As blood and lymph pass through the spleen and lymph nodes, contents are trapped and examined. Recognized cellular matter passes by. Foreign matter is assailed.
The body has a single spleen: an organ found in all vertebrates. The spleen has several functions. One is removing aged or dead red blood cells and recovering the iron from the hemoglobin within a cell. Parts of the spleen act like a supersized lymph node.
There are hundreds of small lymph nodes, scattered throughout the body. Lymph nodes are distributed intelligence centers for the adaptive immune system. Lymph nodes act as customs agents for the body, inspecting blood and lymph cargo.
B and T cells do the inspecting. Anything that is “self” is allowed through. This includes commensal microbiota, which the immune system learns in infancy to accept as part of oneself.
Foreigners set off alarms, activating attack. Infection can come in the form of a virus, bacterium, parasite, or tumor that has infiltrated tissues.
Lymphocytes pick up on chemical signals in the bloodstream. While a body’s own cells commonly call for help, warning signals are sometimes put out by commensal microbes as an assist.
Individual cells and commensal microbes throughout the body, not just lymphocytes, know their neighbors and communicate with each other. Further, cells know not only their own assignments but also the roles of neighboring cells. This extensive distributed knowledge is necessary for a body to function.
Host cells and commensal microbes raise a fuss if an invader is found in the neighborhood. A pathogen is assaulted by a dazzling array of lymphocytes, each lymphocyte bearing a single antibody. An antibody-antigen fit to the right lymphocyte causes the lymphocyte to swell and proliferate.