Immunology

Question 1

Immune system

Answer 1

The immune system is the body’s defense against disease. It consists of white blood cells (leukocytes), which fall into four categories: phagocytes (phagocytosis), granulocytes (inflammation), T lymphocytes (cell-mediated immunity) and B lymphocytes (antibody-mediated immunity)

Question 2

Phagocytosis

Answer 2

Phagocytosis, the digestion of microbes and foreign bodies by phagocytes (large, irregularly shaped white blood cells with a fluid cytoskeleton), is a branch of the non-specific immune system. Phagocytes crawl through blood and tissue fluid (plasma from the capillaries that delivers nutrients) in response to chemicals released by microbes or other WBCs. They change their shape to engulf the microbe, trapping it in a membranous sac called a phagosome. The phagosome fuses with lysosomes, small vesicles containing hydrolytic enzymes, destroying the microbe by breaking it up into its component parts

Question 3

Inflammation

Answer 3

Inflammation is a localised response to infection, driven by granulocytes. Granulocytes release chemicals (histamines and prostaglandins) which stimulate vasodilation (to increase blood flow to the area, warming it up and inhibiting bacterial reproduction), capillary leakage (so phagocytes and granulocytes can enter the tissue fluid), sensory neurone pain impulses (so the brain is aware of the damage) and blood clotting (to seal the wound from pathogens). Inflammation also repairs the wound, creating scar tissue

Question 4

Specific immunity

Answer 4

Specific immunity involves lymphocytes. It is slower than non-specific immunity, but provides long lasting immunity. Cell-mediated responses involve T lymphocytes (from the thymus gland); humoral responses involve B lymphocytes (from the bone marrow)

Question 5

Cell-mediated immunity

Answer 5

A phagocyte destroys a pathogen and places its antigens on its cell-surface membrane, becoming an antigen-presenting cell. Receptors on T helper cells bind to complementary antigens and release chemicals (interleukin-1) that cause other T cells to divide rapidly by mitosis. This clone of cells can develop into memory cells (to enable a rapid future response to reinfection), stimulate phagocytosis and stimulate B cells (producing antibodies). T killer cells also kill infected cells (e.g. for viral infection) by releasing cytotoxic enzymes which make the cell surface membrane freely permeable. Ions and water enter the cell, causing it to burst and preventing further infection

Question 6

Humoral response

Answer 6

Surface antigens of the invading pathogens are taken up by B cells, which present them on their surface. T helper cells (activated by the cell mediated response) attach to the antigens and activate the B cells. These cells divide to form a clone of plasma cells, which produce antibodies that compliment the antigens on the surface of the pathogen. These antibodies can destroy the pathogen (the primary immune response). Some B cells develop into memory cells, which produce specific antibodies in the case of future infection (the secondary immune response)