AS/A-level Biology - Pathogens and Immunology (1)

• Pathogens can be transmitted by droplets, air, direct contact with the skin, through food contamination, the urino-genital system and through the gas exchange system.
• The body prevents entry of pathogens with a thick layer of skin, mucus, stomach acid and the immune system.
• Overcrowded conditions enable diseases to spread rapidly.
• For a microorganism to be considered a pathogen, it must gain entry to the host, colonise the tissues of the host, resist the defences of the host and cause damage to the host tissues.
• Pathogens can enter the host cell by endocytosis or by producing enzymes that breach the host cell membrane.
• Viruses enter cells and ruptures them cause them to release nutrients. They break down nutrients in the cell which starves the cell and they can replicate, causing the cell to burst.
• Bacteria produce cell killing toxins. Exotoxins are proteins that are secreted by bacteria which are engulfed by macrophages and cause the macrophages to produce proteins that alter the body's temperature-regulating mechanisms.
• The period of time between infection and the appearance of the signs and symptoms of a disease is called the incubation period.

Epidemics - Widespread outbreaks of disease.
Pandemics - Epidemics that spread internationally.

• Physical barrier
• Phagocytosis
• Inflammation due to the blood vessels in the affected area becoming more permeable and so more white blood cells and antibodies can come to the infected area.
• Body temperature rises.

Phagocytosis

• They produce chemicals which recognise the antigens are foreign - chemicotaxis.
• This makes them attracted to the pathogens.
• The cell engulfs the bacterium which is then held in the phagocytoic vesicle.
• The Lysosomes fuse with the vesicle contents and discharge their contents.
• Enzymes from the Lysosomes digest the bacteria.
• The waste products are expelled by exocytosis.
• The phagocyte presents the pathogen's antigens on the surface to activate other immune systems cells.

• A T-cell with complimentary antibodies on its surface to the non-self antigens on the phagocytes surface comes along and binds to it.
• The phagocyte then activates the T-cells to divide by mitosis.
• The T-cells release substances to activate the B-cells which, after having bound with the pathogen, start reproducing, becoming plasma cells and start producing antibodies.

Can you differentiate T-Cells and B-Cells in AS/A-level Biology?

T-Cells                      

• Mature in the thymus gland
• Cell-meditated immunity
• T-cells secrete antibodies that remain on the cell
• The whole cell attacks the pathogen

• Mature in the lymph
• Humoral immunity
• B-cells release antibodies
• Antibodies attack the pathogen

Humoral immunity involves antibodies whereas cell-meditated response involves T-cells and phagocytes.

• A protein made by the host's B cells, made of chains of amino acid monomers, linked by peptide bonds.
• The antibody is specific due to the variable region.
• The constant regions are the same in all antibodies.
• The different chains are held together with disulfide bridges.
• Antibodies coat pathogens by forming antigen-antibody so that they are easier to engulf and prevent them from entering the host's cells.
• They bind with and neutralise the toxins produced from the pathogen.

In part 2, you will know more about antigens, primary and secondary responses.

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Drafted by Bonnie (Biology)