The adaptive immune system is the body’s defense mechanism, designed to target and eliminate specific threats. This system is characterized by its ability to recognize particular pathogens, known as specificity. It also possesses memory, allowing for a quicker and more robust response upon subsequent encounters with the same pathogen. Specialized cells, primarily lymphocytes, are responsible for these functions.
B Cells and Antibody Production
B lymphocytes, commonly called B cells, are central to humoral immunity, a defense strategy that focuses on pathogens circulating in body fluids. These cells originate from hematopoietic stem cells in the bone marrow, where they also undergo their initial maturation. During this maturation, B cells develop unique B cell receptors (BCRs) on their surface, which are specialized proteins capable of recognizing specific antigens.
Upon encountering an antigen that matches its BCR, a B cell becomes activated, often with assistance from helper T cells. This activation prompts the B cell to proliferate and differentiate into two main cell types. The first type is plasma cells, which are cells that secrete large quantities of antibodies, also known as immunoglobulins. These antibodies are proteins that circulate in the bloodstream and specifically bind to the foreign antigens, neutralizing them or marking them for destruction by other immune cells.
The second type of cell formed during B cell activation is memory B cells, which are long-lived and persist in the body. Each class of antibody, such as IgM, IgG, IgA, IgE, and IgD, serves distinct roles in immune defense. For instance, IgM is the first antibody produced during an initial infection, while IgG provides more prolonged protection.
T Cells and Cellular Defense
T lymphocytes, or T cells, are the primary cells responsible for cell-mediated immunity, a defense mechanism that involves direct action against infected or abnormal cells. T cells also originate from hematopoietic stem cells in the bone marrow, but they travel to the thymus for their maturation process. In the thymus, immature T cells, called thymocytes, undergo a selection process to ensure they can recognize foreign antigens presented by major histocompatibility complex (MHC) molecules while tolerating the body’s own cells.
There are several types of T cells, each with specialized functions. Helper T cells, identified by the CD4 co-receptor, act as coordinators of the immune response. They recognize antigens presented by MHC class II molecules, which are found on professional antigen-presenting cells like B cells and macrophages. Once activated, helper T cells release signaling molecules called cytokines that stimulate other immune cells, including B cells to produce antibodies and cytotoxic T cells to become active.
Cytotoxic T cells, distinguished by the CD8 co-receptor, are known as “killer cells”. Their role involves recognizing and destroying cells that are infected with viruses or have become cancerous. These T cells identify their targets by recognizing antigens presented on MHC class I molecules, which are found on nearly all nucleated cells in the body. Upon binding to an infected cell, cytotoxic T cells release perforin and granzymes, which induce programmed cell death in the target cell.
Regulatory T cells (Tregs) help to suppress excessive immune responses. These cells play a role in maintaining immune tolerance, preventing the immune system from attacking healthy body tissues, a process that can lead to autoimmune diseases. Tregs can express either CD4 or CD8 markers, and their function is to moderate the activity of other T cell types, ensuring a balanced immune response.
How Adaptive Immunity Remembers
Immunological memory enables the immune system to respond more effectively to previously encountered pathogens. After an initial exposure to a pathogen, known as the primary immune response, the body generates specialized memory B and T cells. These memory cells are long-lived and persist in the body for extended periods.
When the body encounters the same pathogen again, these pre-existing memory cells are quickly activated, leading to a secondary immune response. This secondary response is characterized by its speed, increased strength, and higher specificity compared to the primary response. Similarly, memory T cells are quickly activated and proliferate, providing a rapid cellular defense. The coordinated actions of memory B and T cells provide long-lasting protection, which is the underlying principle behind the effectiveness of vaccines.