Memory Cells & Immunity (CIE A Level Biology)

Developing immunity

• The immune system is activated when a new antigen is encountered
• This launches a primary immune response consisting of a non-specific immune response followed by a specific immune response
  • The primary response occurs the first time an antigen is encountered by the immune system
• Since it is the first time the immune system has encountered the antigen, the numbers of T and B cells with the correct membrane receptors present in the blood will be low
• It will take time for the correct lymphocytes to be activated and to divide and for different cell types to develop
  • It can take several days before plasma cells develop and are able to start producing antibodies against an antigen
  • This is the reason why an infected person will experience symptoms of the disease the first time they contract it
• The primary response produces memory cells which remain in the blood after an infection is over
  • The presence of memory cells means that a person is said to be immune to the pathogen
  • Memory cells can remain in the blood for many years, so the immunity gained from a primary response can be long-term
• Should the immune system encounter the same antigen again in the future it will launch a secondary immune response which will be much faster and stronger than the primary response
  • Memory cells are present in larger quantities than the mature lymphocytes at the start of the primary response, so the correct memory cells are able to detect an antigen, activate and divide by mitosis much more quickly
  • Antibodies are produced more quickly and in larger concentrations in a secondary response
• This will often eliminate the pathogen before the infected person can show symptoms

Immunity graph

The secondary response is much larger and faster than the primary response due to the presence of memory cells in the blood