AQA A Level Biology复习笔记2.5.7 The B lymphocyte Response

B-lymphocytes and the humoral immune response

• B-lymphocytes (B cells) remain in the bone marrow until they are mature and then spread through the body, concentrating in lymph nodes and the spleen
• Millions of types of B-lymphocyte cells are produced within us because as they mature the genes coding for antibodies are changed to code for different antibodies
• Once mature, each type of B-lymphocyte cell can make one type of antibody molecule
• At this stage, the antibody molecules do not leave the B-lymphocyte cell but remain in the cell surface membrane
• Part of each antibody molecule forms a glycoprotein receptor that can combine specifically with one type of antigen
• If that antigen enters the body, B-lymphocyte cells with the correct cell surface receptors will be able to recognise it and bind to it (clonal selection)
• These specific B-lymphocytes divide repeatedly by mitosis (clonal expansion) and differentiate into two main types of cell:
  • Plasma cells
  • Memory cells

   

• These two cell types each have a specific function

The maturation of B-lymphocytes – by the time a child is born, it will have millions of different types of B-lymphocytes, each with a specific antibody receptor

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

During a primary immune response, B-lymphocytes form two types of cell