Why is the trafficking of naive lymphocytes important Outline the stages in the development of the specific immune respons



Our bodies are both fragile and resilient at the same time. being so soft and easily broken, yet keeping us alive even during the worst of conditions, always adapting and always surviving. One of the important factors that keep us alive is our immune system and its ability to adapt to the environment and the pathogens it contains. According to Gene Mayer, PhD, there are two ways wherein the body can protect itself from infection: Innate or nonspecific immune system, and Adaptive or specific immune system. ‘The innate immune system is the first line of defense’ which provides immediate, but nonspecific responses to infection (e.g. skin, mucus, tears, sneezing, etc.). The adaptive immune system is slower and responds only to specific antigens with cells called lymphocytes. Once the adaptive immune system responds to a specific antigen, the body will remember it and will produce antibodies to combat the antigen more effectively every time it comes back.1 There are two main types of lymphocytes involved in adaptive immunity–B lymphocytes (B cells) and T lymphocytes (T cells). These cells are produced in the bone marrow. One of the differences between the two is that the T cells have to migrate to the thymus gland to mature. Stein and Nombela-Arrieta (2005, pp.1-12) state that these lymphocytes that have just matured and have yet to encounter an antigen, are called naive lymphocytes. From the bone marrow and the thymus, the lymphocytes are then trafficked through the bloodstream to the secondary lymphoid organs (SLO), which are the peripheral and mesenteric lymph nodes, spleen and gut-associated lymphoid tissues called the Peyer’s patches. In the SLO, once the naive cells encounter antigens and become activated, they will undergo changes and will eventually leave to combat the infection.2 Kuby wrote that when a pathogen enters the body, it enters the bloodstream and is transported to the lymph nodes and lymphatic organs where antigen presenting cells (APCs) break it down into antigen compounds to be bound to Major Histocompatibility Complex (MHC) molecules. The naive helper T cells become activated once they encounter the antigen bound to the MHC molecules. It then becomes an effector cell (Th) that produces cytokines which activate B cells, T cells, macrophages and other cells included in the specific immune response. Once the Th cells are released into the bloodstream, when they find the antigen, they secrete their cytokines. T cytotoxic cells (Tc) become another effector cell called the cytotoxic T lymphocyte (CTL). This kind of effector cell does not produce cytokines but is designed to destroy infected cells. Once the B cells encounter the antigen, they proliferate and differentiate into antibody-secreting plasma cells whose antibodies attach to the antigen, effectively neutralizing it. Other B cells turn into memory B cells that allow for a faster response to illness once the same pathogen enters the body again.3 Mayer and Nyland (2010) pointed out that ‘since there are relatively few T or B lymphocytes with a receptor for any particular antigen (1/10,000 – 1/100,000), the chances for a successful encounter between an antigen and the appropriate lymphocyte are slim.’4 The chances of encountering the right antigen with the right antibody are maximized with recirculation to the SLOs. The lymphocytes constantly circulate from the lymph organs to the blood via lymphatics if there is no antigen present. ‘It is estimated that 1-2% of lymphocytes recirculate every hour’ (Mayer &amp. Nyland, 2010).5 If it does encounter an antigen, the cells differentiate into the effector cells (B, Th, or Tc) to combat the infection. After the infection, it can go back to the blood stream