AP Bio- Regulation 6: Immunity

Immunity

Evolution always applies!

Animals

Big Questions

Make Sure You Can

What is the purpose of the immune system?

How does the immune system function?

Why do animals have a more developed immune system than other organisms?

Explain the structure and function of all parts of the vertebrate immune system.

Describe the interrelationship between innate and acquired immunity.

Explain how B-cells and T-cells function in the acquisition of specific immunity.

Explain the causes, effects, and treatments of immune system disorders.

Describe how a vaccine works (and why it doesn't sometimes).

Explain why animals are the only organism's with extensive immune systems and how other lineages of life deal with immunity.

The Immune System

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Animals are the only organisms with major immune systems.

All animals possess a degree of innate immunity.

Vertebrates also posses an adaptive immune system that can respond to specific pathogens.

The vertebrate immune system is comprised of many different organs and structures.

The major parts in the specific immune response include the lymphatic system, and the population of leukocytes that patrol the circulatory system.

Immune Disorders

Adaptive Immunity

Innate Immunity

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"non-specific"

"specific"

Allergies

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Cause

Symptoms

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Immune response to non-pathogenic foreign molecules

Humoral

External

Cell-Mediated

Internal

Inflammatory response (often in the respiratory system) due to release of histamine.

Treatment

B-Cell mediated response.

Results in the production of antibodies, specific to particular molecules on particular pathogens (antigens)

T-Cell mediated response.

Results in the activation of particular T-cells, that recognize specific antigens.

The external immune system includes barriers like skin, and mucous membranes, and secretions like mucus and sweat.

Antihistamines, epinephrine (for anaphylaxis)

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T-cell receptors and activity

Adaptive immunity is the responsibility of leukocytes that congregate in the lymphatic system ("lymphocytes")

There are 2 kinds of lymphocytes.

Internal innate immunity is the responsibility of leukocytes that develop from myeloid stem cell precursors (they mature in the bone marrow.

These cells are collectively known as "phagocytes"

Autoimmune Diseases

Skin

Cause

Symptoms

Phagocytes will present antigens to Helper T-Cells.

Helper T-cells will activate specific T-cells.

The activated T-cells differentiate into two populations:

• Cytotoxic T-cells- find cells that express a particular antigen and trigger the death of those cells.

• Memory cells- remain viable and circulating in the lymphatic system in case of future exposure to the same antigen.

Immune response to populations of body cells

Depends on the nature of the disease

Phagocytes

Phagocytes!

Phagocytes will present antigens to Helper T-Cells.

Helper T-cells will activate specific B-cells.

The activated B-cells differentiate into two populations:

• Plasma cells- make and secrete antibodies into the circulatory system
• Memory cells- remain viable and circulating in the lymphatic system in case of future infection by the same pathogen.

Skin presents a water-proof barrier to infection.

Sweat glads make the surface of the skin inhospitable to many microorganisms.

There is a a population of hundreds of commensal species of microorganisms that live on the skin and occupy space that pathogens might otherwise occupy.

The T-cell receptor is similar to the B-cell receptor

Treatment

Populations of white blood cells that patrol the circulatory and lymphatic system.

They phagocytize and digest any material that they do not recognize.

Following phagocytosis, pieces of pathogens are presented to the specific immune system for possible specific response.

Immunosuppresant drugs (sometimes)

Shown: Rheumatoid arthritis, a disease in which the immune system causes inflammation and destruction of connective tissue in the joints of the body.

B-cell receptors and antibodies

B-cells:

• mature in the bone marrow.
• responsible for humoral (antibody) response

T-cells:

• mature in the thymus gland.
• responsible for cell-mediated response

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AIDS

Structurally similar.

Consist of 2 heavy chains and 2 light chains.

Both chains have a constant region and a variable region.

The structure of the variable region is specific to a particular antigen

Symptoms

Cause

Mucus

In order to respond to a particular antigen, a T-cell must interact with a cell that is presenting the antigen complexed with its "Major Histocompatibility Complex" (MHC) protein.

This is called "antigen presentation".

The MHC/Antigen complex is expressed in all phagocytes, cells infected by a particular pathogen, and many cancer cells.

Infection by the HIV virus, which infects Helper T-Cells.

Decrease in Helper T-cell population.

Lack of immune response to "opportunistic infections".

Treatment

Mucus is disgusting and sticky for a reason.

Mucus lines all mucous membranes in the body (which are not as impenetrable as skin).

Mucus contains lysozyme (as do most other fluid secretions) which disrupts bacterial cell walls.

The structure of the antibody gene consists of multiple exons that are randomly combined as B-cells differentiate.

One gene can produce millions of different antibody protein variations.

Antiviral drug therapies.

Palliative care for complications.

The Inflammatory Response

Ciliated Epithelium

When a specific antigen is presented to undifferentiated B-cells, they begin to produce different antigen receptors.

Each B-cell that is produced is screened against the antigen. Only B-cells with reactive antigen receptors are allowed to divide ("clonal selection").

Clonal Selection is Amazing!

The epithelial lining of the respiratory track is highly ciliated.

Together with mucus, this serves as a trap for many pathogens, which are then pushed out of the respiratory tract and into the digestive system.

Both the humoral and cell-mediated immune responses rely upon the action of Helper-T cells to begin the process.

Together, both responses protect the organism against pathogens, infected cells, and cancer cells.

Following recognition, Cytotoxic T-cells will insert a perforin complex into the membrane of cells that express the MHC/antigen complex, which triggers cell death via lysis or apoptosis.

Once a B-cell that has an antigen receptor specific for the antigen is produced, it founds a population of plasma cells which secrete plasma antibodies.

Occurs whenever the skin is ruptured, or the body is otherwise infected.

Cells at the site of the infection will release signaling molecules that recruit populations of phagocytic cells to the area.

These signaling molecules will also increase blood flow to the area, which manifests as swelling and redness.

Vaccination

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Vaccination!

Toll-Like Receptors

Vaccines are a way to expose the immune system to antigens without exposing the body to a functional pathogen.

First discovered by Edward Jenner for smallpox.

Arguably The single most important advancement in human health, ever.

Receptors present on the surface of phagocytes that respond to molecules found in fungi, bacteria and viruses, but NOT found in animal cells.

Antibodies have 3 major effects on pathogens:

• Opsonization: The pathogen is tagged by the antibody for phagocytosis.
• Neutralization: The antibody prevents the pathogen from infecting more cells.
• Complement activation: The complement system forms pores in the pathogen's cell membranes, which triggers lysis.

Unfortunately

Other Organisms

Invertebrates posses a series of antimicrobial peptides that activate Toll-receptors and trigger immune responses.

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Fungi & Plants

Prokaryotes

Vaccines are powerful, but there are many pathogens that can get around vaccination by shifting the structure of their antigens ("antigenic shift").

After the initial exposure ("primary response") to a particular antigen, memory cells remain in the body for subsequent exposures ("secondary response") to the same antigen.

Fungi and Plants rely on the production of a wide variety of chemicals that can cause unpleasant effects in would-be pathogens and predators.

It is thought that restriction enzymes protect prokaryotes from bacteriophage infections.

The "Death Cap" mushroom produces alkaloid chemicals that cause irreversible liver failure in humans.

The Penicillium genus of fungi produce antibiotics (like penicillin) to protect against bacterial infections.

a. By methylating their own restriction sites, prokaryotes can protect against phage infection.

b. Of course, phages can evolve mechanisms to evade prokaryotic defenses.

The diversity of chemicals that plants can produce in response to pathogens is remarkable!

Plants have systemic mechanisms to prevent the spread of viral infections