The Human Immune Response

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Humans have two aspects in their immune system:

• Innate immune system and acquired immune system

• The innate system makes up the first two lines of defence and the third line of defence is the acquired system

1. Innate immune system

- Present in all organisms

- Born with it

- Physical Barriers - skin, mucous membranes, tears)

- Also the presence of blood cells which identify and fight off foreign particles

First Line of Defence (FLOD):

• Physical and chemical barriers to stop pathogens entering the body

• Skin makes up most of the FLOD - sweat and oil makes it hard for pathogens to colonise

• Mucous membranes line the openings of the body - (throat, lungs... etc) - sticky to catch pathogens and cough them back up

• Other components: Skin, mucous membranes, Cilia, urinary and vaginal tracts, Alimentary canal, tears, urine.

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Second Line of Defence (SLOD):

• Inflammatory response and white blood cells (WBC)

• If the pathogen passes the first line of defence it meets the internal structures the body uses to fight off disease

- Include WBC and MHC proteins (major histocompatibility complexes)

MHC - Type one

- Specialised proteins that every cell has

- Unique to every organism

- Determines which cells belong in the body

- If the cell does not have the correct MHC, it is marked by the immune system for destruction.

- This is how pathogens are detected

WBC

- 5 different types

• Neutrophils, eosinophils, basophils, monocytes and lymphocytes.

- Basophils - inflammatory response

• When tissue is damaged and chemicals are released causing basophils and mast cells to release histamine and heparin.

• Histamine causes blood to flow to injured area. Heparin stops blood clotting.

• Helps WBC reach infected site

• If the inflammation is bad enough, pyrogen is released which raises the temperature of the body, causing fever.

• The heat stops it spreading and prevents it reproducing or surviving.

• Prostaglandins help direct blow flow to injured area.

- Neutrophils and Monocytes

• Called Phagocytes - ingest and destroy other cells

• Neutrophils are single-celled phagocytes which can mature into large cells called macrophages

• Neutrophils can inject one pathogen (macrophages up to 100) and can die due to bacterial toxins - causing puss.

- Eosinophils

• Kill macro parasites (worms)

• Help in an allergic reaction

- Lymphocytes

• Found in lymphatic system

• Responsible for acquired immune system

• Lymphatic system filters body tissue fluid and nodes trap pathogens where lymphocytes and macrophages attack them

- The Complement System

• Made up 20 proteins, formed in the liver

• Activated by one of three pathways (Classical Pathway, MB Lectin pathways and Alternative pathway)

Classical Pathway - recruitment of inflammatory cells

MB-Lectin Pathway - Opsonisation of pathogens

Alternative Pathway - Killing of pathogens

• Once activated they recruit inflammatory cells, signal phagocytes to consume pathogens, or break down a cell wall to connect it

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2. Acquired immune system

Third Line of Defence:

• Made up of two different types of lymphocytes:

- T-lymphocytes and B-lymphocytes

• T-Lymphocytes

- Mature in the thymus

- Responsible for attacking and destroying pathogens that have entered into specific cells

- 4 main types:

• Cytotoxic T-cells - produce toxic substance that destroy cells that have been invaded by a pathogen

• Helper t-cells - helps b-cells divide rapidly

• Suppressor t-cells - turns off immune response and antibody production when they are no longer needed

• Memory t-cells - Recognises antigen if it is re-introduced, providing long-term immunity

The Humoral Response

• B-Lymphocytes

- Do not mature in the thymus, often in bone marrow

- Responsible for identifying, signalling and attacking pathogens in the blood and tissue fluid surrounding cells (humoral response)

- Congregate in lymph nodes, tonsils, spleen, appendix and other specialised areas

- Produce antibodies made from small proteins called gammaglobulins whenever an antigen is detected by the MHC

- The gammaglobulins create 5 different types of of antibodies (immunoglobulins)

• Stick to the pathogen, block essential sections of the pathogen, make multiple pathogens clump together, neutralise pathogen toxins or dissolve the pathogen.

• MHC - Type 2

- Enables immune system to recognise condition of cell - eg, cancerous, infected... etc

- Facilitate binding by the T-Lymphocytes

• When a b-cell encounters and antigen it will divide rapidly

• This is to create many copies of the correct antibody producing plasma cells

• The plasma cells mature and begin to release the correct antibody which will bind with the antigens on the pathogen.

• A second type of b-cell is also formed through division called a memory cell

• They exist long after the pathogen has been destroyed to create new plasma cells should it ever return

- This is what gives humans acquired immunity

The Cell-Mediated Response

• Antigens stimulate the production of T-cells in the thymus gland.

• Bind to antigens and facilitate an immune response including attracting phagocytes, preventing foreign matter from leaving the site, activating monocytes, and killing the pathogen by punching holes in its membrane

• Some T-Lymphocytes also become memory cells

•T-cells and B-cells sometimes work together and sometimes work independently

• Helper t-cells help b-cells whereas cytotoxic t-cells do not need b-cell activation to perform their work

• Certain diseases attack lymphocytes: eg. HIV attacks t-cells

• Some diseases caused because the lymphocytes malfunction and accidentally begin attacking the healthy cells. These are called autoimmune diseases.