Transcript of Types of Immune Cells Part 2: Myeloid and Lymphoid Lineages

In the previous tutorial we began to discuss  immune cells, and we did this by going over   immune cell function in a general way.  Now it’s time to get more specific about   the different types of immune cells, and which  of these functions is performed by each type.  Most immune cells originate in the bone marrow and  develop from hematopoietic stem cells, which also   give rise to red blood cells and platelets. To  become a white blood cell, a hematopoietic stem   cell can follow one of two major differentiation  tracks, those being the myeloid lineage and the   lymphoid lineage. Hematopoietic stem cells develop  through these lineages into fully developed   immune cells by following specific sets of  molecular cues in the bone marrow. Historically,   many of these cells were characterized based on  how they appeared under a microscope. However,   we now have molecular tools to define cells based  on the proteins they express on their surfaces,   and thus based on which genes  are turned on inside the cell.  Let’s start with the myeloid-derived innate immune  cells, which include macrophages, monocytes,   granulocytes, and dendritic cells. As we’ve  mentioned previously, macrophages are the   professional eaters of the immune system. They are  very good at killing pathogens by phagocytosis,   the process by which a pathogen is engulfed  and ingested. Macrophages are also important   for keeping tissues free of cellular debris  by phagocytosing dead and dying self cells.   Macrophages that live in these  tissues under normal conditions   are called tissue-resident macrophages, and  these cells originate in the fetal yolk sac,   rather than the bone marrow. Almost every  tissue has a specific type of tissue-resident   macrophage which performs important maintenance  roles during homeostasis. During inflammation,   a process we will describe at length a bit later,  cells can also enter tissues from the bloodstream   as monocytes, which then differentiate  into macrophages. Macrophages are also   important for secreting cytokines  that can shape the immune response.  Next, we have a family of immune cells  called granulocytes. This name comes   from the fact that these cells are preloaded  with granules full of antimicrobial chemicals   that they can release during an infection  to kill pathogens. Neutrophils, eosinophils,   basophils, and mast cells are all  types of granulocytes. Neutrophils,   eosinophils, and basophils are also called  polymorphonuclear leukocytes, or PMNs,   because of the unique, multi-lobed shape of  their nuclei that can be seen under a microscope.  Neutrophils are the most numerous immune cell type  in the blood, and the first type of immune cell to   arrive on the scene when tissues become infected.  They are extremely good at phagocytosis and   killing pathogens, especially bacteria, but  they have a very short lifespan compared to   other types of white blood cells. These are also  the cells which produce the pus that we can see   during an infection. Eosinophils and basophils are  much less common in the blood than neutrophils.   Eosinophils release many cytokines and other  chemical factors that are important for promoting   wound healing and tissue repair. Basophils  are the least common, but the largest type   of granulocyte in the blood. These secrete  cytokines and other signaling molecules.   Mast cells are similar to basophils, but do not  have a multi-lobed nucleus and are usually found   in connective tissue instead of circulating  in the bloodstream. Mast cells, eosinophils,   and basophils are important for killing large  parasites that are too big to be destroyed by   phagocytosis. However, these cells are also  responsible for promoting allergies and asthma.  Next up, dendritic cells also are capable  of phagocytosis, but unlike macrophages   and the granulocytes, they aren’t usually  heavily involved in pathogen clearance.   Instead, dendritic cells patrol tissues for  signs of infection so they can take antigens   back to structures called lymph nodes,  which we will discuss in the next tutorial,   and activate T cells that will be able to  specifically neutralize that kind of pathogen.   In this way, dendritic cells are critical  for linking the innate and adaptive immune   systems. There are also innate immune cells  that are derived from the lymphoid lineage,   and these cells are called innate lymphoid  cells. This group of cells includes   natural killer cells, which recognize and  kill cancer cells and virally infected cells.   There are many other kinds of innate lymphoid  cells, which sense microbial cues and stress   signals, allowing them to quickly respond  to a diverse range of microbial threats.  Innate immune cells are effective at identifying  and clearing different pathogens, but they can   only recognize generalized features of pathogens,  not specific ones, and they can’t differentiate   between good and bad microbes. This is where  adaptive immune cells come in. Adaptive immune   cells also derive from the lymphoid lineage, and  are called lymphocytes. First up are B cells.   The main role of these is to make antibodies,  which are incredibly specific Y-shaped proteins   that can bind to, and coat pathogens  and foreign particles. Antibody binding   blocks pathogens from entering cells and  marks them for destruction by phagocytes.   However, B cells require help from  T cells to become fully activated.  There are 3 main types of T cells, and  they are defined by their functional role,   as well as their expression of identifying  surface proteins, CD8 or CD4. Here,   CD stands for cluster of differentiation, which  is a designation that immunologists use to name   various molecules that are expressed on the  surface of immune cells. We will see a variety of   CD numbers throughout this series. Killer T cells  express CD8, and are able to very specifically   identify and kill self cells that are cancerous  or infected. Helper T cells express CD4 and are   called “helper” cells because they are important  for helping to activate B cells. They are also   critical for secreting cytokines to guide the  immune response, depending on the type of threat.   Regulatory T cells also express CD4, and these  cells secrete cytokines to tone down the strength   of the immune response. They can even kill killer  T cells if they start getting out of control.   After encountering a certain pathogen once,  T and B cells can live for years in the body,   so the next time they see that pathogen,  these cells are able to mount a faster   and stronger reaction, allowing the body  to control the infection more quickly.  So to summarize, the immune system is made up of  billions of myeloid-derived and lymphoid-derived   white blood cells that circulate in  the bloodstream and patrol tissues   for signs of damage or infection. These cells  communicate with each other through cytokines   and chemokines to carry out their diverse array  of effector functions which help destroy pathogens   and heal damaged tissue. Once again, each of  the cell types we’ve gone over here will get its   own tutorial later in the series, but we  have some work to do before we get there,   so next up let’s investigate the lymphatic system.