C3.2 Defense against disease

What are pathogens?

Pathogens are organisms that cause infectious diseases, including viruses, bacteria, fungi, and protists.

How do the skin and mucous membranes act as primary defences?

• Skin: Tough outer layer and acidic secretions (lactic/fatty acids) inhibit bacterial growth.

• Mucous membranes: Produce mucus with lysozyme to kill bacteria (e.g., in trachea, vagina).

Describe the steps of blood clotting.

1. Platelets release clotting factors at the wound.

2. Cascade reactions produce thrombin.

3. Thrombin converts fibrinogen to fibrin.

4. Fibrin forms a mesh, trapping blood cells to create a clot/scab.

Compare innate and adaptive immunity.

• Innate: Non-specific (e.g., phagocytes).

• Adaptive: Pathogen-specific (e.g., lymphocytes); develops over time.

How do phagocytes destroy pathogens?

1. Engulf pathogens via endocytosis.

2. Lysosomes fuse with vacuole, digesting the pathogen.

3. Phagocytes move to infection sites (amoeboid movement).

What is the role of lymphocytes in antibody production?

• B-lymphocytes produce Y-shaped antibodies.

• Antibodies bind to specific antigens on pathogens (hypervariable regions).

• Each B-cell makes one antibody type; clones expand during infection.

What are antigens, and how do they trigger immunity?

• Antigens are surface molecules (proteins/glycoproteins) on pathogens.

• They trigger antibody production; recognized as "non-self" by lymphocytes.

How are B-lymphocytes activated?

1. Macrophages display pathogen antigens via MHC II.

2. Helper T-cells bind to macrophages, then activate matching B-cells.

What happens after B-cell activation?

• Activated B-cells divide (clonal expansion) to form plasma cells (antibody factories).

• Some become memory B-cells, enabling rapid response to reinfection.

How does HIV lead to AIDS?

• HIV destroys helper T-cells, crippling antibody production.

• AIDS results from opportunistic infections (e.g., Kaposi’s sarcoma).

Why do antibiotics work on bacteria but not viruses?

• Antibiotics target prokaryotic processes (e.g., cell wall synthesis).

• Viruses lack metabolism; they hijack host cells (no targets for antibiotics).

How does antibiotic resistance develop?

• Resistant bacteria survive treatment and spread (e.g., MRSA).

• Mitigation: Limit antibiotic use, improve hygiene, develop new drugs.

What is zoonosis? Give examples.

Diseases transmitted from animals to humans (e.g., tuberculosis from cattle, rabies from dogs, COVID-19).

How do vaccines work?

• Contain antigens/mRNA to trigger primary immune response.

• Memory cells provide long-term immunity (secondary response).

What is herd immunity?

• Occurs when enough people are immune, reducing pathogen spread.

• Formula: (1−1/R)×100%(1−1/R)×100% (e.g., measles requires 93% vaccination).