The immune system is composed of specialized cells called lymphocytes that use proteins to target, kill and remove infectious agents from the body. The proteins are either attached to the outside of the cell or secreted by the cells, where they can attach to bacteria, fungi or viruses in order to kill them directly or target them to be killed. The immune system is composed of an intricate network of lymphocytes and proteins, all of which are specialized for their specific roles in protecting you from infection.
B cells and Antibodies
B cells are a type of lymphocyte that grow in the bone marrow. B cells make proteins called antibodies, which can either be attached to the outside of the cell or secreted to bind to microbes in the body. Each B cell makes one specific type of antibody. For example, the antibody from one B cell binds the flu virus, while the antibody of another B cell only binds bacteria. B cells secrete antibodies that bind microbes and target them for killing by other cells.
T cells and Cytokines
T cells are lymphocytes that grow in an organ called the thymus. These cells can either directly kill infected cells or help B cells make antibodies. T cells perform both of these functions through proteins called cytokines. There are many different types of cytokines, some of which can bind to an infected cell and poke holes in it in order to kill it. A dead infected cell cannot infect another healthy cell, which is one way the immune system limits an infection. Other cytokines help to activate B cells to secrete antibody proteins.
Macrophages and TLRs
Macrophages are specialized cells of the immune system that "eat" infectious microbes in order to kill them and/or remove them from the body. One way they detect a microbe is recognizing antibodies that are bound to the infectious agent. Another way they detect microbes is to use TLR proteins that are bound to the outside surface of the macrophages. TLRs bind to proteins that are present on most bacteria, viruses and/or fungi. Triggering TLR proteins signals a macrophage to eat the microbe in front of it.
Dendritic Cells and MHC proteins
Dendritic cells also eat infectious agents, but not specifically to kill them. These cells process proteins from microbial agents and present them to T cells, which in turn become activated and start secreting cytokines. Dendritic cells take up microbes all over the body and, once a microbe is detected, the cell travels to a lymph node in order to present the protein to T cells. Dendritic cells use MHC proteins in order to present the microbial proteins to the T cell. An analogy is that the MHC protein is the plate that serves the microbial proteins to T cells.
