The immune system protects the body from pathogens such as viruses, bacteria, fungi, and parasites. This cheat sheet summarizes the major defenses used to recognize, attack, and remember disease-causing agents. Students need these ideas to understand infection, vaccination, allergies, autoimmune disease, and public health.
It is designed as a quick reference for biology review, labs, and test preparation.
The immune response begins with barriers and innate defenses, then may activate adaptive immunity. Innate immunity includes skin, mucus, inflammation, fever, phagocytes, and natural killer cells. Adaptive immunity depends on B cells, T cells, antibodies, antigens, and memory cells.
Key disease concepts include transmission, incubation period, herd immunity, R0, and vaccine effectiveness.
Key Facts
- Innate immunity is fast and nonspecific, while adaptive immunity is slower at first but highly specific to particular antigens.
- An antigen is a molecule recognized by immune receptors, and an antibody binds a matching antigen using a specific lock-and-key fit.
- B cells can differentiate into plasma cells that secrete antibodies and memory B cells that respond faster after re-exposure.
- Helper T cells activate B cells, cytotoxic T cells, and macrophages by releasing signaling proteins called cytokines.
- Cytotoxic T cells kill infected body cells by recognizing antigen fragments displayed on MHC I proteins.
- Phagocytes such as macrophages and neutrophils engulf pathogens using phagocytosis: pathogen + phagocyte -> phagosome -> digestion.
- Basic reproduction number R0 means the average number of new infections caused by one infected person in a fully susceptible population.
- Herd immunity threshold can be estimated by H = 1 - 1/R0, where H is the fraction of the population that must be immune.
Vocabulary
- Pathogen
- A disease-causing agent such as a virus, bacterium, fungus, protist, or parasite.
- Antigen
- A molecule or molecular pattern that the immune system recognizes as foreign or abnormal.
- Antibody
- A Y-shaped protein made by plasma cells that binds a specific antigen and helps neutralize or mark it for destruction.
- Inflammation
- A local innate immune response that increases blood flow and brings immune cells to damaged or infected tissue.
- Vaccine
- A preparation that safely exposes the immune system to an antigen so memory cells form without causing the full disease.
- Memory Cell
- A long-lived B cell or T cell that responds rapidly when the same antigen enters the body again.
Common Mistakes to Avoid
- Confusing antibiotics with antivirals is wrong because antibiotics target bacteria, not viruses.
- Saying innate immunity has no recognition is wrong because innate cells recognize broad pathogen patterns, even though they are not antigen-specific like adaptive cells.
- Thinking antibodies directly kill all pathogens is wrong because antibodies mainly neutralize, clump, or label targets for other immune defenses.
- Assuming vaccines give instant immunity is wrong because the body needs time to activate lymphocytes and produce memory cells.
- Mixing up autoimmune disease and immunodeficiency is wrong because autoimmune disease is an immune attack on self, while immunodeficiency is weak or missing immune function.
Practice Questions
- 1 A disease has R0 = 4. Use H = 1 - 1/R0 to estimate the herd immunity threshold.
- 2 If one infected person infects an average of 2.5 people in a fully susceptible population, what is the R0 value?
- 3 A patient receives a vaccine booster and produces antibodies much faster than after the first dose. Which immune cells explain this faster response?
- 4 Explain why a person can have inflammation at a cut before the adaptive immune system has produced specific antibodies.
Understanding Immune System & Disease
Inflammation is a controlled local response, not simply a sign that the body is failing. Damaged cells and immune cells release chemical signals. These signals widen nearby blood vessels and make vessel walls leakier.
More fluid enters the tissue, causing redness, warmth, swelling, and sometimes pain. White blood cells can then leave the blood and move toward the damaged area. Complement proteins in blood can coat microbes, attract immune cells, or damage some microbial cell membranes.
Fever can slow the growth of certain pathogens and increase immune activity, though a very high fever can be dangerous. Inflammation must switch off after the threat is controlled. If it continues too long, it can harm healthy tissue.
A strong adaptive response depends on communication between cells. Dendritic cells often collect pieces of a pathogen at an infection site and carry them to a lymph node. There they show antigen fragments to helper T cells using MHC class two proteins.
A helper T cell only responds if its receptor fits that displayed fragment and it receives extra activation signals. This safety check lowers the chance of attacking harmless substances or the body's own cells. Activated helper T cells multiply into many matching cells.
They direct other immune cells with cytokines. Infected cells display internal antigen fragments on MHC class one proteins. Cytotoxic T cells inspect these displays and can trigger an infected cell to die before it releases many new viruses.
Antibodies do more than stick to pathogens. Some block a virus or toxin from attaching to body cells. Some join pathogens into clumps that are easier for phagocytes to remove.
Others mark a target so complement proteins or immune cells can destroy it. Antibodies are especially useful against microbes outside cells. They cannot directly reach viruses that are already reproducing inside a cell, which is one reason cytotoxic T cells matter.
Vaccines expose the immune system to a safe form, piece, or instruction from a pathogen. This gives selected lymphocytes time to form memory without the usual risks of the full disease.
Protection can fade when antibody levels fall or when a pathogen changes enough to avoid old antibodies. Booster doses remind memory cells and raise protection again.
Disease spread depends on more than the pathogen itself. It depends on how people mix, how long an infected person can transmit, how much immunity exists, and whether the microbe survives in air, water, food, or on surfaces. Incubation period means the time between infection and symptoms.
A person may sometimes spread an infection before feeling ill, so symptom checks alone cannot stop every outbreak. The basic reproduction number describes spread in a population with no immunity. In real communities, the effective reproduction number changes as immunity and behavior change.
For learning, separate infection from disease. Infection means a pathogen has entered and reproduced.
Disease means the infection has caused noticeable harm or symptoms. This distinction helps explain asymptomatic transmission, testing, and public health advice.