At the site of infection, damaged white blood cells and mast cells release histamines that cause arterioles to dilate and capillaries to become more permeable. Blood flow to the area increases and plasma, white blood cells and antibodies leak out into the tissue. This causes oodema (swelling) and often pain. This enables white blood cell to attack. It also causes heat and redness. The locally raised temperature reduces the effectiveness of pathogen reproduction in the area.

Fever – causes the hypothalamus to reset to a higher body temperature which will combat infection – reduce the ability of many pathogens to reproduce effectively and also the specific immune response works better at higher temperatures.


Lysosome action:

An enzyme found in tears, sweat, saliva and nasal secretions, it destroys bacteria by breaking down the bacterial cell walls – causing lysis. Lysozymes protect the body from harmful bacteria in the air and food.



A chemical released from cells which inhibits/stops protein synthesis in viruses – prevents the virus from multiplying.



White blood cells engulf, digest and destroy bacteria and foreign material which is too large to diffuse in the form of vesicles released from the cell-surface membrane.

These phagocytes include neutrophils (most abundant, first to arrive, leave blood capillaries by squeezing between capillary walls, manufactured in bone marrow, short lived), lymphocytes and monocytes which become macrophages (circulate in blood for a few days before they move into the tissue, manufactures in bone marrow, settle in lymph nodes).

  • Phagocytes attach themselves to the surface of the pathogen by recognising their antigens
  • Pathogen engulfed by phagocytes by vesicles (cytoplasm moves around the pathogen) – forms a phagosome
  • Enzymes in lysosomes join the phagosome and release their contents (fusion)
  • The enzymes digest and break down the pathogen
  • Discharge of waste materials/indigestible material
  • The phagocyte presents the pathogen’s antigens. It sticks the antigens on the surface to activate other immune system cells


The immune system is the specific response of the body to invasion by pathogens. It enables the body to recognise anything that is non-self and remove it from the body efficiently.

The immune system has 4 key characteristics:

  • It can distinguish ‘self’ from ‘non-self’
  • It is specific – responds to specific foreign cells
  • It is diverse – recognise ~10 million different antigens
  • It has immunological memory – once responded to a pathogen, you can respond rapidly if you meet it again