Content Reviewers:Antonia Syrnioti, MD, Viviana Popa, Lisa Miklush, PhD, RNC, CNS, Gabrielle Proper, RN, BScN, MN
Immunity is the ability of the body to fight pathogens, like viruses, bacteria, and fungi; but, also, foreign substances, like toxins and chemicals. Now, the immune system consists of two main branches: innate immunity and adaptive immunity. Innate immunity is the first line of immunity, that we are born with; it is fast, meaning that it responds within several minutes to hours; it’s non-specific, therefore it does not differentiate one pathogen from another; and finally, it’s short-lived, meaning it does not retain the memory of previous infections. On the flip side, adaptive immunity is the second line of defense, that is acquired throughout life; it is slower and takes time to respond; but it’s also specific, so it recognizes different pathogens; and long-term, so it doesn’t forget a previous exposure to a pathogen.
Now, let's cover the physiology of the adaptive immune response, which can be further subdivided into active and passive adaptive immunity.
In the active adaptive immune response, antibodies are produced by the client’s own immune system, following exposure to a particular antigen. In this case, two mechanisms can be employed: the humoral response, where antibodies are secreted by B cells; and the cell-mediated response which is carried out by T cells. Both B and T cells are a type of white blood cell called lymphocytes, and they’re produced in the bone marrow. B cells are called “B” cells because they mature in the bone marrow, while T cells are called “T” cells because they mature in the thymus.
Now, when mature B cells leave the bone marrow they’re still naive, meaning they haven’t been exposed to an antigen yet; so they usually circulate around in the blood or find their way to the lymph nodes or spleen. Once they encounter an antigen, they become memory B cells, or plasma cells, which secrete specific antibodies against the antigen.
Now T cells usually mature and differentiate into either helper T cells, or cytotoxic T cells in the thymus. A particular subset of T helper cells, called regulatory T cells, live up to their name by regulating the immune response. This means they can inhibit or suppress the immune response as needed, such as at the end of an infection and they reduce the risk of autoimmune diseases by helping distinguish between self and non-self antigens. These naive T cells are then released into the blood and end up in the same places as the B cells. All these types of cells interact tightly with each other to ensure an adequate and specific immune response.
Now, the key element in the adaptive immune response is specificity. This means that B and T cells mount a particular type of response suited to the specific pathogen. See, when a pathogen, like a bacteria or virus enters the body and runs into antigen-presenting cells. This includes macrophages, dendritic cells, and even naive B cells. These APCs will engulf and digest the pathogen and the fragments are then presented on the APC’s surface via proteins called major histocompatibility complex class II, or MHC II. Now these fragments serve as antigens, which are anything that could trigger an immune response, and they can be presented to T helper cells.
Now, once the APC presents its antigen to T helper cells, it causes the T helper cells that recognize the antigen to activate and proliferate; this results in an army of T helper cells that release cytokines, which are signalling molecules that attract other immune cells to the site of inflammation. These can be part of the adaptive immune system, like cytotoxic T cells, or killer T cells; or innate immune cells, like neutrophils and natural killer, or NK cells.
Cytotoxic T cells can recognize the pathogen just like the helper T cells, so it will go directly to the pathogen and release perforin to make pores in its membrane; and then inject a cytotoxin directly into the invader, inducing apoptosis, or programmed cellular death.