Pathogen – a disease causing organisms

– May be:

– viral (HIV, chicken pox)

– bacterial (TB, strep)

– fungal (athletes foot)

– Protozoan (malaria = plasmodium)

– Flatworm

– Roundworm

How to get infected?

– airborne; waterborne; food; insect transmission; STI; skin

Barriers to infection – skin, mucous membranes

Specific defenses – production of antibodies to a specific antigen

Antigen – a molecule (protein) recognized as foreign by the immune system

Antibody – a globular, y-shaped protein that recognizes an antigen.

– produced by B-cells (a type of leukocyte)

Antibiotics – used to treat bacterial infections.

– Inhibit the function of bacterial (prokaryotic) proteins, preventing protein synthesis. Will not affect a virus, because a virus uses host cells to reproduce.

– Bacteria have short reproductive time, so can easy evolve to a resistance to an antibiotic. -> Super bugs

Production of Antibodies

  1. Pathogen invades body
  2. Antigen detected by macrophage
  3. Macrophage engulfs pathogen, and displays the antigen on the outside of its membrane
  4. T-Cells detect the antigen -> activate B-cells.
  5. B-cells produce antibody. B cells clone themselves
  6. Antibody binds to antigen “neutralizing it”
  7. Memory T-cells and B-cells are left behind. These reduce the response time to a second exposure.

Types of T-Cells

Helper T-cell – activate B-cells to produce antibodies

Killer (cytotoxic) T-cell – destroys infected body cells.

E.g. – Cancer cells.

Memory T-cell – remains in body to recognize antigen again

Suppressor T-cell – shuts down immune response


– Stimulates antibody production and formation of memory cells without causing the disease.

– Uses a weakened or dead pathogen.

– If person becomes exposed, immune response will be quick and person will probably not be infected again.

Immunization – deliberate exposure to the pathogen to produce memory cells against it.

Different classes of antibodies -> proteins known as immunoglobulins

– antibodies are y-shaped        – 2 heavy chains (long)

– 2 light chains (short)

Class of antibody depends on the form of the heavy chain.

IgA – alpha *

IgG – gamma * * most common

IgE – epsilon

IgD – delta

IgM – mu

Monoclonal Antibodies

– Antibodies obtained from single B-cell clones.

– Produced in a lab using hybridoma cells

Uses – to prevent organ rejection

– to kill/inhibit cancer cells

– detect pregnancy (HCG)

– blood typing (blood will clot if antigen and antibody mix)

– produce vaccines


1. Active – due to production of antibodies after exposure.

– once you have the disease you are unlikely to be infected a second time (measles, chicken pox)

2. Passive – immunity due to acquisition of antibodies

– E.g. – placenta or serum injection

– takes no time to develop

3. Natural – immunity due to infection

4. Artificial – immunity due to vaccination

The process of blood clotting is also another type of defense.  Blood is too precious a liquid to allow it to escape in large amounts.  As a result blood clots where there is a cut.  But it is just as important that blood not clot at other times.

Clotting Process

  1. Blood reacts with air and platelets
  2. Damaged cells trigger release of enzyme thrombokinase
  3. Proteins in blood bind with factor X (10) and Ca2+
  4. Causes prothrombin (inactive) -> thrombin (active)
  5. Thrombin causes fibrinogen (soluble) -> fibrin (insoluble)
  6. Fibrin traps blood cells forming a CLOT.

Benefits and Dangers of Vaccination


  • “herd immunity”  almost everyone in the community is immune against a disease.
  • Eradication of a disease – e.g. smallpox


  • Live vaccines could be passed out through feces and infect others
  • Some people do not respond to a vaccine and are not immune – malnutrition can prevent antibodies from being formed
  • A virus like the common cold mutates regularly making previously formed antibodies useless.
  • MMR Measles/Mumps/Rubella – a useful vaccine that protects against two most lethal childhood diseases Mumps and Measles.
author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

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