section 2, chapter 16: defense against pathogens

Lymphatic System & Defense Against Infection

Section 2, Chapter 16

Pathogens that infiltrate the body may cause infection.

Pathogens Include:• Bacteria, Protozoa, Fungi, Viruses, Ect.:

Defense against pathogens falls under two categories

1. Innate Defenses• General defenses against many type of pathogens• They function the same way regardless of the number of exposures.

2. Adaptive Defenses (Immunity)• More specific defense – target specific pathogens• Carried out by lymphocytes

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Species Resistance – pathogens of one species often cannot infect a different species.

Innate Defenses

• 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.

Cellular Defense

Innate Defenses

• 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.

Chemical Barrier

• 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.

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.

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.

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

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

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.

Origin 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.

Types of Lymphocytes

B-Cells contribute to antibody-mediated defense (humeral response).• Plasma cells – activated B-cells that secrete antibodies• Memory B-cells – remember a previous threat.

lymphocyte

lymphocyte

lymphocytelymphocyte

lymphocytelymphocyte

lymphocyte lymphocyte

antigen

T-Cells and B-Cells are covered with identical copies of a receptor that binds for an antigen.

When the lymphocyte's receptor matches an antigen precisely, the lymphocyte makes many identical copies, forming clones.

Lymphocytes Clones receptors

Cell proliferates

Cloned cells can now mount an assault on any cell possessing that antigen.

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.

T-Cells and Cell-Mediated Immunity

Helper T-Cells – Stimulates antibody production by B-cells.• They also activate cytotoxic T-cells.

Activated 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.

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

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.

B-Cells and Antibody-Mediated Immunity

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

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

End of Section 2, Chapter 16