OM NAMO BIOLOGY: Immune System

Defense System

White cells (phagocytes) contain digestive enzymes within lysosomes
- Neutrophils primarily engulf bacteria
- Macrophages engulf larger particles; including old and infected red blood cells
Found in blood, lymph systems and tissues
Squeeze through gaps in the walls of venules to enter tissues
This allows them to move faster to tissues infected with pathogens

Molecule that stimulates an immune response
Usually proteins (polysaccharides, nucleic acid, lipids can also act as antigens) and other inorganic molecules important forself-recognition
Self-antigen
- Only found on the host's own cells and does not trigger an immune response
- As these are proteins, their structure depends on the amino acid sequence
- The gene for this sequence is highly polymorphic, having several alleles at each loci
- There is great genetic variability between individuals
- Thus, Antigen is different in other people → would cause an immune response
- There is only 1:4 change that siblings will possess an identical antigen
Non-self-antigen
- Found on cells entering the body (e.g. bacteria, viruses, another person's cell)
- Will cause an immune response

Secreted by B-lymphocytes and produced in response to a specific (foreign) non-self antigen
B-lymphocyte's receptor site matches the non-self-antigen
Each antibody is produced by one type of B-lymphocyte for only one type of antigen
An antibody is Y-shaped
- The two ends of the Y are called the Fab fragments
- The other end is called the Fc fragment
- Fab fragment is responsible for the antigen-binding properties
- Fc fragment is the effector component and triggers the immune response
B cells divide and form memory cells and antibody-secreting plasma cells:
- Agglutination makes pathogens clump together
- Antitoxins neutralise toxins produced by bacteria
- Lysis digests bacterial membrane, killing the bacterium
- Opsonisation coats pathogen in protein that identifies them as foreign cells

Lymphocytes undergo maturating before birth, producing different types of lymphocytes
Humoral response - B lymphocytes
- Produce and release antibodies into blood plasma
- Produce antibodies from B plasma cells
- Recognize foreign antigen directly
Cellular response - T lymphocytes
- Bind to antigen carrying cells and destroy them and/or activate the humoral response
- Recognize foreign antigens displayed on the surface of normal body cells
Primary response produces memory cells which remain in the circulation
Secondary response new invasion by same antigen at a lower state. Immediate recognition and distraction by memory cells -faster and larger response usually prevents harm

Response for pathogens not entering our cells (e.g. bacterium)
Each B-lymphocyte recognizes only one specific antigen / need T-helper cell to be activated
Maturation / B-cells develop to give many different variants / specific immune system responds to
any type of pathogen entering the body
Primary response:
- Pathogen is ingested by macrophages / macrophage displays the pathogens surface non-self antigen on its surface (antigen presentation)
- It then joins with specific T-helper cells and B lymphocytes that have membrane receptors and are complementary in shape to the non-self antigen
- T-helper cells will release cytokines to activate selected B-cell/lymphocyte
  - Secretes antibodies of the same type into the blood
  - Divided by mitosis to produce a clone
  - Cells grow to form plasma cells producing masses of free antibodies
- Some of the cells remain in the blood as memory cells.
Secondary response = new invasion by same antigen at lower state. Immediate recognition and distraction - faster, larger response usually prevents harm. Antibodies are produced more rapidly and in larger amounts

Virus enter cell and more difficult to remove
No antibodies involved / work directly on the infected cell by destroying it
Special proteins called Major Histocompability Complex (MHC) are present on all human cells
Non-self antigen interacts with MHC as human cell becomes infected by a pathogen

Specific T-lymphocyte recognises specific non-self antigen only with a chemical marker next to it (MHC)
Activated T-lymphocytes multiply by mitosis and enter circulation
Cells differentiate into different types of cell
- Cytotoxic T-Cells destroy pathogens and infected cells by enzyme action, and secrete chemicals which attract and stimulate phagocytes
- Helper T-Cells stimulate the activity of the cytotoxic T-Cells and B-lymphocytes by releasing chemicals (cytokines andinterleukins). Destroyed by HIV
- Suppressor T-Cells switch off the T and B cell responses when infection clears
- Memory T-Cells Some activated T-Cells remain in the circulation and can respond quickly when same pathogen enters body again

IMG 3-12-3
Table 3-12-3: Different types of immunity

Active (Antibodies made by the human immune system, long term acting due to memory cells)	Passive (Given-Antibodies, short term acting)
Natural	- Response to disease - Rejecting transplant	- Acquired antibodies (via placenta, breast milk)
Artificial(immunisation)	- Vaccination (Injection of the antigen in a weakened form)	- Injection of antibodies from an artificial source, e.g. anti venom against snake biter
Differences	- Antibody in response to antigen - Production of memory cells - Long lasting	- Antibodies provided - No memory cells - Short lasting

Vaccine containing dead pathogens. Antigen is still recognised and an immune response made
- Salk polio vaccine (Polio vaccine is injected)
- Influenza
- Whooping cough
Vaccine containing a toxin
- Diphteria
- Tetanus
Vaccine containing an attenuated (modified or weakened) organism which is alive but has been modified so that it is not harmful
- Sabin polio vaccine (Taken orally, often sugar pumps)
Purified antigen - genetically engineered vaccine
- Hepatitis B (A gene coding for a surface protein of the hepatitis B virus has been inserted into yeast cells which produce the protein when grown in fermenters)