Primary and Secondary Immune Response

  • Immune response evolves in several stages.
  • Most of the antigens require antigen presentation. Antigen-presenting cells (or APC) ingest Ag, process it and present to T helper cells in complex with HLA antigens.
  • During the inductive phase, APC activates T helpers by means of direct contact and cytokine production.
  • T helpers differentiate into Th1 or Th2.
  • These subpopulations re-route immune response towards cell-mediated reactions (Th1) or to humoral immunity activation (Th2).
  • Effector phase is characterized by T cells cytotoxic reactions, the transformation of B cells into plasma cells, which produce specific antibodies.
  • They inactivate and eliminate the antigen.
  • At the same time suppressor mechanisms evolve, which confine immune reactions.
  • Resting long-living specific T- and B-memory cells remain capable of future extensive reactivity in case of secondary contact with the same or similar antigen.

From that point, there is primary and secondary immune response.

Primary immune response

  • It is developed after a short latent period.
  • IgM is shown to be major antibodies, produced at primary immune reactions (in 2-3 days). IgG start to rise in 5-7 days after stimulation.
  • The amplitude of the primary response is not very high.
  • In 2-3 weeks it declines but trace amounts of specific antibodies and immune cells are maintained in the body.
  • Finally, immune memory cells are formed.

Secondary immune response

  • It is characterized by intensive proliferation of specific T- and B cells, followed by the high rate of antibody production predominantly of IgG class.
  • The elevated levels of IgG tend to persist much longer than in the primary response.
  • The affinity of T cell receptors and antibodies is increased. All these events promote rapid antigen elimination.

Primary and Secondary Immune Response