Difficulty: Intermediate | Prerequisites: Basic cell biology, blood components
This material covers the body's defence systems, from the physical barriers and innate responses that act within minutes, through to the highly specific adaptive immune cells that remember past invaders for years. It sits at the intersection of anatomy (lymphatic organs), cell biology (white blood cell types), and physiology (inflammation, fever, antibody production). You should already be comfortable with basic cell structure, blood components, and the circulatory system. If those feel shaky, revisit your earlier notes before diving in.
Your body uses a layered defence: first-line barriers (skin, microbiome), then innate immune cells (neutrophils, mast cells, NK cells), and finally adaptive immunity (T cells and B cells that target specific pathogens). The lymphatic system drains excess fluid, houses immune cells, and filters pathogens. Vaccines train your adaptive immune system without causing disease, and antibiotic misuse drives resistance.
Tonsils
Masses of lymphatic tissue at the back of the throat that trap pathogens entering through the mouth and nose. Removal is recommended only for children with more than 6 to 7 throat or ear infections. Think of them as bouncers at the entrance to your airway.
Spleen
An abdominal organ that filters blood, removes old red blood cells, and stores immune cells. Adults who lose their spleen (commonly from car injuries) are more vulnerable to infections and may need antibody injections. In simple terms, it is a blood-filtering station packed with white blood cells.
Thymus
A gland behind the sternum where T cells mature. It is largest in childhood and shrinks steadily from roughly age 15 onward (line V on a size-vs-age graph). Think of it as a training academy for T cells that slowly closes its doors as you age.
Lymph capillaries
Thin-walled vessels with one-way flaps that collect excess interstitial fluid and return it to the blood. Pathogens can slip through these flaps easily, which is how infections enter the lymphatic system.
Edema
Swelling caused by excess fluid in tissues. When lymph vessels are removed (as in breast cancer surgery), fluid cannot drain properly and the affected limb swells.
Hemocytoblast
The multipotent stem cell found in red bone marrow that can differentiate into red blood cells, platelets, and all white blood cell types. Think of it as the ancestor cell of every blood cell in your body.
Microbiome
The community of microorganisms living on and in your body. It is part of your first line of defence and is enhanced by a healthy diet and exercise, not by antibiotics (which can actually disrupt it).
Natural killer (NK) cells
Innate immune cells that destroy virus-infected cells and tumour cells. In obese individuals, NK cells accumulate more lipid and become less effective at attacking cancer cells.
Platelets
Cell fragments involved in clotting. When a cut introduces bacteria, platelets can bind bacteria to trap them, trigger fibrin formation, and stimulate neutrophils and activate dendritic cells, so they do all of these functions simultaneously.
Complement system
A group of proteins, produced mainly by the liver, that punch holes in bacterial membranes. Fatty liver disease impairs complement production. Think of complement as a set of molecular drill bits that bore into bacterial walls.
Interferon
A signalling protein released by virus-infected cells that warns neighbouring cells and stimulates white blood cell attack. Bacteria trigger complement, not interferon. In simple terms, interferon is the alarm signal a virus-infected cell sends out before it dies.
Mast cells
Innate immune cells that are usually the first to arrive at a localised infection site, where they trigger an inflammatory response by releasing histamine and other chemicals.
Inflammation
A localised immune response characterised by redness, heat, swelling, and pain. A localised infection causes nearby lymph nodes to swell as immune cells multiply inside them.
Fever / pyrogens
Fever occurs when muscle contractions raise body temperature. The higher temperature stimulates white blood cell metabolism, making the immune response faster. Pyrogens are the chemical signals that reset the body's thermostat.
Dendritic cells
Antigen-presenting cells that engulf pathogens, break them down, and load antigen fragments onto MHC receptors in the endoplasmic reticulum before displaying them on the cell surface for T cells to recognise.
MHC (major histocompatibility complex)
Cell-surface receptors that display antigen fragments. Dendritic cells load antigen fragments onto MHC inside the endoplasmic reticulum, then present them on the membrane to activate T cells.
Killer T cells (cytotoxic T cells)
Adaptive immune cells that release perforin to rupture infected or cancerous cells. They are the most numerous immune cells during an active infection.
Memory T cells
Long-lived T cells that persist after an infection clears. Six months after infection, memory T cells become the most common T cell type, ready to mount a faster response if the same pathogen returns.
Plasma B cells
Differentiated B cells that produce large quantities of antibodies. During a secondary infection (second exposure to the same pathogen), plasma B cells produce the largest number of antibodies.
Antigens
Molecules (usually proteins) on the surface of pathogens that the immune system recognises as foreign. Fragments of antigens, not antibodies, are loaded onto MHC receptors for display.
Antibodies
Y-shaped proteins produced by plasma B cells that bind to specific antigens, marking pathogens for destruction. Only B cells produce antibodies; T cells do not.
Autoimmune disease
A condition in which the immune system attacks the body's own tissues. Triggers include viral infections, exposure to maternal antigens, and weak T regulatory (Treg) cells.
T regulatory (Treg) cells
T cells that suppress immune responses to prevent the immune system from attacking the body's own tissues. Weak Treg cells are one trigger for autoimmune disease.
Antibiotic resistance
The ability of bacteria to survive antibiotic treatment. Best practices to prevent resistance include eating antibiotic-free meat, completing full courses of prescribed antibiotics, and not requesting antibiotics for viral infections.
Acellular vaccines
Vaccines made from parts of a pathogen rather than whole organisms. They are the safest regarding infection risk and side effects, but they are usually more expensive to produce than live vaccines.
Live vaccines
Vaccines made from weakened (attenuated) pathogens. They produce a strong immune response but carry a small risk of causing infection, particularly in immunocompromised individuals.
Lymph capillaries have one-way flaps that allow fluid (and pathogens) to enter easily
The lymphatic system returns excess interstitial fluid to the bloodstream and transports immune cells
Lymph nodes swell during localised infections as immune cells multiply inside them
The thymus is where T cells mature; it shrinks from about age 15 onward
The spleen filters blood and stores immune cells; loss of the spleen weakens immunity and may require antibody injections
Tonsils trap pathogens entering the mouth/nose; removal is indicated only after 6+ throat/ear infections
Removing lymph vessels (e.g. during breast cancer surgery) causes edema (swelling) in the affected limb because fluid can no longer drain
All blood cells (red blood cells, platelets, white blood cells) derive from hemocytoblasts in red bone marrow
Platelets are multifunctional at wound sites: they trap bacteria, trigger fibrin clotting, stimulate neutrophils, and activate dendritic cells
Physical barriers: skin, mucous membranes, cilia
The natural microbiome is part of the first line of defence and is supported by healthy diet and exercise (not by antibiotics)
Mast cells are typically the first immune cells to arrive at an infection site and trigger the inflammatory response
NK cells are innate immune cells that kill virus-infected and cancerous cells; in obese individuals, NK cells accumulate excess lipid and become less effective
Complement proteins (produced by the liver) attack bacteria by punching holes in their membranes; fatty liver disease impairs complement production
Interferon is released by virus-infected cells (not bacteria-infected cells) to warn neighbouring cells and stimulate WBC attack
Inflammation causes redness, heat, swelling, and pain at the infection site
Fever results from increased muscle contractions; the elevated temperature stimulates white blood cell metabolism, accelerating the immune response
Uses both non-specific and specific immune systems together under normal circumstances
Dendritic cells process antigens and load antigen fragments onto MHC receptors in the endoplasmic reticulum, then display them on the cell surface
Helper T cells coordinate the immune response; killer T cells release perforin to rupture infected/cancerous cells
Killer T cells are most numerous during active infection; memory T cells dominate six months later
Plasma B cells produce antibodies; the largest antibody response comes from plasma B cells during a secondary (not primary) infection
Viruses with rapid mutation rates produce many new antigens, allowing them to evade the specific immune system
Autoimmune disease can be triggered by viral infections, exposure to maternal antigens, or weak Treg cells
Acellular vaccines are safest but most expensive; live vaccines are cheaper but carry a small infection risk
Antibiotic resistance is worsened by: taking antibiotics for viral infections, stopping antibiotics early, using broad-spectrum antibiotics unnecessarily, and consuming meat raised with routine antibiotics
Best practice: eat antibiotic-free meat, complete full antibiotic courses, and use narrow-spectrum antibiotics when possible
Regular moderate exercise promotes a strong immune response; high-fat diets, type II diabetes, and chronic stress weaken it
TB is not easily killed by macrophages (it can survive inside them)
TB can infect tissues beyond the lungs
TB has evolved many drug-resistant strains
No highly effective vaccine currently exists for TB
Students often think antibiotics help fight colds or viral infections. They do not. Antibiotics target bacteria only, and misuse drives resistance.
Students confuse antigens and antibodies. Antigens are the foreign molecules the immune system recognises. Antibodies are the Y-shaped proteins your B cells make to bind those antigens. Fragments of antigens (not antibodies) are loaded onto MHC.
Students assume the primary immune response produces the most antibodies. The secondary response (second exposure) is faster and produces far more antibodies from plasma B cells.
Students think interferon is released in response to all pathogens. Interferon is specifically released by virus-infected cells. Bacterial infections trigger complement instead.
⚠️ Know which immune cells arrive first (mast cells), which kill infected cells (killer T cells via perforin), and which produce antibodies (plasma B cells).
⚠️ Understand the difference between primary and secondary immune responses, and that the secondary response produces more antibodies.
⚠️ Be able to read forest plots (Fig. B): if the 95% confidence interval crosses 1.0, the result is not statistically significant.
⚠️ NK cells in obese individuals have higher lipid content and are less effective. This comes directly from Fig. F.
⚠️ Antibiotic resistance is a recurring exam topic. Know the best practices (complete courses, avoid unnecessary use, eat antibiotic-free meat).
⚠️ Antigens are loaded onto MHC in the endoplasmic reticulum of dendritic cells, not the lysosome.
⚠️ The thymus shrinks with age (line V on a descending graph). The spleen is linked to antibody injections, not anaemia.
True or false: The spleen is where T cells mature. (False, that is the thymus.)
Fill in the blank: Virus-infected cells release ______ to warn neighbouring cells. (Interferon.)
True or false: Plasma B cells produce the most antibodies during a primary infection. (False, the largest antibody response is during a secondary infection.)
Fill in the blank: The stem cell in red bone marrow that gives rise to all blood cells is called a ______. (Hemocytoblast.)
True or false: Mast cells release perforin to kill infected cells. (False, killer T cells release perforin. Mast cells trigger inflammation.)
Q: A breast cancer patient has had lymph vessels surgically removed from one arm. What symptom will she most likely experience in that arm, and why?
A: Edema (swelling), because the loss of lymph vessels prevents proper fluid drainage from the tissues.
Q: Which white blood cell type releases perforin to destroy an infected or cancerous cell?
A: Killer T cells (cytotoxic T cells).
Q: During a secondary infection by chickenpox, which cells produce the largest number of antibodies?
A: Plasma B cells.
Q: A patient has fatty liver disease. Which immune function is most likely to be impaired?
A: Complement attack of bacteria, because complement proteins are produced by the liver.
Q: Name three possible triggers for autoimmune disease.
A: Viral infection, exposure to maternal antigens, and weak Treg cells.
Q: Why do viruses with rapid mutation rates pose a particular challenge to the specific immune system?
A: Rapid mutations produce many new antigens, so the immune system's existing antibodies and memory cells cannot recognise the new variants. The virus evades the specific immune response.
Q: What is the best practice for reducing antibiotic resistance when it comes to food choices?
A: Eat antibiotic-free meat, if you eat meat.
Q: An infant is given antibiotics within the first 24 months of life. Based on meta-analysis data (Fig. B), what is the effect on their risk of childhood overweight/obesity?
A: A significantly increased risk of becoming overweight or obese (overall OR 1.11 with 95% CI that does not cross 1.0).
This material connects directly to the respiratory system unit, because the lungs are a major site of pathogen entry and immune defence (alveolar macrophages, TB infection). Understanding how obesity impairs NK cells also ties into the cancer biology section, where immune surveillance of tumour cells is a key concept. The microbiome material links back to earlier digestive system content and forward to discussions of antibiotic resistance in public health.
Immune system, lymphatic system, innate immunity, adaptive immunity, non-specific immunity, specific immunity, first line of defence, second line of defence, third line of defence, T cells, B cells, killer T cells, cytotoxic T cells, helper T cells, memory T cells, Treg cells, regulatory T cells, plasma B cells, antibodies, immunoglobulins, antigens, MHC, major histocompatibility complex, dendritic cells, macrophages, neutrophils, mast cells, natural killer cells, NK cells, complement system, interferon, inflammation, inflammatory response, fever, pyrogens, lymph nodes, spleen, thymus, tonsils, hemocytoblast, bone marrow, edema, lymphedema, autoimmune disease, autoimmunity, vaccines, acellular vaccine, live vaccine, antibiotic resistance, microbiome, tuberculosis, TB, perforin, phagocytosis, BIO 118, biology 118, University of Washington