IGCSE Biology: Disease and Immunity
A pathogen is a disease-causing organism. It is a tiny organism (microorganism) that can only be seen with a microscope.
| Pathogen Group | Examples |
|---|---|
| viruses - not cells and are much smaller than bacteria | influenza (flu), common cold, HIV/AIDS, polio, measles |
| bacteria - prokaryotes that only have a strand of DNA that produces toxins | cholera, syphilis, tuberculosis, E. coli |
| protozoa - parasitic eukaryotes | malaria, amoebic dysentery |
| fungi - saprotrophic and can spread easily | athlete’s foot, ringworm |
Note: HIV is the virus that causes AIDS, but not all HIV will lead to AIDS.
Diseases can either be transmissible or non-transmissible.
- non-transmissible diseases
- do not involve infection or disease-causing agent
- may be due to environmental factors e.g
- diet
- physical inactivity
- substance abuse
- or generic faults e.g sickle cell anaemia
- transmissible diseases
- can be caught by infection or disease-causing agent
- can be direct or indirect
| Direct Transmission | Indirect Transmission |
|---|---|
| blood, bodily fluids, skin-to-skin contact such as intercourse | contaminated surfaces, food, air, animals (vectors) |
The body has a variety of defence mechanisms against diseases. The body has mechanical and chemical barriers.
- mechanical barriers are physical and external. They include:
- hair nostrils: trap dust carrying pathogens
- skin: a thick layer of outer cells containing keratin that makes it difficult for pathogens to penetrate
- blood clots: when skin is cut, blood clots form to prevent pathogens from entering the wound (using fibrin)
- chemical barriers are created through internal processes. They include:
- mucus in the respiratory system: trap pathogens which can then be swept back up to the throat by cilia and swallowed
- hydrochloric acid (HCl) in the stomach: low pH kills bacteria
- white blood cells: digest pathogens or create antibodies that disable/kill pathogens
Types of Immunity
Active immunity is when a person is exposed to a pathogen and their immune system has created its own antibodies and memory cells. Not all lymphocytes fight the pathogen and store information about it for later exposure. It is long-lasting and, while the initial immune response is slow, the secondary response is fast.
Natural immunity is exposure and infection of the pathogen. Artificial immunity is via vaccination.
Passive immunity is when a person has been given antibodies made by another organism. No lymphocytes are stimulated and thus, antibodies and memory cells are not made. It is short-lasting.
Natural immunity is like breastfeeding mothers for their babies. Artificial immunity is when antibodies are injected into a person’s blood.
Mode of Action Against Diseases
Vaccines work by injecting a small amount of the inactive pathogen into the body. The pathogen still has an antigen on its surface to help white blood cells detect the needed antibodies to lock and destroy the antigen. Other antibodies become memory cells so that if a person were to be exposed to the virus, the antibodies are mass-produced before re-infection of the person.
Antibiotics are substances that slow down or stop the growth of bacteria. These can be taken to cure the diseases by killing the pathogens, but only cure bacterial diseases and not viral ones.
- Antibiotics damage the bacterial cells by restraining their cellular processes but do not damage the host cell
Viruses cannot be cured by antibiotics as they reproduce inside the host cells. It is very difficult to develop antiviral drugs as they might damage the host cell whilst killing the virus.
Antiviral drugs only slow down viral development and viruses change their antigens quickly which means new drugs have to be generated regularly.
Antibodies
White blood cells (also known as leukocytes) are responsible for fighting pathogens in the body. Antibodies are a type of protein, with a particular shape that is designed to fit right into another molecule.
- When an antibody recognises a specific site (antigen) on a pathogen, it binds to the antigen and ‘marks‘ the pathogen for destruction
Humans have billions of different antibody molecules each with a unique binding site. Each antibody binds to a specific antigen. This is known as antibody specificity.
Once the antibody has bound to the antigen, the antibody:
- alerts a phagocyte (a cell capable of ‘engulfing’ other particles)
- phagocyte then’ ‘engulfs‘ the pathogen and kills the antigen-antibody complex OR
- starts a series of reactions in the blood that produces enzymes to destroy the pathogen
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