CIE A Level Biology复习笔记11.1.3 Primary Immune Response

• Lymphocytes are another type of white blood cell
• They play an important part in the specific immune response
• They are smaller than phagocytes
• They have a large nucleus that fills most of the cell
• They are produced in the bone marrow before birth
• There are two types of lymphocytes (with different modes of action). The two types of lymphocytes are:
  • B-lymphocytes (B cells)
  • T-lymphocytes (T cells)

B-lymphocytes

• 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

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

• 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
• During an immune response, these B-lymphocytes then form two types of cell:
  • 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 antigen is relatively slow and is known as a primary immune response

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

T-lymphocytes

• Immature T-lymphocytes leave the bone marrow to mature in the thymus
• Mature T-lymphocytes have specific cell surface receptors called T cell receptors
• These receptors have a similar structure to antibodies and are each specific to one antigen

The maturation of T-lymphocytes – some become helper T cells and others become killer T cells

• T-lymphocytes are activated when they encounter (and bind to) their specific antigen that is being presented by one of the host’s cells (host cells being the human’s own cells)
• This antigen-presenting host cell might be a macrophage or a body cell that has been invaded by a pathogen and is displaying the antigen on its cell surface membrane
• These activated T-lymphocytes (those that have receptors specific to the antigen) divide by mitosis to increase in number (similar to the clonal selection and clonal expansion of B-lymphocytes)
• These T-lymphocytes differentiate into two main types of T cell:
  • helper T cells
  • killer T cells
• Helper T cells release cytokines (hormone-like signals) that stimulate B-lymphocytes to divide and develop into antibody-secreting plasma cells. Some helper T cells secrete cytokines that stimulate macrophages to increase their rates of phagocytosis
• Killer T cells attach to the antigens on the cell surface membranes of infected cells and secrete toxic substances that kill the body cells, along with the pathogen inside

Helper T cells and killer T cells carry out different functions during an immune response