CIE A Level Biology复习笔记11.1.4 Memory Cells & Immunity

• During an immune response, B-lymphocytes form two types of cell: plasma cells and memory cells
• Memory cells form the basis of immunological memory – the cells can last for many years and often a lifetime
• There are two types of immune response:
  • Primary immune response (responding to a newly encountered antigen)
  • Secondary immune response (responding to a previously encountered antigen)

Primary immune response

• When an antigen enters the body for the first time, the small numbers of B-lymphocytes with receptors complementary to that antigen are stimulated to divide by mitosis
• This is known as clonal selection
• As these clones divide repeatedly by mitosis (the clonal expansion stage) the result is large numbers of identical B-lymphocytes being produced over a few weeks
• Some of these B-lymphocytes become plasma cells that secrete lots of antibody molecules (specific to the antigen) into the blood, lymph or linings of the lungs and the gut
• These plasma cells are short-lived (their numbers drop off after several weeks) but the antibodies they have secreted stay in the blood for a longer time
• The other B-lymphocytes become memory cells that remain circulating in the blood for a long time
• This response to a newly encountered pathogen is relatively slow

Secondary immune response

• If the same antigen is found in the body a second time, the memory cells recognise the antigen, divide very quickly and differentiate into plasma cells (to produce antibodies) and more memory cells
• This response is very quick, meaning that the infection can be destroyed and removed before the pathogen population increases too much and symptoms of the disease develop
• This response to a previously encountered pathogen is, relative to the primary immune response, extremely fast

During a secondary immune response, memory cells that remained in the blood divide very quickly into plasma cells (to produce antibodies) and more memory cells

• T-lymphocytes also play a part in the secondary immune response
• They differentiate into memory cells, producing two main types:
  • Memory helper T cells
  • Memory killer T cells
• Just like the memory cells formed from B-lymphocytes, these memory T cells remain in the body for a long time
• If the same antigen is found in the body a second time, these memory T cells become active very quickly