Immune System Vocabulary

Pathogens

• Pathogens are foreign invaders that can cause harm.
• A pathogen is typically a living organism that enters the body.
• The host is the organism that the pathogen enters.
• Virulence is the relative degree of harm a pathogen can cause.
• Pathogens include bacteria, viruses, fungi, and parasites.

Bacterial Pathogens

• Examples include Salmonella and E. Coli, which commonly cause foodborne illnesses.
• Other bacterial infections include strep throat, C. diff, and Tuberculosis (TB).

Viral Pathogens

• Viruses are non-living pathogens.
• They consist of a DNA or RNA genome wrapped in a protein coat called a capsid.
• Some viruses have an outer envelope.
• Viruses cause illnesses such as COVID-19, the flu, and chickenpox.

Fungi and Parasites

• Common fungal infections include ringworm and yeast infections.
• Parasitic infections common in the US include tapeworms and pinworms.

Summary of Pathogens

• Pathogens come in various forms, including non-living types.
• They cause illnesses in hosts with varying degrees of severity.
• Virulence indicates how damaging a specific pathogen is.

Innate Immunity

• The innate immune system includes:
  • Physical barriers: Skin and organ mucosal layers.
  • Chemical barriers: Stomach acid and lysozymes in the eye.
  • Innate response: Inflammatory response cells.
• The adaptive immune system includes:
  • Adaptive response: Cell-mediated and humoral responses.
  • Cell-mediated response: T-lymphocytes.
  • Humoral response: B-lymphocytes.
• Immune system linkage: Dendritic cells connect the innate and adaptive immune systems.

Physical Barriers

• Skin is a resilient barrier preventing microbe entry.
• Mucous membranes lining respiratory and digestive tracts trap microbes.
• Stomach acid kills many pathogenic organisms.
• Urine flushes microbes away from the urethra.

Pathogen Recognition

• PAMPs (pathogen-associated molecular patterns) on the pathogen's surface are recognized as foreign.
• PAMPs include carbohydrate, polypeptide, and nucleic acid signatures.
• Recognition initiates an immune response.
• Immune cells engulf the pathogen and produce proteins for pathogen destruction.

White Blood Cells

• Neutrophils, eosinophils, basophils, dendritic cells, and macrophages are part of the innate immune response.
• B cells and T cells are part of the adaptive response.
• Natural Killer (NK) cells play roles in both innate and adaptive immunity.

Innate Immune System Attacks

• Phagocytic cells (neutrophils, macrophages, and eosinophils) engulf pathogens.
• Eosinophils target pathogens too large for other phagocytes, like parasites.
• Basophils target parasites and are involved in allergic reactions.

Natural Killer Cells

• NK cells kill anything recognized as foreign, including virus-infected cells and cancer cells.

Defensive Proteins

• Interferons are produced by a host cell in response to viral invasion.
• The infected cell dies, but interferons recruit NK cells and help neighboring cells produce antiviral proteins.
• Complement proteins, produced in the liver, poke holes in bacterial cell membranes.

Inflammatory Response

• Inflammation is most obvious around wounds.
• Hallmarks: redness, heat, pain, and swelling.
• Damaged cells release chemical alarms like histamine and prostaglandins.
• Increased blood flow brings in macrophages and neutrophils.
• Swelling “seals” the area to prevent pathogen spread.
• NSAIDS inhibit prostaglandin production to reduce inflammation.

Pyrogens and Fever

• Pyrogens are substances released from bacteria, viruses, and body cells that raise the body’s thermostat in the hypothalamus.
• This causes fever.
• Fever makes the environment less hospitable for pathogens.
• Phagocytic cells increase activity in response to pyrogens.

Summary of Innate Immunity

• Innate immune system includes physical and chemical barriers to prevent pathogen entry and acts as a primary defense line.
• Phagocytes, natural killer cells, interferons, the complement system, the inflammatory response, and pyrogens neutralize threats.

Adaptive Immunity

• Dendritic cells link the innate and adaptive immune systems by acting as antigen-presenting cells.
• They engulf pathogens but save a bit of the pathogen's protein or sugar (antigen).
• This antigen is displayed to other immune system cells, acting as a “wanted poster.”

Antigens and MHC Markers

• Pathogens have antigens that the immune system recognizes as foreign.
• Dendritic cells engulf the pathogen and use an antigen to “show” the immune system what has invaded.
• The MHC marker signals “friend” and prevents the dendritic cell from being attacked.

T and B Cells

• Helper T cells (CD4) receive information from dendritic cells and inform cytotoxic T cells and B cells about the pathogen.
• Cytotoxic T cells (CD8) directly attack infected cells.
• B cells produce antibodies that neutralize pathogens.

Delay in Immune Response

• The immune system initially encounters a delay when dealing with a new pathogen.
• Antigen-presenting cells must find the appropriate (naïve) helper T cell.
• They present the “wanted poster” to every helper T cell until the correct match is found.
• The correct T cell has an antigen-binding site matching the antigen presented by the pathogen.
• Helper T cells then activate specific cytotoxic T cells and antibody-producing B cells.
• This search process causes a delay in the immune system’s response.

Replication of Immune Cells

• Once the appropriate cells are found, they are replicated to deal with the pathogen.
• Cytotoxic T cells use perforins to poke holes in target cells.
• B cells produce antibodies that neutralize antigens.
• Antibodies are Y-shaped proteins specific to a particular antigen.

Immunological Memory

• After the first encounter with a pathogen, the adaptive immune system stores memory helper T cells, memory cytotoxic T cells, and memory B cells.
• Upon subsequent encounters, the immune system recognizes the pathogen and generates a faster and stronger response.
• The innate immune response remains unchanged regardless of the number of encounters.

Viral Mutation

• Viruses mutate frequently, changing the antigens presented on their surface.
• The immune system may no longer recognize the mutated virus, leading to reinfection.
• This is why updated vaccinations are needed for viruses like the flu.

Antibody Neutralization

• Antibodies produced by B cells neutralize viral particles.

Adaptive Immunity Summary

• The adaptive immune response is slower-acting, longer-lasting, and more specific than the innate response.
• Antigen-presenting cells display antigens via MHC molecules to complementary naïve T cells.
• In response, T cells differentiate and proliferate, becoming helper T cells.
• After the appropriate T cell is found, cytotoxic T cells and antibody-producing B cells are produced.
• Memory cells persist after a primary exposure; re-exposure causes memory cells to differentiate into effector cells, creating a fast, robust response.

Vaccination

• Louis Pasteur worked with chicken cholera, leading to a vaccine in 1880.
• Thomas Francis and Jonas Salk developed the first flu vaccine in 1945.
• Giving a dead/weakened form of the pathogen primes the immune system.

Types of Vaccines

• Inactivated (killed): Contains a killed microbe (e.g., Flu shot).
• Attenuated (live): Contains a weakened live virus (e.g., MMR).
• Subunit: Contains a specific protein that invokes an immune response (e.g., Shingles).
• Toxoid: Targets a toxin made by the pathogen (e.g., Tetanus).
• Conjugate: Contains a specific carbohydrate that invokes an immune response (e.g., Pneumonia).
• mRNA: mRNA codes for protein to invoke immune response (e.g., COVID-19).

Immune Priming

• The immune system is primed for stronger activation after initial exposure to a pathogen.
• Memory B and T cells already know what to do.
• The secondary response is faster and stronger.

Herd Immunity

• Herd immunity occurs when a large percentage of a population is immune to an infection.
• This reduces transmissibility and protects individuals who are not immune.

Allergic Reactions

• Allergic reactions are an unnecessary immune response to something the body mistakes as a pathogen.
• Common allergens are pollen, animal dander, and shellfish.
• Histamine, released during allergic reactions, is the main cause of symptoms.
• Histamine causes widening of blood vessels, leading to swelling, and airway constriction by contracting smooth muscle.
• Adrenaline (epinephrine) constricts blood vessels and relaxes airways to counteract these symptoms.