• 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