Biological Systems MCAT Practice Exam 2026 – Complete Study Resource

The primary immune response refers to the initial reaction of B cells when they encounter a specific antigen for the first time. During this process, the B cells undergo activation, proliferation, and differentiation into plasma cells and memory B cells. Plasma cells are responsible for producing antibodies that are specific to the antigen, while memory B cells remain in the body to provide long-lasting immunity.

During the primary response, it typically takes time for the B cells to recognize the antigen and mount an effective response, resulting in a delayed but significant production of antibodies. This process distinguishes the primary response from subsequent exposures to the same antigen, which would trigger a more rapid and robust secondary immune response due to the presence of these memory B cells that can quickly produce antibodies.

Clonal selection is a pivotal mechanism within the immune response but is not a standalone response. It describes how specific B cells are selected to expand and differentiate based on their affinity for the antigen. Memory cell activation refers to the process where previously formed memory B cells respond to future exposures to the same antigen, leading to a quicker immune response. These concepts are integral to understanding the immune response but represent different components or stages of the immune system's reaction to pathogens.