IMMUNE SYSTEM

🧠 Lymphatic System

Lymphatic System: Functions
Removes excess fluid from tissues, absorbs fats, and protects against invaders.

Primary Lymphoid Organs
Bone marrow: most immune cells are made here.
Thymus: T cells mature here.

Secondary Lymphoid Organs
Lymph nodes: filter and store immune cells.
Spleen: stores immune cells.
Tonsils: trap pathogens from mouth and nasal cavity.
Appendix: contains lymphoid tissue.
Bone marrow: also listed as primary.

🦠 Infectious vs Noninfectious Disease

Infectious Disease
Caused by pathogens (bacteria, viruses, fungi, parasites) and can be spread.

Noninfectious Disease
Not caused by pathogens. Cannot be spread. Caused by genetics, lifestyle, or environment.
Examples: cancer, diabetes, cardiovascular disease, chronic respiratory disease.

🧫 Pathogens

Bacteria: Description
Prokaryotic single-celled organisms. Lack nucleus and membrane-bound organelles. Produce toxins and compete for nutrients. Treated with antibiotics.

Structure of Bacteria
Plasmid DNA, capsule, fimbriae, flagella.

Fungi: Description
Eukaryotic (have nucleus). Single-celled or multicellular (molds, yeasts). Cause disease by spore inhalation or direct contact.

Structure of Fungus
Hyphae form mycelium, have chitin cell walls, contain organelles, reproduce via spores.

Viruses: Description
Non-living pathogens. Reproduce only in host cells. Treated with antivirals or prevented by vaccines.

Structure of Virus
Genetic material (DNA or RNA), protein coat (capsid), surface proteins for host entry.

🧬 Influenza & Resistance

Influenza A Virus
Labeled by H (haemagglutinin) and N (neuraminidase) antigens.
18 H types × 11 N types = 144 combinations. Changes yearly due to genetic reassortment.

Influenza Infection Process
Virus enters cell, uses host machinery to replicate, new viruses assemble and burst out.

Prevention
Vaccines yearly due to antigenic variation.
Antivirals target replication.

Antibiotic Resistance
Occurs when bacteria mutate and survive antibiotic use. Caused by overuse/misuse. Reduces treatment effectiveness. Superbugs emerge.

🛡 Innate Immune Response (Non-Specific)

First Line of Defence: Physical Barriers
Skin: prevents entry.
Cilia: sweep out pathogens.
Mucous: traps pathogens.

First Line of Defence: Chemical Barriers
Stomach acid: kills microbes.
Urine: acidic, flushes microbes.
Secretions: lysozyme in tears, saliva, etc.

First Line of Defence: Expulsion
Coughing, sneezing, vomiting, diarrhoea.

Natural Flora
Good bacteria that outcompete pathogens.

Second Line of Defence: Overview
Activated if first line fails. Includes inflammation, phagocytes, NK cells, cytokines, fever.

Phagocytes
White blood cells (e.g. macrophages) that engulf and digest pathogens.

Natural Killer (NK) Cells
Destroy cells without MHC markers (e.g. virus-infected, cancer cells).

Cytokines
Chemical messengers that attract immune cells, interfere with viral replication, and activate immune responses.

Chemokines
Control cell movement and immune cell release from bone marrow.

Fever
Raises body temperature (~40°C) to inhibit pathogen survival and activate immune cells.

🧠 Adaptive Immune Response (Specific)

Antigen
A substance that triggers an immune response and leads to antibody production.

MHC (Major Histocompatibility Complex)
Cell surface marker used to present antigens and identify self vs non-self.

💉 Humoral Response (B Cells)

Humoral Response: Overview
Involves B cells and antibodies. Targets extracellular pathogens.

B Cell Activation: Step 1
An antigen-presenting cell (APC), such as a macrophage, engulfs a pathogen and presents its antigen on an MHC.

B Cell Activation: Step 2
A helper T cell (CD4+) binds to the antigen-MHC complex and becomes activated.

B Cell Activation: Step 3
The activated helper T cell releases cytokines that help identify and select the B cell that has a matching antibody receptor for the antigen.

B Cell Activation: Step 4
The selected B cell undergoes clonal expansion, differentiating into plasma cells (which produce antibodies) and memory B cells.

Plasma Cells
Produce and secrete antibodies to neutralize or destroy antigens.

Memory B Cells
Remain in body long-term, respond faster on re-exposure.

Antibody Roles
Bind antigens to neutralize pathogens, mark them for phagocytosis (opsonization), or activate complement system.

🔥 Cell-Mediated Response (T Cells)

Cell-Mediated Response: Overview
Involves T cells. Targets infected or abnormal cells.

T Cell Activation: Step 1
An infected or abnormal cell displays a foreign antigen on an MHC.

T Cell Activation: Step 2
A cytotoxic T cell (Tc) with a matching receptor binds to the antigen-MHC complex.

T Cell Activation: Step 3
Helper T cells may also assist by releasing cytokines that enhance Tc cell activation.

T Cell Activation: Step 4
The cytotoxic T cell becomes fully activated and releases cytotoxic granules to induce apoptosis in the infected or abnormal cell.

💉 Vaccination & Immunity

Vaccination: How It Works
Injects weakened/dead pathogen or antigen.
Triggers antibody and memory cell production.
Faster, stronger response on later exposure.

Types of Vaccines
Live attenuated, inactivated, subunit.

Memory B Cells in Vaccines
Enable faster, larger antibody response upon re-exposure.

Natural Active Immunity
Body produces antibodies after encountering a pathogen naturally.

Natural Passive Immunity
Antibodies received from mother (placenta, breast milk).

Artificial Active Immunity
Vaccination causes body to make antibodies and memory cells.

Artificial Passive Immunity
Injected antibodies (e.g. antivenom) provide immediate protection.

👥 Herd Immunity & Thresholds

Herd Immunity
When a large portion of the population is immune, it indirectly protects non-immune individuals.

Immunity Threshold Equation
1 - (1 ÷ reproduction number)

⚖ Bioethical Approaches

Consequence-Based Ethics
Focuses on maximizing positive outcomes and minimizing harm.

Duty-Based Ethics
Follows rules/duties even if consequences are negative.

Virtue-Based Ethics
Prioritizes what a “good” person would do — moral character over outcomes or rules.