What is a Primary Immune Response?

Immune response to new antigens is known as primary response. When a new pathogen enters in to the body, immune system takes little time to mount response.

During this period injmune system learns about the pathogen and gets ready to eliminate it from the body.

The preparation period that is the period between entry of pathogen and expression of immune response is known as “latent period”. Latent period in humoral immunity indicates activation and proliferation of naive B cells to produce plasma cells and memory cells.

Plasma cells secrete antibodies and memory cells store information of the pathogen. Generally IgM type antibodies with a half life of 5 days are produced in primary response.

Concentration of secreted antibodies is different at different stages of latent period.

In the initial, stage antibodies against the new antigens are almost absent, and this period is known as “Lag phase”. Antibodies gradually rise from zero stage to a maximum and this raising period is referred as “Log phase”.

After attaining maximum, antibody levels remain constant for some time and the period with constant antibody levels is referred as “plateau phase”. Maintenances of Abs at constant level in plateau phase indicate production of Abs proportionate to the loss.

Plateau phase is accompanied by a decline phase, indicating reduction of Abs.

Presence of antigen is necessary for the production of plasma cells. Since the antigens are removed by the action of antibodies, B cells fail to get stimulation from antigens and stops producing plasma cells.

The number of plasma cells producing Abs gradually decreases due to their short life. Death of plasma cells producing antibodies and drop in the formation of plasma cells leads to a reduced Abs level in the decline phase. If a graph is plotted for the Abs titer against time in an immune response, it gives a typical sigmoid curve with an extended decline phase as shown in the figure.

Duration of latent period in primary immune response varies from several hours to days. It depends on many factors such as type of antigen, amount of antigen entered, rout of entry, health status of the individual etc. For instance latent period is few hours for pneumococcal antigen but 2- 3 weeks for diphtheria.