Pathogen Types: Bacteria, Viruses, Fungi and Parasites

Types of Pathogens

• Bacteria are single-celled organisms that can multiply quickly with Salmonella and E. coli infections as examples. Entry occurs through contaminated food/water or touching infected surfaces.
• Viruses are infectious agents needing a host to reproduce like Norovirus (stomach flu) and Influenza (flu). They enter by inhaling airborne particles or touching the mouth, nose, or eyes after touching infected surfaces.
• Fungi are organisms like mold and yeast, leading to infections like Candida (yeast infections) and Tinea (athlete's foot), most often contracted by wearing damp clothing or walking barefoot.
• Parasites live on or inside hosts, with Giardia lamblia (causing diarrhea) and Plasmodium (malaria) as examples. Entry occurs by drinking untreated water or via insect bites (e.g., mosquitoes).
• Protists are single-celled organisms spreading through water, like Giardia (intestinal infection) and Plasmodium (malaria), entering the body through contaminated water or insect bites.
• Prions are abnormal proteins causing rare brain diseases, such as Creutzfeldt-Jakob disease (CJD), which affects the nervous system. Entry includes eating infected meat, such as from animals with mad cow disease.

Jungle Rot (Bacterial/Fungal Infection)

• Bacteria (Staphylococcus, Streptococcus) or Fungi (Trichophyton, Candida) can cause jungle rot.
• Breaks in the skin (cuts, blisters, abrasions) exposed to humid, dirty environments provide an entry pathway.
• The infection causes skin ulcers, sores, and infections, possibly leading to tissue breakdown; severe cases spread to deeper tissues, causing systemic illness.
• The immune system responds through white blood cells, causing inflammation, pus formation, and swelling.
• Untreated infections overwhelm the immune system and leading to cell death, sepsis, or gangrene.

Common Cold (Viral Infection)

• Viruses like Rhinovirus, Coronavirus, and Adenovirus are common pathogens
• Respiratory droplets from sneezing, coughing, or talking and touching contaminated surfaces, then the eyes, nose, or mouth are the entry pathways.
• Symptoms such as runny nose, sore throat, sneezing, and congestion occur when the virus infects mucous membranes of the nose and throat, triggering inflammation.
• Cytokines released by white blood cells cause fatigue and a mild fever, the immune response eventually clears the virus in about 7-10 days.

First Line of Defense: The Skin

• The skin physically blocks pathogens and uses biochemical processes to destroy or repel them.

Physical Protection

• The epidermis, the outermost layer, has tightly packed, dead cells filled with keratin to make it tough and waterproof.
• The continuous shedding of skin removes pathogens that stick to it.

Biochemical Protection

• Sebum contains fatty acids that maintain a low skin pH (~5.5), which inhibits bacterial growth.
• Sweat contains salt and lysozymes that break down bacterial cell walls.

Role of Each Layer in Defense

• Epidermis (Outer Layer): Acts as a barrier; shedding removes attached microbes.
• Dermis (Middle Layer): Provides structural support, mounts immune responses using macrophages and dendritic cells, and houses blood vessels
• Hypodermis (Deepest Layer): Absorbs shock, provides insulation, and regulates immune responses by storing immune cells.

Second Line of Defense: Innate Immune Response

• The innate immune response is immediate and non-specific
• Inflammation: Blood flow increases, bringing immune cells to the site.
• Phagocytes (Macrophages & Neutrophils) engulf and destroy invaders.
• Fever raises body temperature to slow pathogen growth.
• Antimicrobial proteins (Interferons, Complement System) help fight infections.

Third Line of Defense: Adaptive Immune Response

• The adaptive immune system activates if a pathogen survives the second line of defense and creates immunity.
• B Cells (Humoral Response) produce antibodies that neutralize pathogens, and memory B cells ensure a faster response if the pathogen returns.
• T Cells (Cell-Mediated Response): Helper T cells activate B cells and other immune cells, and cytotoxic T cells destroy infected cells.

Effects of Burns on Skin Defense Layers

• 1st Degree: Affects the epidermis, with a slight loss of barrier and mild inflammation, but the immune response remains intact.
• 2nd Degree: Affects the epidermis and part of the dermis; blisters form, and open wounds increase infection risk.
• 3rd Degree: Skin is destroyed from the Epidermis to the Dermis and Hypodermis, with a severe infection risk due to the loss of the barrier and immune cells.
• 4th Degree: Affects all skin layers, including muscle, bone, and tendons; an absence of immune defense leads to a risk of high infection and dehydration.

How Inflammation and Fever Defend the Body

• Inflammation occurs when the body detects injury or infection; it contains the threat, removes damaged cells, and begins healing.
• Blood vessels widening (vasodilation) causes more immune cells to reach the area.
• Increased cell permeability allows white blood cells (like neutrophils) to enter tissues to fight the damage.
• Inflammation traps and eliminates pathogens, removes damaged tissue, and promotes healing.
• A fever is when the immune system raises the body's temperature to slow down infections, controlled by the hypothalamus.
• Pyrogens signal the hypothalamus to increase body temperature, and a higher temperature slows bacterial and viral growth.
• Fever increases the production of interferons and makes white blood cells work more efficiently.

Sepsis

• Sepsis is a life-threatening condition with widespread inflammation, organ failure, and dangerously low blood pressure caused by the body's response to an infection.
• Early symptoms include fever/low temperature, fast heart rate, rapid breathing, and confusion.
• Severe symptoms are low blood pressure, reduced urination, difficulty breathing, and cold/clammy skin.
• Septic Shock leads to organ failure, unresponsive low blood pressure, and unconsciousness.
• Untreated local infections spread to the bloodstream (bacteremia), leading to widespread inflammation and leaky blood vessels.
• Severe sepsis results in oxygen deprivation causing organ failure, septic shock leads to multiple organ failure and death.
• Treatment requires immediate IV antibiotics, fluids, oxygen, and possibly surgery. Septic shock has a high mortality rate of ~40-60%, making early detection critical.

What is Lymph?

• Lymph is a clear to slightly yellowish fluid that circulates through the lymphatic system, similar to blood plasma but lacking red blood cells.
• Lymph contains lymphocytes (T cells & B cells) to fight infections and carries proteins & nutrients, waste & toxins, and excess fluid.
• Lymph moves via skeletal muscle contractions, valves in lymphatic vessels, respiratory movements, and smooth muscle contractions.

What is a Lymph Node?

• Lymph nodes contain B and T lymphocytes and act as traps for foreign invaders and cancer.
• Enlargement occurs when fighting infection
• Fluid circulates in lymphatic vessels and returns to the vena cava after filtering.

Lymphatic System

• A network of vessels that interact with the cardiovascular system (releases fluid into interstitial space).
• Lymphatic vessels carry lymph fluid (blood plasma with WBCs) toward the heart.
• Fluid is absorbed by vessels, filtered, and redistributed to blood.
• Lymphocytes (WBCs) are made and circulated.
• Lymph nodes are rounded masses with lots of lymphocytes to filter lymph fluid.
• Lymph vessels empty into ducts that drain into veins.

Primary Lymph Organs

• The thymus has high concentrations of WBCs that can destroy toxins.
• Bone marrow produces blood cells, including leukocytes.

Secondary Lymph Organs

• Secondary lymph organs localize antigens and activate the specific immune system.
• The spleen filters blood, cleans it of bacteria/viruses, and destroys old red blood cells, and also acts as a platelet and blood storage.
• Lymph nodes remove foreign material from the lymphatic stream and produce lymphocytes.
• Tonsils remove bacteria and foreign pathogens entering the throat.
• Lymph vessels return excess fluid from blood (lymph) and tissue to the bloodstream.
• the filters blood and removes substances, fights infection and disease.

How Does a Lymph Node Function?

• Lymph nodes are filters for the lymphatic system, trapping pathogens or activating an immune response, containing white blood cells (WBCs) such as macrophages, T cells, and B cells to fight infections.
• Lymph fluid enters the lymph node through afferent vessels, where macrophages and dendritic cells capture and break down pathogens.
• Next, T cells and B cells recognize antigens to initiate an immune response, resulting in B cells producing antibodies for infections.
• Clean lymph exits via efferent vessels and returns to circulation.

Where Are Lymph Nodes Found?

• Lymph nodes are located throughout the body, with major clusters in the cervical (neck), axillary (armpits), inguinal (groin), mesenteric (abdomen), and popliteal (behind the knees) areas.
• Cervical nodes fight infections in the head & throat.
• Axillary nodes protect the chest, arms, and upper body.
• Inguinal nodes defend against infections in the legs & lower body.
• Mesenteric nodes monitor the intestines.
• Popliteal nodes filter lymph from the legs.

Why do lymph nodes get swollen?

• Lymph nodes swell (lymphadenopathy) when they are actively fighting an infection or reacting to an immune response.
• Causes include bacterial, viral, fungal & parasitic infections, autoimmune diseases (Lupus, rheumatoid arthritis), and cancer (Lymphoma, metastasis from other cancers).
• Swelling occurs b/c immune cells multiply rapidly to fight the infection, causing inflammation.

Key Parts of a Lymph Node:

• Afferent Lymphatic Vessels: Bring lymph into the node.
• Capsule: Protective outer layer.
• Subcapsular Sinus: The first filtering region.
• Cortex: Contains B cells in germinal centers.
• Paracortex: Contains T cells for antigen recognition.
• Medulla: Filters debris and contains macrophages.
• Efferent Lymphatic Vessel: Carries filtered lymph out.

Lymph Pathway (How Lymph Travels)

• Tissue Fluid → Lymphatic Capillaries (absorbs excess fluid).
• Lymphatic Vessels → Lymph Nodes (filters pathogens).
• Larger Lymphatic Trunks (thoracic or right lymphatic duct).
• Subclavian Vein → Bloodstream (returns clean fluid to circulation).

What is a Leukocyte?

• A leukocyte (WBC) defends the body against infections, foreign invaders, and abnormal cells as part of the innate and adaptive immune systems.
• Leukocytes are made in bone marrow and found in the blood, lymphatic system, and tissues. Their lifespan varies from hours (neutrophils) to years (memory cells).

How Do Leukocytes Protect the Body?

• Recognizing invaders: Leukocytes detect pathogens (bacteria, viruses, fungi, parasites) or abnormal cells (cancer).
• Destroying threats: Some WBCs engulf invaders (phagocytosis), others release toxic chemicals to kill infections, and some produce antibodies to tag and neutralize pathogens.
• Remembering Past Infections: Memory cells store information for a faster response in the future.

Types of Leukocytes & Their Functions

• Neutrophils are the first responders that engulf & kill bacteria and fungi.
• Lymphocytes (T & B Cells): T cells kill infected cells; B cells make antibodies.
• Monocytes differentiate into Macrophages, which engulf pathogens and dead cells and activate other WBCs.
• Eosinophils attack parasites and are involved in allergic reactions.
• Basophils and Mast Cells release histamine, trigger inflammation, & allergic responses.

Summary of Leukocytes

• Leukocytes (WBCs) defend the body against infections & abnormal cells by engulfing/killing invaders.
• Different types specialize in bacteria, viruses, parasites, and allergies, and memory cells help create long-term immunity.

Relationship between Lymphatic and Cardiovascular Systems

• They work together to maintain fluid balance and immunity.
• During fluid exchange, blood plasma leaks out from capillaries into tissues (interstitial fluid); the excess fluid enters lymphatic capillaries, becoming lymph.
• Lymph rejoins the bloodstream via the subclavian veins, it transports nutrients and waste, and works with the cardiovascular system to return fluid and fight infections.
• The lymphatic system screens fluid for pathogens while the cardiovascular system delivers immune cells through the blood.

Memory Cells: The Key to Long-Term Immunity

• Are white blood cells which facilitates the immune system to recognize past infections allowing the body to respond faster if the same returns.
• Store memory pathogens, activate cytotoxic T cells → find infected cells to destroy.
• Memory B Cells: store memory pathogens and activates plasma B cells→ antibodies

Antigen vs Antibody

• ANTIGENS are recognized as a foreign body and proteins found on the outside of pathogens
• "Disease starters” from viruses and disease-causing bacteria are unique to each pathogen.
• Lymphocytes recognize antigens as foreign to produce antibodies specific to them.
• ANTIBODIES are proteins specific to antigens, produced by B-Cells.
• Antibodies destroy the antigen and is consumed by macrophages, while memory cells remember past pathogens making antibodies if attacked again, giving immunity.

Nonspecific Defense System

• The first line of defense includes skin-barrier (pH and sebum), mucous and reflex membranes, saliva, tears (enzymes), normal gut flora.
• The second line of defense includes white blood cells (phagocytes), inflammation, fever, and antimicrobial proteins.
• Phagocytic cells surround and kill pathogens by engulfing and antimicrobial proteins-kill bacteria

Specific Defense System-The Immune system

• Attacks foreign substances that make it passed the nonspecific defenses using lymphatic tissues and body fluids.
• Lymphocytes: Mature in B and T lymphocytes in the lymph nodes and spleen and identify pathogens.
• Antibodies fight pathogens that provide long-term immunity.
• Involves B cells and T cells to recognize and attack pathogens: B cells make antibodies, destroys infected cells

Cell-Mediated vs. Humoral Immune Response (Third Line of Defense)

• Humoral (B lymphocytes): produce antibodies in response to antigens which deactivated and are destroyed marked by cells (macrophages and killer cells)
• B cells “remember” specific pathogens in the memory cells
• Cell Mediated-T cells (T Lymphocytes): cytotoxic to kill pathogens in flagged cells and helper cells stimulate other immune system cells
• Different functions: T Cells fight infections inside cells while B Cells fight infections in body fluids
• B Cells use antibodies T cells kill infected cells directly

Acquired Immunity Defense

• Active: Acquired after infection and recovery or from a vaccination
• Passive: Acquired by a child from their mother via the placenta and breastfeeding.

Stages of Illness

• Incubation Stage (Silent Phase): The pathogen enters and multiplies in your body, but you don't feel sick yet. Duration varies by disease.
• Prodromal Stage (Early Symptoms): Mild symptoms (fatigue, aches, slight fever) appear as your immune system starts responding. You are contagious.
• Illness Stage (Peak of Sickness): Full symptoms develop (fever, coughing, sore throat). The pathogen is at its strongest, and your immune system fights hardest.
• Decline Stage (Recovery Begins): Your immune system starts overcoming the infection. Symptoms lessen, but weakness and contagiousness may persist.
• Convalescence Stage (Full Recovery): Your body heals, energy returns, and memory cells form for future protection. Immunity depends on the disease.

Tears, mucus, cilia, stomach acid, gut and skin flora, saliva?

• First-line defense is innate and consists of physical, chemical, and biological barriers that prevent pathogens from entering the body
• Tears: contain lysozyme to break down and wash away bacterial cell walls and pathogens.
• Mucus: traps bacteria, viruses, and dust, preventing from reaching deeper tissues.
• Cilia: tiny hair-like cells in the respiratory tract sweep mucus to remove pathogens of lungs
• Stomach Acid (HCI): kills ingested bacteria so it prevents infections
• Gut skin beneficial bacteria kills harmful microbes: this is a prevention
• Saliva: includes lysosomes to wash and prevent bacteria from growing

Pathway of Light Into the Eye & How the Brain Processes Vision

• Cornea: Light enters eye and bends light to help focus it.
• Aqueous Humor: Light passes through clear fluid that nourishes the eye and maintains pressure.
• Pupil & Iris: Controls the amount of light being adjusting by the irs expanding through pupil.
• Lens: Focuses image by changing shape so it helps land correctly on retina.
• Vitreous Humor: Light moves through gel-like substance that helps shape.
• Retina (Photoreceptors): Retina contains rods (for low-light, black-and-white vision) and cones (for color vision).
• Optic Nerve is signals going to your brain.
• Brain (Visual Cortex in the Occipital Lobe): Brain flips image right-side up, processes details like shape, color, and movement, and combines both eyes' input for depth perception.

How do eye structures protect?

• Cornea & Sclera: Protects and helps focus light
• Eyelids, Eyelashes & Tears: Keep out dust, debris, and bacteria.
• Pupil & Iris: Controls light to enters
• Lens & Ciliary Muscles: Focuses light onto the retina for clear vision.
• Retina (Rods & Cones): Detect light and color, sending signals to the brain.
• Optic Nerve: Transmits visual information to the brain.
• Vitreous Humor: Maintains eye shape and absorbs shocks.
• Protective Reflexes: Prevents infections and damage such as tear and blinking.

Optometrist, Optician, and Ophthalmologist

• Optometrist is a doctor of optometry: eye exams , diagnoses conditions (glasses, infection) and can not treat surgery.
• Optician gets certificate or associate degree: fit glasses and prescribes prescriptions from optometrist or ophthalmologists but cant diagnose or treat
• Ophthalmologist is a doctor to diagnoses and treats eye problems or surgery (removing of lens): specializes in retinal diseases