The Adaptive Immune System
The adaptive immune system (adaptive meaning it can adapt to a specific threat) is the body’s second line of defense against pathogens. It is a specific defense system (meaning that its defense mechanisms directly target a specific pathogen) that is called into action when innate defenses are overcome. The specificity of the adaptive immune system comes from its ability to recognize a pathogen by its antigens. Antigens are molecular structures, such as proteins or polysaccharides (complex sugars) found on the surface of a pathogen, that are able to trigger the immune system to respond.
The main cells of the adaptive immune system are B cells and T cells. If a pathogen invades the body, it is detected by an antigen-presenting cell, such as a dendritic cell. These cells display antigens from the pathogen on their cell surface and travel to the lymph nodes where they present the antigen to T cells, activating them. These T cells then activate B cells to produce Y-shaped proteins called antibodies that attach to the pathogen and either neutralize it (by preventing it from entering other cells), or tag it for other immune cells to destroy. Other T cells are able to destroy cells that have already been infected. Some T cells and B cells become memory cells that protect against infection in the future. If the same pathogen invades the body again, these cells remember it and respond quickly and efficiently, preventing infection. This protection is called immunity.
Watch the video from Crash Course to learn more about the adaptive immune system (the video below will only show you the best parts, but feel free to watch the whole thing if you'd like!).