG – Effective against bacteria, viruses, and toxins
• May cross the placenta (anti-Rh antibodies)
IgM – Produced in response to antigens in food and bacteria
• Examples include then anti-A and anti-B antibodies
• Coagulation
IgA – In exocrine secretions
• Breast milk, tears, gastric juices, ect.
IgE – Associated with allergic reactions
IgD – Receptors on the surface of B-cells
• Important in activating B-cells

19. End of Section 2, Chapter 16