Primary and Secondary Responses

• Primary immune response


  Responding to a newly encountered antigen
• Secondary immune response


  Responding to a previously encountered antigen
• Primary immune response


  1. Clonal selection and expansion of specific T cells and B cells

  2. Synthesis of antibodies
• Antibodies do not begin to appear in the blood until roughly 10 to 17 days after the foreign antigen first entered the body
• This time delay is why we often experience symptoms of a disease when we are first exposed to a pathogen
• Clonal expansion of T and B cells

  Form memory cells
• Memory cells


  • Can last for many years and often a lifetime
  • Form the basis of immunological memory
• Secondary immune response


  1. B memory cells recognise the antigen

  2. B memory cells divide quickly and differentiate into plasma cells and more memory cells

  3. More memory cells can be selected and more antibodies produced quickly
• Secondary immune response


  Extremely fast compared to primary immune response
• Secondary immune response


  T memory cells (helper and killer) become active quickly
• Immunological memory (made possible by memory cells) is the reason why catching certain diseases twice is so unlikely
• Some infections such as the common cold and influenza are caused by viruses that are constantly developing into new strains, so the primary immune response must be carried out each time before immunity can be achieved