Immunity and Defense Mechanisms: Key Concepts in Microbial Resistance

Immunity

Ability to ward off disease

Susceptibility

Lack of resistance to a disease

Innate immunity

Defenses against any pathogen; rapid, present at birth

Adaptive immunity

Immunity or resistance to a specific pathogen; slower to respond, has memory component

First line of defense

Skin and mucosal membranes

Skin

Largest organ of the body, or largest and most impactful protection against microbes

Mucosal membranes

Trap and prevent microorganism entry into the respiratory and digestive canal

First line of defense: Physical factors

Physical factors of defense include skin and mucosal membranes.

Lacrimal apparatus

Drains tears and washes the eye.

Epiglottis

Prevents microorganisms from entering the lower respiratory tract.

Earwax

Prevents microbes from entering the ear.

Urine

Cleans the urethra via flow.

Vaginal secretions

Move microorganisms out of the vaginal tract.

Peristalsis, defecation, vomiting, diarrhea

Mechanisms that help expel pathogens from the body.

First line of defense: Chemical factors

Chemical factors include sebum, lysozyme, and low pH environments.

Sebum

Forms a protective film and lowers the pH of skin.

Lysozyme

Destroys bacterial cell walls and is found in perspiration, tears, saliva, and urine.

Low pH of gastric juice

Destroys most bacteria and toxins in the stomach.

Low pH of vaginal secretions

Inhibits microbes.

First line of defense

Microbiome

Microbial antagonism

Normal microbiota compete with pathogens via competitive exclusion

Competitive advantage

For space and nutrients

Normal microbiota functions

Produce substances harmful to pathogens

Normal microbiota role

Alter conditions that affect pathogen survival

Normal microbiota prevention

Prevent the overgrowth of harmful microbes

Normal microbiota and immunity

Play an important role in the development of the immune system

Commensalism

One organism benefits while the other (host) is unharmed

Opportunistic pathogens

E. coli, S. aureus, S. epidermidis, Enterococcus faecalis, Pseudomonas aeruginosa

Probiotics

Live microbial cultures administered to exert a beneficial effect

Prebiotics

Chemicals (nutrients) that selectively promote the growth of beneficial bacteria

Probiotics and upset stomach

Studies show that taking probiotics due to upset stomach relieves symptoms

Second line of defense

Commonly associated with inflammation and fever, also includes phagocytes and antimicrobial substances.

Phagocytes

Defensive cells that ingest microorganisms or other substances.

Phagocytosis

The ingestion of a microorganism or other substance by a cell.

Neutrophils

Highly phagocytic; most active in early stages of infection; can leave the blood and enter infected tissue, first responders.

Fixed macrophages

Residents in tissues and organs (liver, lungs).

Free (wandering) macrophages

Roam tissues and gather at sites of infection, acting as the clean-up crew.

What does the second line of defense target?

It targets pathogens and foreign substances through innate immunity.

What are Natural Killer (NK) Cells responsible for?

Killing infected cells by releasing perforin, causing their membranes to perforate and rupture.

What types of cells do NK Cells target?

Cells infected by a virus and cancer cells.

How do NK Cells recognize their target cells?

By altered protein expression on the surface of infected or disturbed cells.

Why are NK Cells considered part of innate immunity?

Because they operate without antigen-specific cell surface receptors.

What is the distinction between leukocytes and lymphocytes?

Leukocytes are white blood cells that protect against illness and disease, while lymphocytes are a specific type of leukocyte.

What role do phagocytes play in the immune system?

Phagocytes are a type of leukocyte that helps protect the body against illness and disease.

WBC count

Sign and degree of infection can be monitored by WBC count

Healthy range of WBC for adults

4,000-11,000 WBC per ul of blood

Healthy range of WBC for children

5,000 to 19,000 WBC per ul for children 2 months to 6 years old

Leukopenia

Low white blood cell count

Leukocytosis

High white blood cell count

Inflammation

Local or systemic defensive response triggered by damage to tissues

Causes of tissue damage

Microbial infection, physical agents (heat, electricity, sharp objects), or chemical agents (acids, bases, gases)

Signs and symptoms of inflammation

Pain, redness, immobility, swelling (edema), heat

Effect of dilated blood vessels

More heat

Increased permeability of blood vessels

Permits leucocytes and defensive chemicals through

Acute inflammation

Develops rapidly, lasts for few days to few weeks.

Chronic inflammation

Develops more slowly, lasts months to years; may be severe and progressive.

Functions of inflammation

Destroys injurious agent or limits its effects on the body; repairs and replaces tissue damaged by the injurious agent.

Fever

Abnormally high body temperature (38C or higher, 100.4F or higher).

Hypothalamus in fever

Driven by cytokines that reset hypothalamus to create a fever until cytokines are eliminated.

Cytokines

Signaling proteins regulating immunity and inflammation; can be produced by immunity or infected cells.

Concerning fever temperatures

Temperature of 103F is concerning; 104F should be treated.

Hydration during fever

Important to drink water and electrolytes.

What is considered a defense mechanism in the second line of defense?

Fever

How do phagocytes and T cells respond to higher temperatures?

They work better at a slightly higher temperature.

What effect does a higher temperature have on antimicrobial substances?

It intensifies the effect or production of other antimicrobial substances.

What is one benefit of higher temperatures in the context of pathogens?

Higher temperatures may slow the growth of pathogens.

How does an increased metabolic rate during fever affect the body?

It speeds up repair processes.

What are some complications associated with fever?

Tachycardia, acidosis, dehydration, seizures, coma.

What role does the complement system play in the second line of defense?

It contributes to inflammation, promotes phagocytosis, and other innate immune responses to target microbes.

What are interferons?

A group of signaling proteins made and released by host cells in response to the presence of several viruses.

What is the function of iron-binding proteins in the immune response?

They restrict iron from pathogens, interfering with their metabolic processes.

What are antimicrobial peptides?

Short peptides produced in response to protein and sugar molecules on microbes, with a broad spectrum of activity.

Genetic resistance

Confers a selective survival advantage

Sickle cell trait and Plasmodium falciparum

Example of genetic resistance related to malaria

Age

Very young and elderly are more susceptible to disease

Healthy protocols

Practices such as hand hygiene, sneeze and cough hygiene, safer sex practices

Adaptive immunity

Defenses that target a specific pathogen after exposure

Ability to distinguish 'self' from 'nonself'

Characteristic of adaptive immunity

Primary response

First time the immune system combats a particular foreign substance

Secondary response

Later interactions with the same foreign substance; faster and more effective due to 'memory'

What type of immunity produces antibodies?

Humoral immunity

What are antibodies designed to combat?

Foreign molecules known as antigens

What type of cells are B cells?

Lymphocytes (a type of white blood cell)

Where are B cells created and matured?

In red bone marrow

What do antigens cause the production of?

Antibodies

What are some examples of antigens?

Components of invading microbes or foreign substances such as capsules, cell walls, flagella, fimbriae, toxins, viral capsids, and viral spikes

What are epitopes in relation to antigens?

Specific regions (antigenic determinants) on the antigen that antibodies interact with

What type of immunity is associated with T cell lymphocytes?

Cellular immunity (cell-mediated immunity)

Where do T cell lymphocytes mature?

In the thymus

What do T cell receptors (TCRs) recognize?

Antigenic peptides processed by phagocytic cells

What is the primary function of T cells in the immune response?

To target infected cells and secrete cytokines

What are cytokines?

Protein messengers produced in response to a stimulus

What role do interleukins (ILs) play in the immune system?

They communicate between leukocytes

What is the function of chemokines?

To induce migration (chemotaxis) of leukocytes toward infection

What do interferons (IFNs) do?

They interfere with viral infections of host cells

Cytotoxic T cells

Target and kill infected cells as well as destroy tumor cells, activated by antigens.

Helper T cells

Signal other cells to attack pathogen; most important T cell.

Suppressor T cells

Regulate and reduce activity of other T cells as needed.

Humoral immunity

Fights invaders and threats outside cells, such as extracellular bacteria and toxins.

Cellular immunity

Attacks antigens that have already entered cells, such as viruses and some intracellular bacteria.