Primary Immune Response (CIE A Level Biology)

• Lymphocytes are another type of white blood cell
• They play an important part in the specific immune response
• They are smaller than phagocytes and they have a large nucleus that fills most of the cell
• They are produced in the bone marrow
• There are two types of lymphocytes with different roles in the immune response
  • B-lymphocytes, or B cells
  • T-lymphocytes, or T cells
• The action of lymphocytes is triggered by exposure to non-self antigens:
  • When a pathogen enters the body its antigens are displayed by antigen presenting cells
  • These antigens may be on the surface of macrophages after phagocytosis, on the surface of infected cells, or on the surface of the pathogens themselves

B-lymphocytes

• B-lymphocytes remain in the bone marrow until they are mature and then spread through the body, concentrating in lymph nodes and the spleen
• Millions of different B-lymphocyte cells are produced within the bone marrow
• Once mature, each different B-lymphocyte cell has a different antibody molecule on its surface
  • 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 receptor that can combine with complementary antigens from pathogens; these receptors are known as antibody receptors, as they are made of antibodies
  • Each different B cell has an antibody receptor that is specific to one type of pathogenic antigen; this is why the action of B cells is part of the specific immune response

B Cell Maturation Diagram

Mature B cells have many different antibody receptors on their cell surface membranes

B lymphocytes in the specific immune response

• The arrival of non-self antigens in the body triggers the following series of events in B lymphocytes
  • The B lymphocytes, with antibody receptors that are complementary to the new antigen, come into contact with the new antigen
  • The B lymphocytes bind to their complementary antigens and are stimulated to divide by mitosis
    • This process of finding and stimulating the correct lymphocytes is known as clonal selection
    • T helper cells play an important role in initiating the division of B cells at this stage (see below)
  • The B lymphocytes divide repeatedly by mitosis, producing many cloned B lymphocytes with complementary antibody receptors
    
    • The cloning of B lymphocytes at this stage is known as clonal expansion
  • The cloned B lymphocytes develop into
    • Plasma cells that secrete antibodies
      • The antibodies are complementary to the antigens that initiated the immune response 
    • Memory cells that remain in the blood, providing immunity; these are known as B memory cells

Primary Immune Response Diagram

During a primary immune response, B-lymphocytes divide by mitosis, resulting in the production of plasma cells and memory cells

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 type of antigen

T Cell Maturation Diagram

Mature T cells have many different types of T cell receptors on the cell surface membranes

T lymphocytes in the specific immune response

• The T lymphocytes undergo a similar process to that seen in B cells
  • T-lymphocytes are activated when they encounter and bind to their specific antigen
    • This is clonal selection
  • These activated T-lymphocytes divide by mitosis to increase in number
    • This is clonal expansion
  • These T-lymphocytes differentiate into different types of T cell:
    • T-helper cells
      • These cells release cytokines, a group of chemicals that
        • stimulate B-lymphocytes to divide
        • cause an increase in the rate of phagocytosis by macrophages
    • T-killer cells
      • These cells attach to antigens on the cell surface membranes of infected body cells and secrete toxic substances that kill the infected cells
• This response to a newly encountered antigen is known as a primary immune response
  • Primary responses are slow due to the time taken for clonal selection and expansion

T helper cells are involved with the activation of B cells, while T killer cells kill infected body cells