Pathogens and Disease - The Immune System (SL)

Assessment Statement

6.3.1Define a pathogen

The Immune System in the human body is carried out by the Lymphatic System, in conjunction with the Cardiovascular System and bone marrow.

Define a pathogen:

There are many different pathogens that affect humans:

  • Viruses – chicken pox, polio, flu, West Nile, ebola
  • Bacteria – tetanus, strep throat
  • Fungi – athlete’s foot (tinea), keratitis (inflammation of the eye cornea)
  • Protozoa – malaria, sleeping sickness, trichomoniasis
  • Flatworms – bilharzias (worms in your blood)
  • Roundworms – elephantiasis (threadworms living in your lymphatic system)

The big three are:

  • Tuberculosis – bacterial (mycobacterium tuberculosis)
  • AIDS – viral (HIV – human immunodeficiency virus)
  • Malaria – protozoan infection (plasmodium sp. [species])

How do these things get in your body?

  1. Airborne
  2. Droplets in the air
  3. Breathe in and pass through the lung epithelial
  4. Water-borne
  5. Water contaminated by feces
  6. Drink-enters the intestines
  7. Food-borne
  8. Food contaminated by feces
  9. Eat-goes into intestines
  10. Insect-borne or animal-borne
  11. Animal or insect bite
  12. Injected directly into the blood stream
  13. Sexually Transmitted
  14. Pathogens transmitted during intercourse
  15. Pass through skin into the blood
  16. Direct contact
  17. Spread by skin contact/saliva
  18. Pass through skin into digestive tract

We have an excellent weapon against these types of disease – antibiotics!

How antibiotics work

Assessment Statement

6.3.2Explain why antibiotics are effective against bacteria and not viruses

Antibiotics only work on prokaryotic cells and the cells in our bodies are eukaryotic cells.

Both fungi and bacteria act as decomposers and sometimes compete with one another for food. Some microorganisms produce chemicals called antibiotics to inhibit growth of their competition.

Antibiotics listed in the table below interfere with bacterial metabolic processes such as DNA replication, transcription, translation, ribosome function and cell wall formation. This is why antibiotics can be used in the human body. Protein synthesis is similar in both types of cell, but not exactly the same. Prokaryotic cells have a cell wall and eukaryotic cells do not. Some antibiotics block biochemical pathways in the bacterium, causing death – usually blocking protein synthesis. Some stop the reproduction of the cells and allows the white blood cells to catch up, while others kill the bacterial cells by dissolving the cell walls and membranes.

Some of the names familiar to you -aminoglycosides, chloramphenical, erythromycins (eyedrops), clindamycin.

Below is a table of familiar antibiotics

Organism / Antibiotic Produced / Mechanism of action
Fungi
  1. Penicillium sp.
  1. Cephanlosporium sp.
  1. Streptomyces
/

Penicillin

Cephalosporin
Tetracycline
Streptomycin
Erythromycin /
  1. Inhibits cell wall synthesis
  2. Inhibits cell wall formation
  1. inhibits translation

Bacteria
  • Bacillus sp.
/
Polymyxin
Bacitracin / Disrupts cell membranes
Disrupts cell wall synthesis

Resistance is an issue, as the plasmids, can move from one bacterium to another, carrying several drug resistant genes.

But antibiotics are not useful on viruses because they do not carry the mechanism to reproduce or have cell wall (as we saw earlier). Since viruses use our body cells to reproduce, any chemical would be harmful to our cells. We need a vaccine, which we will go over later.

Preventing pathogens from entering our body - Lines of Defence

Assessment Statement

6.3.3Outline the role of skin and mucous membranes in defence against pathogens

6.3.4Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues

6.3.5Distinguish between antigens and antibodies

6.3.6Explain antibody production

The ability to resist infection by a disease is termed immunity. Non-specific immunity allows the body to resist infection by a wide range of pathogens. An example is the skin.

The best line of defence is to prevent the pathogen from entering the body. This is done in many ways:

Skin

If there is not a cut or an opening of any kind, it is impossible for pathogens to penetrate. Our skin has two layers, the dermis and the epidermis. The dermis is alive and contains the sweat glands, capillaries, sensory receptors and dermal cells that give structure and strength to our skin. The epidermis, is constantly being replaced, as underlying dermal cells die and move upwards. This layer is effective as a barrier because it is not truly alive.

The skin oil, sebum, is secreted by the sebaceous glands in the skin. The sebum is a protective film over the skin. It maintains skin moisture and lowers the skin pH.

Perspiration flushes microbes away and contains lysozyme, an enzyme, which breaks down bacterial cell walls.

Mucous

On the epithelial layers, the fluid is emitted which traps bacteria and prevents further entry in internal cavities that are exposed to the external environment. Mucous membranes secrete mucous. They are found in the:

  • Trachea
  • Nasal Passages
  • Urethra
  • Vagina

The sticky mucous traps the bacteria, as do tiny hairs. Cilia in the bronchial tubes, sweep the mucous to the digestive system, through the mouth and throat, or triggers a spasm of the bronchioles, called coughing. In addition, the cells that secrete mucous also secrete an enzyme called lysozyme (same as the perspiration). The enzyme does chemical damage to the pathogen.

Other lines of defence are:

  1. Stomach – gastric juice is acidic and destroys bacteria and most bacterial toxins.
  1. Eyes – tears have the same chemicals as sweat, the flush out the bacterial.
  1. Vagina – muscous and an acidic environment
  1. Urethra – flow of urine cleanses the duct
  1. Rectum – Anus – defecation cleanses the areas of pathogens (Wash your hands)
  1. Vomiting – enough said!

This along with good hygiene prevents most bacteria from entering our body.

If microorganisms get past the physical barriers and enter the body, phagocytic cells engulf them by endocytosis. The microorganisms are then digested by the enzymes in the lysosomes of the phagocyte. This is a very non-specific response to pathogens, or cell mediated immune response (CMI). If this did not happen, every pathogen and foreign body could make humans sick.

What happens when pathogens do get in?

We have another line of defence. There are two types, a very non-specific response to pathogens, or cell mediated immune response (CMI) and a Specific Immune Response or Antibody-mediated immune Response (AMI). There are several parts to the AMI response.

Role of phagocytic leucocytes. (SL)

Leucocytes, or white blood cells, are the cells in our blood stream that help us fight off pathogens that enter our bodies and also provide us with an immunity for many pathogens we encounter a second time. All of the leukocytes, along with the erythrocytes (RBC) and platelets are produced on the bone marrow.

One type of leucocyte that gets involved very early in the process of fighting off pathogens is a macrophage. They are large white blood cells that are able to change their shape a surround an invader to take it in by phagocytosis, like an amoeba. Since they can change their shape, they are able to squeeze their way in and out of small This allows them to move in and out of the blood stream to go after pathogens.

When a macrophage does meet a cell, it recognizes whether the cell is a natural part of the body, “self” or a foreign invader, “not self”. The macrophage recognizes and reads the proteins and other molecules on the surface of the pathogen. If it is “not self”, it will be ingested, regardless of the protein on the cell. This is an example of CMI Response.

Along with the white blood cells, there are antibodies and antigens, so your body can recognize foreign bodies, antigen, and try to neutralize and get rid of them, using antibodies.

Antigen – a molecule or particle recognized as foreign by the immune system, that can trigger an immune response.

Antibody – Immunoglobulin - a globular protein that recognizes an antigen

Antibodies have a T or Y shape made from polypeptide chains. The antibody is a globular protein, that belong to a group of glycoproteins (remember the glycoproteins on the cell membrane?)

When antibodies are produced, it is for a specific antigen, and this is a AMI Response.

The antibodies are produced by B-Cells. Along with the B-Cells, there are T-Cells that can be divided into:

  1. Helper T cells
  2. Cytotoxic T-Cells
  3. Suppressor T-Cells
  4. Memory T-Cells.

The helper T-Cells are floating around and when they encounter a foreign pathogen, ingest it and take it to the B-Cells. The B-Cells make the antibody, and make more cells to release the antibodies. Some of the antigen / antibody combination is kept for later, by the Memory T-Cells. The Suppressor Cells slow down the immune response, as the pathogen is removed, as the Cytotoxic T-Cells directly kill the pathogen, along with macrophages. The process of the response to the antigen will be explained later.

Some of the outcomes of the binding of the antibodies to antigens include: