4.1.9 Primary & Secondary Immune Responses

• During clonal expansion T and B cells form memory cells
• Memory cells form the basis of immunological memory – the cells can last for many years and often a lifetime
• There are two types of immune response:
  • Primary immune response (responding to a newly encountered antigen)
  • Secondary immune response (responding to a previously encountered antigen)

The primary immune response

• The primary immune response has a considerate time delay
  • It takes considerable energy and time for:
    • The clonal selection and expansion of specific T cells and B cells
    • The 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
• Some of the B cells differentiate during clonal expansion to become plasma cells and memory cells
  • Plasma cells are short-lived

• A portion of the selected T cells also differentiate into memory cells
• Memory cells remain circulating in the blood for a long time and allow for a rapid secondary immune response

B memory cells and the secondary immune response

• If the same foreign antigen is found in the body a second time, the B memory cells recognise the antigen
• B memory cells divide very quickly and differentiate into plasma cells (to produce antibodies) and more memory cells
• This response is very quick, meaning that the infection can be destroyed and removed before the pathogen population increases too much and symptoms of the disease develop
• This response to a previously encountered pathogen is, relative to the primary immune response, extremely fast
  • The response is quicker because there are more memory cells present to be selected than there were  cells within the original clone (that existed prior to the first infection)
  • More memory cells can be selected and so more antibodies are produced within a short time period

During a secondary immune response, B memory cells that remained in the blood divide very quickly into plasma cells (to produce antibodies) and more memory cells

The concentration of antibodies during the primary and secondary immune responses

T memory cells and the secondary immune response

• T-lymphocytes also play a part in the secondary immune response
• They differentiate into memory cells, producing two main types:
  • Memory helper T cells
  • Memory killer T cells

• Just like the memory cells formed from B-lymphocytes, these memory T cells remain in the body for a long time and provide long-term immunity

• If the same antigen is found in the body a second time, these memory T cells become active very quickly