immunity

Person gets a splinter
- triggers a non specific inflammatory response in the region on puncture
- histamine chemical is released
- dilate the capillary in diameter and it becomes leaky
- tissue around the splinter becomes red and swollen as plasma leak in tissues

Damaged or infected tissue release proteins and other chemicals
- attracts phagocytes to the area
- advantage of leaky capillaries - white blood cells (phagocytes/neutrophils) leave capillaries easily
- phagocytes engulf the dead cell and bacteria

Cells of the immune system

• All orignate from the bone marrow
• All are leukocytes = white blood cells
• 2 main groups:
• phagocytes - neutrophils an macrophages
• lymphocytes - B and T lymphocytes
• Phagocytes - engulfs dead cells and invading micro-organism
• Neutrophils
• short live
• smaller than macrophages
• travel around the body in the blood
• squeeze through walls of capillaries
• have a lobed nucleus
• Monocytes/macrophages
• large and long lived
• circulate in blood as monocytes which then settles in organs such as lungs lymph nodes
• develop into macrophages that remain in the organ
• remove foreign matter from organs
• initiate specific immune response

Immune system

2 Parts:

• Non specific immunity
• 2nd line of defence
• immediate attack against a variety of 'attackers'
• present from birth
• does not distinguish between different pathogens
• same response each time
• e.g. phagocytosis (engulfing) and inflammation
• specific immunity
• 3rd line of defence
• body's response is tailored for a specific 'attacker'
• involves lymphocytes and antibodies
• response is slow when it meets the pathogen for the first time
• involves white blood cells particularly B lymphocytes which produce chemicals called antibodies which target specific pathogens

Antigens

• all cells have surface markers called antigens
• antigens are molecules that stimulate an immune response causing the production of antibodies 
• antibodies are specific to antigen
• the body can recognise antigen as self or non-self (foreign)
• our own antigen do not stimulate a immune response

Antibodies

• Y shaped molecules
• made of 4 polypeptide chains held together by disulphide bridges
• 2 identical short or light chains
• 2 identical long or heavy chains
• consists of two regions
• constant region - same in all antibodies
  enables antibodies to attach to phagocytes and helps in the process of phagocytosis
• variable region, differs from one type to another - result of amino acid sequence which is complementary to shape of antigen
• hinge region allows the molecule to bend so the branches of the Y shaped molecule can move further in order to allow attachment to more than one antigen

Lymphocytes

• involved in specific immune response
• have a large nucleus which fills most of the cell
• 2 types:
• B lymphocytes
• matures in the bone marrow
• T lymphocytes
• mature in the Thymus gland
• both must be mature before they can take part in the immune response.
• during maturation both B and T lymphocytes  develop receptors on their cells surface membrane.
• these specific receptors are able to combine with specific antigen in much the same way that an enzyme combines with a specific substrate
• Humans can produce several million different types of B and T cells (initially only a few of each types)
• B and T cells circulate between the blood and lymph so are distributed around the body where they may come in contact with antigens

Maturation of B and T cells

• In the bone marrow immature B and T cells divide by mitosis
• B cells matures in the bone marrow
• mature B cells each with a different antibody receptor on cell membrane
• T cells mature in the thymus
• mature T cells each with a different T cell receptor on the membrane

B lymphocytes

• many different types of each capable producing an antibody (=protein) that acts against a particular antigen
• when a pathogen enter the body, B cells with the appropriate receptors bind to the antigens = clonal selection.
• causing them to divide rapidly by mitosis to form a large clone of identical cells = clonal expansion.
• The selected B cells differentiate into 2 types of cells
• plasma cells
• produce antibodies, several thousand/second
• antibodies are secreted into the blood
• plasma cells are short lived (several weeks)
• memory cells
• long live (several years)
• remain dormant in the blood
• ready to respond immediately to further attacks by antigen
• providing an immunological memory.

The cells of the immune system originates from bone marrows where stem cells divide by mitosis to produce cells that differentiate into lymphocytes and phagocytes. Immature T lymphocytes migrate to the thymus gland where they mature. Mature B lymphocytes and mature T lymphocytes circulate and enter lymph nodes. During an immune response some B lymphocytes differentiate into plasma cells and secrete antibodies.