• T-Helper cells (a type of lymphocyte that responds to specific antigens) and mature?B cells?(another type of lymphocyte) have specific receptors located on their cell surface membranes
  • These receptors have a?similar structure to antibodies?and are each?specific to one antigen
  • Note that lymphocytes are a type of white blood cell involved in the specific immune response; there are several different types of lymphocyte
• When?phagocytes?engulf pathogens, they?present the pathogen antigens?on their own cell surface membrane
  • A cell with non-self antigens on its surface membrane is known as an antigen presenting cell
• The T-helper cell with the?complementary receptor proteins to the antigen?will bind to the antigen and become?activated?by the phagocyte
• Activated T-helper cells?then bind with?complementary receptors?on the surface membrane of specific?B-lymphocytes
• On binding, the?T-helper cells?releases?signalling proteins?and?activate these B-cells

Antigens activate complementary T-helper cells which go on to activate complementary B-cells

• During an immune response, B-lymphocytes?mature?to form?two types of cell:?plasma cells?and?memory cells
• Plasma cells?produce large volumes of?antibodies?specific to the single type of antigen that triggered the immune response
• The cells are specialised with large amounts of?rough endoplasmic reticulum?which promotes?protein synthesis?to make the required antibodies
• As B-cells only produce one type of antibody, only a?small proportion of the genes are expressed?in the nucleus

• Clonal selection?involves?identifying and activating a B-cell?with the?complementary receptor?to the target antigen
• Clonal expansion?can then occur
  • The?activated B-cell divides by?mitosis?to create many clones of itself
    • Each?clone?will produce?the exact same antibody, complementary to the target antigen
• Some of these mature B-lymphocytes differentiate into?plasma cells
• The other B-lymphocytes become?memory cells?that? remain and circulate in the blood
  • Whilst the antibodies produced by the plasma cells are only present for a matter of weeks or months, memory cells form the basis of?immunological memory?– the cells can last for?many years?and often a lifetime

The primary and secondary immune response

• • A?primary immune response?occurs in response to a?newly encountered antigen
    • This is a relatively?slow response?as the immune system takes time identifying the complementary antibody for each new antigen it encounters
    • The infection may result in?symptoms?being presented?whilst the immune system identifies and manufactures the correct antibodies
  • Secondary immune response?in response to a?previously encountered antigen
    • The memory cells with the correct antibody, are already circulating in the blood so the?response is?more rapid,?producing more antibodies than the primary response, in a much shorter time frame
    • Symptoms do not develop?as the pathogen can be destroyed before significant cell damage occurs

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.? Whereas a primary response occurs much more slowly.