- When an antigen is introduced for the first time in a host, it activates T and B cells.
- Activation of B- cells produces memory cells and plasma cells.
- Plasma cells produce antibodies whereas memory cells are long lived and remain in blood for a long time.
- This immune response induced for the first time in host is called primary immune response.
- There are different phases during the development of the immune response.
- Duration from the entry (injection) of antigen to the appearance of first antibody in blood is called lag phase which is normally of 4-7 days in a primary immune response.
- T and B-cells are activated during this phase for the production of antibodies.
- The level of antibody increases continuously, reaches the peak level and then declines.
- If same antigen is injected into the same host for the second time in life, secondary immune response is induced.
- Secondary immune response occurs due to the persistence of memory cells in the blood.
- These memory cells directly get converted into plasma cells when activated by antigen.
- In this case, activation and mitosis of T and B cells are not required because of which the lag phase is shorter (1-3 days) in secondary immune response.
- Usually, 100 times more antibodies are produced in secondary immune response.
- Unlike in primary immune response, IgG is produced instead of IgM in secondary response.
- Antibodies produced in secondary response have greater affinity for antigen than the antibody produced in primary immune response.