The primary immune response involves
- an immediate increase in the concentration of antibodies, followed by an immediate and sharp decline.
- a slow rise in the concentration of antibodies, followed by a rapid decline.
- an immediate increase in the concentration of antibodies, followed by a slow decline.
- a slow rise in the concentration of antibodies, followed by a gradual decline.

The correct answer is- a slow rise in the concentration of antibodies, followed by a rapid decline.

Explanation:

The first exposure of antibody in an organism generates the primary response. Initially, for some days after the exposure of antigen, there is no antibody detection in the blood and it is called the latent phase.  

After the latent or lag phase, the antibodies start accumulating and reach to the peak between 7-10 days after exposure. So it takes a longer time to establish immunity in primary response.

After the immunity reaches the peak it declines rapidly that means higher immunity does not stay for a longer time. This phase is called the declining phase. Therefore, the correct answer is- a slow rise in the concentration of antibodies, followed by a rapid decline.

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