Topic 2: Cells : Antigens and the immune system

What is a disease?

A persistent deviation from a person's usual appearance ,body function or behaviour

Communicable (or infectious) diseases

Those that can be passed from one organism to another. They are caused by microorganisms known as pathogens

Helminths

Parasitic worms e.g. Flutes, roundworms and tapeworms. Tend to live in the gastrointestinal tracts. Release microscopic eggs which excreted in faeces which contaminate water and soil to spread

Plasmodium (malaria causing Protozoa)

Spreads when mosquito drinks blood. Destroys red blood cells. Carried by female mosquitoes (vectors)

Fungi

Airborne or contaminated food e.g. Athletes foot, ringworm

Prions (acellular - proteins)

Causes incorrect folding of proteins, very progressive especially in nervous system, spread by consuming infected tissue

How do bacteria cause disease?

Produce toxins that damage body cells

How do viruses cause disease?

Use host cells to replicate before bursting out and destroying cells.

How do Protoctists cause disease?

Take over cells and break them open

How do fungi cause disease?

Release enzymes to digest living cells to destroy them and some produce toxins

How do bacteria and fungi spread disease?

Direct contact, consuming them, sexual contact

How do viruses spread disease?

Airborne, direct contact, inhaled or enter through wounds e.g cuts

Non-self cells or molecules

Those that the immune system has not had exposure to during maturation or does not normally encounter in the body

Lymphocytes

Cells that identify the presence of pathogens and potentially harmful foreign substances in the body and to then destroy or neutralise them to prevent harm

How can lymphocytes distinguish between pathogens and self- cell?

Each type of cell has specific molecules on its surface that identify it. These molecules are usually proteins ,as their 3D tertiary structures enable lots of unique and identifiable shapes to be made

Recognising self-cells

• You have 10 million different types of lymphocytes in your body - each one can recognise a different shaped antigen

• Lymphocytes are made when you are a foetus. When you are a foetus in the womb you are unlikely to be exposed to any cells other than self cells. The lymphocytes complementary to the antigen on self-cells will die or production will be suppressed. This is to prevent lymphocytes from attacking your own cells

• The only remaining lymphocytes one complementary to pathogenic and non- self cells. The same process occurs after birth in the bone marrow. Any new lymphocytes made in the bone marrow which are complementary in shape to antigens on self-cells will be destroyed

Sometimes this process doesn't work properly and lymphocytes which will attack sef-cells are produced

This is what causes the symptom of autoimmune diseases

Antigens detected

If a non-s elf cell is detected, a response will be triggered to destroy the cells. These different surface molecules enable them to identify: pathogen, cells from other organisms of the same species, abnormal body cells and toxins

Non-self: pathogens (e.g. Bacteria, fungi or viruses such as HIV)

Immune system recognises antigens as being foreign and activates cells to destroy the pathogens

Non-self: cells from other organisms of the same species (harmful for those with organ transplant)

There cells may have different antigens to the body's cells and so are identified as being foreign

Non-self: Abnormal body cells (e.g. Cancer cells)

Trigger an immune response

Non-self: Toxins (some pathogens release toxins into the blood , such as cholera)

Can be recognised by the immune system

What is an antigen?

Antibody generating molecule. Can be found on the surface of cells. Anything that the immune system will recognise as non-self or foreign

Antigen molecules can be (the most effective are large and complex)

Proteins, polysaccharides, glycoproteins, glycolipids and a free molecule e.g. a toxin

Epitopes

Pathogens posses highly specific antigenic determinants

Antigen variability

Pathogenic DNA can mutate frequently. If a mutation occurs in the gene which codes for the antigen, then the shape of the antigen will change. Any previous immunity to this pathogen is no longer effective, as all the memory cells in the blood will have a memory of the old antigen shape. This is known as antigen variability. The influenza virus mutates and changes its antigens very quickly and this is why a new flu vaccine has to be created each year

Antimicrobial proteins

Chemical proteins produced to kill pathogens or reduce their ability to reproduce

First line of defence e.g. Barriers (barriers to infection)

A type of Non-specific defence mechanism - immediate

Skin

(Dead cells containing the protein keratin) hard for microbes to digest, sloughed off constantly. Acts as a physical barrier to block pathogens from entering the body. Chemical barrier by producing sebum, an oily, antimicrobial substance that lowers pH to inhibit the growth of pathogens

Mucous membranes

Produce mucus to trap microbes in trachea, bronchi and bronchioles. Parts of the ears, nose, throat and digestive tract are lined by mucus membranes. These membranes secret mucus to trap pathogens and use lysozymes to destroy them

Cilia

Microbes trapped in mucus are transported away from lungs by beating cilia

Tears

Washes eyes and contains lysozyme enzymes that digest microbes

Saliva

Flushes bacteria away, also contain lysozymes

Expulsive reflexes

Coughing and sneezing are methods for expelling foreign objects (or pathogens) from the gas exchange system. Vomiting and diarrhoea expel the contents of the gut along with any pathogens present from digestive system

Blood clotting and Wound repair

A cut to the skin provides a possible entry for pathogens and so blood clots act quickly to seal any wounds. The clot dries out to form a scab that blocks entry to the body. After a scab has formed, the skin is capable of repairing itself to reform its physical barrier. Epidermal cells underneath the scab divide while damaged blood vessels regrow and collagen fibres are used to provide strength to new tissue. Helps to prevent blood loss. Once the epidermis is the required thickness the scab breaks off and the wound is healed

Cuts damages blood vessel→ formation of fibrin network →

Traps more platelets and blood cells → forms clot → protects would while tissue below regenerates

Antimicrobial proteins

Chemical proteins produced to kill pathogens or reduce their ability to reproduce

Where do white blood cells come from?

Stem cells in the bone marrow

The lymphatic system

How cells of the innate immune system can move around the body

Cytokines

Cell signalling molecules that aid cell to cell communication in immune responses and stimulate the movement of cells towards sites of inflammation, infection and trauma. Recruit more phagocytes to site of infection. Can increase body temperature (helps prevent pathogens from reproducing). Also stimulate T helper cell. Released when phagocyte engulfs a cell

Inflammation

Combusts of swelling, heat, redness and pain at site of infections or wounds. Generally occur when an infection is relatively localised. Mast cells (found in connective tissue below the skin and around blood vessels), in addition to basophils releases histamines. Histamines cause blood vessels in the area, particularly arterioles ( branch of an artery) to dilate, causing local heat and redness and capillary was are made leaky resulting in fluid (plasma) containing WBCs -mainly neutrophils and antibodies to be forced out of the capillaries, causing swelling and pain

Inflammation can be triggered by damaged tissue which release chemicals affecting blood vessels in two ways:

• Blood vessels dilate which increases blood flow to the area, making it hotter to prevent pathogens from reproducing

• Blood vessel walls become more permeable so that they start to leak tissue fluid, causing swelling and isolating any pathogens in the damaged tissue

Non-specific defence mechanisms

Act quickly to defend the body, but respond in the same way for all pathogens (physical and chemical barriers and phagocytosis)

Specific defence mechanisms

These are slower to defend the body, but produce a specific response for each pathogen (cellular response and humoral response)