The Immune System

Chapter 37

Infectious Diseases

Section 1

Pathogens Cause Infectious Disease

• An infectious disease is caused when a pathogen is passed from one organism to another, disrupting homeostasis in the organism’s body.– Agents called pathogens are the cause of infections.

Some but not all types are bacteria, viruses, protozoans, fungi and parasites.

Spread of Disease

• For a pathogen to spread, it must have both a reservoir and a way to spread.

• A disease reservoir is a source of the pathogen in the environment.

• Reservoirs might be animals, people or inanimate objects such as soil.

Human Reservoirs

• Humans are the main reservoir for pathogens that affect humans.

• They might pass indirectly or directly from other humans.

• Pathogens that cause colds, influenza and sexually transmitted diseases, such as HIV can be passed without a person knowing he or she is infected.

Animal Reservoirs

• Influenza and rabies are examples of human disease that are caused by pathogens passed to humans from other animals. – Influenza can affect pigs and various types of birds.– Rabies can infect domestic dogs and many wild

animals such as bats, foxes, skunks and raccoons.

Other Reservoirs

• Some bacteria found only in soil contains tetanus bacteria.– This can be found in deep wounds.

• Contaminated water or food is another reservoir for pathogens.– Sewage problems– Food left out on the counter in contact with flies.

Transmission of Pathogens• Direct Contact:

– Major transmission of pathogens. Diseases such as colds, infectious mononucleosis, herpes and sexually transmitted diseases (STDs) are caused by direct contact.

• Indirect Contact– Some pathogens can be passed indirectly through air. When a

person coughs or sneezes they may pass along tiny mucous droplets. By cleaning dishes, utensils and countertops with detergent as well as washing hands can severely limit the exposure to pathogens.

• Vectors– Most common vectors are arthropods, which include biting insects

such as mosquitoes and ticks. Diseases like Lyme disease, malaria and West Nile are spread by mosquitoes.

Symptoms of Disease

• After replication of the infected disease it can kill living cells and release harmful toxin chemicals into the bloodstream.

• Outward symptoms may include fever, chills, shortness of breath, rash, sneezing and coughing to name a few.

Treating & Fighting Diseases

• A medical professional may prescribe a drug to help the body fight a disease.

• One type of prescription drug is an antibiotic. – It is a substance that can kill or inhibit the growth of

other microorganisms. – However in the last 60 years bacteria have become

resistant to antibiotics.

The Immune System

Section 2

Nonspecific Immunity

• At the time of birth the body has a number of defenses in the immune system that fight off pathogens.

• These defenses are nonspecific because they are not aimed at a specific pathogen.

• Nonspecific immunity helps slow the progression of the disease, it is the first line of defense.

(Examples of Non-Specific) Barriers• Skin:

– The simplest way of preventing foreign organisms from entering the body.

– This major line of defense is the unbroken skin and its secretions. – By forming a barrier, the layers of dead skin cells help protect against

invasion by microorganisms.• Chemical:

– Saliva, tears, and nasal secretions contain the enzyme lysozyme, which breaks down bacterial cell walls which kills pathogens.

– Mucus blocks airways within the body which stimulates coughing and sneezing. Eventually it is eliminated from the body.

– Hydrocholric acid from the stomach aids in digestion, but also kills many microorganisms found in food that could cause disease.

Nonspecific responses to invasion• Cellular defense:

– One cellular defense is phagocytosis. • White blood cells, especially neutrophils and macrophages, surround and internalize

foreign microorganisms. The phagocytes release digestive enzymes and other harmful chemicals which destroy the pathogen.

– Complement proteins also aid in the process.

• Interferon: – Virus infected cells secrete a protein called interferon, which binds to

neighboring cells and stimulates these cells to produce antiviral proteins which can prevent viral replication.

• Inflammatory response:– When pathogens damage tissue, chemicals are released by both the

invader and the cells of the body.– These chemicals attract phagocytes to the area, increase blood flow and

make blood vessels more permeable to allow WBC’s to escape to the infected area. In this area there might be pain, heat and redness.

Specific Immunity• Lymphatic system: – It includes organs and cells that filter lymph and blood and destroy foreign

microorganisms. – It also absorbs fat.– Lymph is the fluid that leaks out of capillaries to bathe body cells.

• Lymphatic organs:– Lymphocytes are a type of WBC that is produced in red bone marrow. – Lymphatic organs include: lymph nodes, tonsils, spleen, thymus and

diffused lymphatic tissues found in mucous membranes in the intestine, respiratory, urinary and genital tracts.

– The lymph nodes filter the lymph and remove foreign materials from lymph.

– The spleen stores blood and destroys damaged RBC’s.– The thymus gland produces T-cells.

B Cell Response

• Antibodies are proteins produced by B lymphocytes that specifically react with a foreign antigen.– An antigen is a substance foreign to the body that causes an

immune response; it can bind to an antibody or a T-cell.• In the lymph nodes, the macrophage, with the

processed antigen on its surface, binds to a type of lymphocyte called a helper T cell.– The helper T cell activates antibody secretion in B cells and

another type of T cell.

What the helper T-cell does…• The activated helper T cell reproduces, binds processed

antigens, and attaches to a B cell.• The new helper T cell continues the process of binding

antigens, attaching to B cells, and reproducing.• Once an activated helper T cell binds to a B cell holding

an antigen, the B cell begins to manufacture antibodies that specifically bind to the antigen.

• The antibodies can enhance the immune response by binding to microorganisms, making them more susceptible to phagocytosis and by initiating the inflammatory response.

T Cell Response

• Once helper T cells are activated by an antigen, helper T cells can also bind to and activate a group of lymphocytes called cytotoxic T cells.

• Cytotoxic T cells destroy pathogens and release chemicals called cytokines.– This chemical stimulates the cells of the immune

system to divide and recruit immune cells to an area of infection.

Passive Immunity

• Temporary protection occurs when antibodies are made by other people or animals and are transferred or injected into the body.

• Passive immune therapy is available for people who have been exposed to hepatitis A and B, tetanus, and rabies.

Active Immunity

• Active immunity occurs after the immune system is exposed to disease antigens and memory cells are produced.– Memory cells are long-living cells that are exposed to the

antigen during the primary immune response. These cells help protect the body by reducing the likelihood of developing the disease if exposed again to the same pathogen.

• Immunization (vaccination) is the deliberate exposure of the body to an antigen so that a primary response and immune memory cells will develop.

Immune System

• The immune system and the circulatory system work together to defend the body against antigens.

Properties of Viruses･ no membranes, cytoplasm, ribosomes, or

other cellular components･ they cannot move or grow･ they can only reproduce inside a host cell

(therefore not considered “living”)･ they consist of 2 major parts - a protein

coat(capsid), and hereditary material (DNA or RNA)･ they are extremely tiny, much smaller than a

cell and only visible with advanced electron microscopes

Parasytic Nature

･ Obligate intracellular parasites

･ Specific to their hosts (human, dog, some can cross species)

･ They can only attack specific cells.

-The common cold is a virus that specifically attacks cells of the respiratory track (hence the coughing and sneezing and sniffling). HIV virus specifically attacks white blood cells

Active Immunity: Vaccines

• Artificially acquired active immunity

• Consists of: dead or weakened bacteria or viruses, OR a toxoid (a toxin from an infectious organism that is chemically altered to avoid toxic effects)

• Vaccines are typically injected or inhaled

Vaccines• Include antigens that stimulate an immune

response without producing severe symptoms of the disease

• Vaccines work by stimulating antibody production without causing disease

• Common Vaccinations: typhoid fever, cholera, whooping cough, tetanus, polio, chickenpox, measles, mumps, rubella, influenza, hepatitis (A & B), bacterial pneumonia

Immune System Failure

• The two most likely diseases to decrease the effectiveness of the immune system are HIV and AIDS.

– HIV- Human immunodeficiency virus

– AIDS- Acquired immunodeficiency syndrome• When infected with HIV the CD4+ cells (like helper T

cells) become HIV factories. • Over time, the number of helper T cells in an infected

person decreases, making a person less able to fight disease.