25.17 : Cells of the Adaptive Immune Response

The T and B lymphocytes of the adaptive immune system develop from common lymphoid progenitor cells in the bone marrow. These progenitors give rise to precursors that eventually develop into both T and B lymphocytes. As these precursors mature, they gain the ability to detect and respond to foreign antigens in the body, a process known as immunocompetence. Additionally, these precursors acquire self-tolerance, a process that ensures they do not react to self-antigens. This intricate system helps maintain a healthy immune response and protect the body from harmful invaders.

Maturation of B and T Cells

During the development of the immune system, B cell precursors undergo a crucial maturation process within the bone marrow. During this process, they develop B cell receptors (BCRs) that can bind to specific antigens. On the other hand, T cell precursors migrate to the thymus, where they mature and develop T cell receptors (TCRs) that can recognize foreign antigens on infected body cells. This maturation process involves rigorous selection to ensure self-tolerance, eliminating cells that strongly react to self-antigens.

Seeding Secondary Lymphoid Organs

Following their maturation process, the newly formed but still naive T and B lymphocytes leave the thymus and bone marrow. They then travel to secondary lymphoid organs, such as the spleen and lymph nodes, or circulate in the bloodstream. These secondary lymphoid sites provide an optimal environment for encountering antigens.

Activation of Naive Lymphocytes

Naive T and B lymphocytes become activated upon encountering their specific foreign antigens. Naive T cells require assistance from antigen-presenting cells (APCs) like dendritic cells, macrophages, or B cells to become activated. These APCs process and present antigens on their surface, facilitating the activation of T cells. B cells can be directly activated by antigens but often require additional signals from helper T cells.

Clonal Expansion and Differentiation

Once activated, lymphocytes rapidly proliferate to form a large population of cloned cells with identical antigen specificity. This clonal expansion produces effector cells and memory cells. Effector cells, such as plasma cells from B lymphocytes and cytotoxic T cells, are primarily responsible for clearing the infection. Meanwhile, a subset of the cloned cells differentiates into memory cells. These memory cells persist long-term in the body, providing a rapid and robust response if the same antigen is encountered again in the future.