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Immune System

Protects from bacterial and viral pathogens, toxins, even cancerous cells


Immunity in Cellular Slime Molds

Composed of many individual amoeboid cells living in unison as a “slug”
Sentinel cells – circulate throughout the slug and engulf bacteria and toxins
Eventually remove themselves from the body of the slug


Immunity in Drosophila

Contain cellular receptors capable of recognizing common components of pathogenic microbes
Pathogen-associated molecular patterns (PAMPs)
Trigger an immune reaction
Receptors for PAMPs found in diverse organisms.
May have been one of earliest cellular receptors that evolved for pathogen recognition


innate immunity!

-Recognizes microbial invaders quickly, but shows no signs of an increased response upon repeated exposure
Both Immunity in Drosophila & Immunity in Cellular Slime Molds illustrate a type of defense


Adaptive immunity

Results in the production of receptors on surface of white blood cells that bind to a foreign antigen
Stimulates lymphocytes to increase in number, resulting in an increased response to specific antigens and immunological memory
Originally developed in an ancestor that gave rise to the jawed vertebrates
Precise mechanism causing adaptive immunity in ancestor not known



The capability of removing or killing foreign substances, pathogens, and cancer cells from the body.


Innate Defenses

Do not distinguish one type of threat from another
Are fully functional without previous exposure to invaders
Occur immediately or shortly after infection occurs


Types of innate immune defenses!

Physical and chemical barriers to entry
Inflammatory response
Phagocytes and natural killer cells
Protective proteins such as complement and interferons


Physical and Chemical Barriers

Skin and mucous membranes lining the respiratory, digestive, and urinary tracts
Cilia lining the respiratory tract sweep mucus and particles into the throat
Antimicrobial molecules in secretions of oil glands, mucous membranes, and the stomach


Examples of Antimicrobial molecules

Lysozyme, in mucus, an enzyme that lyses bacteria
Acidic pH of stomach kills microbes


Inflammatory Response

Localized tissue response to injury
Damaged cells and mast cells release histamine which causes capillaries to dilate and become more permeable.
Enlarged capillaries cause skin to redden.
Swelling stimulates free nerve endings, causing pain.
Neutrophils and monocytes migrate to the site of injury.


How do monocytes and neutrophils work in inflammatory response.

Monocytes differentiate into macrophages.
Macrophages release colony-stimulating factors, stimulating production and release of white blood cells.
Neutrophils, dendritic cells, and macrophages phagocytose pathogens



Maintenance of an elevated body temperature
In some instances, a fever may be beneficial.
It’s the body’s way of informing us that something is wrong.
Certain bacteria or viruses may not survive as well at higher temperatures.
Some immune mechanisms work better at higher body temperatures.



Leave the bloodstream and phagocytize bacteria
Release antimicrobial peptides and bacteria-digesting enzymes
Generate free radicals which kill engulfed bacteria



Phagocytic cells
Also mount an attack against parasites that are too large to be consumed via phagocytosis


Macrophages and dendritic cells

Engulf and destroy pathogens
Stimulate T cells in lymph nodes, which initiate adaptive immune responses


Natural killer (NK) cells

Large, granular lymphocytes
Kill virus-infected cells and cancer cells by cell-to-cell contact
Virus-infected cells, lacking a self molecule (MHC-1) may be recognized and killed.
Numbers don’t increase after stimulation, like lymphocytes.



A collection of plasma proteins that “complement” certain immune responses
Must be activated by pathogens


complements help destroy pathogens in three ways

Enhanced inflammation
Bind to pathogens coated with antibodies to ensure phagocytosis
Form a membrane attack complex that produces holes in the surface of some bacteria and viruses
--Fluids entering bacterial cell or virus cause bursting



Cytokines that affect the behavior of other cells
Produced by virus-infected cells
Bind to receptors of non-infected cells
Causes them to produce substances that interfere with viral replication
Used to treat certain cancers and viral infections, such as hepatitis C


Adaptive Immune Defenses

Also known as acquired immunity
Because adaptive defenses are not inborn
Take 5–7 days to become activated but last for years
Involve three steps


3 steps of adaptive immune defense!

Recognition of an antigen
Response to the antigen
Memory of the antigen



An antigen is any substance that stimulates the immune system to react.



Are capable of “recognizing” and binding to specific antigens
Have antigen receptors on their plasma membrane
The receptor protein’s shape allows it to combine with a specific antigen.
Pathogens, cancer cells, and transplanted tissues and organs bear antigens the immune system recognizes as nonself.


Adaptive immunity is primarily the result of

B cells and T cells


B cells

B-cell receptors bind directly to antigens.
B cells give rise to plasma cells.
Plasma cells produce and secrete antibodies.


T cells

T-cell receptors bind to antigens presented by antigen-presenting cells.
Helper T cells regulate specific immunity.
Cytotoxic T cells kill virus-infected cells and cancer cells.


Clonal selection theory

The antigen selects which lymphocyte will
Undergo clonal expansion
Produce more lymphocytes
Most of the cloned lymphocytes become plasma cells that produce specific antibodies.
Some of the cloned lymphocytes become memory B cells.
If the same antigen enters the system again, memory B cells quickly divide and give rise to more lymphocytes capable of quickly producing antibodies.


Antibodies (immunoglobulins)

Consist of two heavy and two light polypeptide chains in a Y shape
Both types of chains have variable and constant regions.
Neutralize pathogens by coating their antigens, preventing them from binding to receptors on cells
Attract white blood cells that move in for the kill
Immune complexes may be engulfed by neutrophils or macrophages or may activate the complement system.


Class is determined by the structure of the antibody’s constant region.!

IgG – Main type of antibody in circulation!
IgA – Main type secreted in milk, tears, and saliva
IgM – The first antibodies produced; also indicate infection
IgE – Bound to receptors on eosinophils and mast cells in tissues