Which immune response occurs first after an antigen encounters a lymphocyte?

The first immune response that occurs after an antigen encounters a lymphocyte is the proliferation of the lymphocyte. When a lymphocyte, such as a B cell or a T cell, recognizes its specific antigen, this recognition triggers a series of intracellular signaling pathways that activate the lymphocyte. This activation leads to the lymphocyte undergoing clonal expansion, which is the process of proliferation, where the activated lymphocyte rapidly divides and produces many identical copies of itself.

This expansion is crucial because it increases the number of immune cells available to combat the antigen effectively. These newly formed lymphocytes are either effector cells, which actively respond to the antigen, or memory cells, which will be retained for future encounters with the same antigen. The proliferation stage is essential for the body to mount a robust and effective immune response against pathogens.

Migration to the infection site and the formation of memory cells occur later in the immune response. Initially, the focus is on generating a sufficient number of responding lymphocytes, which then contribute to the subsequent steps of the immune response. Immediate destruction of the antigen is not typically characteristic of the initial phase after lymphocyte activation, as it requires the coordinated action of the expanded lymphocyte population along with other immune components.