Session 1: Introduction: Which Cells Are the Players?
Transcript of Part 1: The Cells of the Immune System
00:00:07.12 The immune system is responsible 00:00:09.06 for fighting infection and disease. 00:00:10.29 It is comprised of many specialized cell types, 00:00:13.16 all which work together to keep your body healthy. 00:00:16.24 In this short video, 00:00:18.11 you will be introduced to the major cellular players of the immune system. 00:00:23.10 Let’s start by introducing the two major arms of the immune system: 00:00:26.00 innate and adaptive immunity. 00:00:28.25 Innate immunity serves as the first line of defense 00:00:31.14 and is a more general immune response. 00:00:33.27 You might recognize some of the classic symptoms of innate immunity, 00:00:36.24 such as fever and inflammation. 00:00:39.09 Adaptive immunity allows for very specific detection 00:00:42.16 and elimination of pathogens. 00:00:45.04 However, it takes a longer time to ramp up than innate immunity. 00:00:48.14 Importantly, the adaptive immune system provides immunological memory -- 00:00:52.23 the ability of our immune cells to remember previous infections and clear them more quickly in the future. 00:01:00.17 All immune cells develop from a single pluripotent cell 00:01:03.07 in the bone marrow, 00:01:04.20 the hematopoietic stem cell. 00:01:06.07 Hematopoietic stem cells give rise to 00:01:08.09 lymphoid and myeloid progenitors, 00:01:09.24 each of which differentiate into a range of immune cell types. 00:01:13.27 We refer to the cells on the top as the lymphoid lineage, 00:01:16.18 whereas the cells on the bottom are classified as the myeloid lineage. 00:01:20.27 The myeloid progenitors also give rise to red blood cells and platelets, 00:01:24.09 although we won’t focus on those here. 00:01:27.25 Let’s now look closely at the lymphoid lineage. 00:01:31.08 The lymphoid progenitor differentiates into three cell classes. 00:01:35.09 We’ll start by introducing B cells. 00:01:38.02 Upon activation, mature B cells differentiate into memory cells or plasma cells. 00:01:43.09 Plasma cells are the immune cells that are responsible for secreting antibodies, 00:01:47.10 an important component of adaptive immunity. 00:01:52.05 Natural killer cells are cytotoxic cells of the innate immune system. 00:01:56.10 They detect virus-infected cells and kill them. 00:02:00.29 T cells arise from a common progenitor. 00:02:03.22 There are many types of mature T cells, 00:02:05.25 but the best known ones -- those which we’ll cover here -- include: 00:02:10.08 memory T cells, cytotoxic T cells, 00:02:12.11 and helper T cells. 00:02:14.22 Like memory B cells, 00:02:16.15 a fraction of mature T cells remain in the body as memory T cells 00:02:19.09 to help the body mount a faster immune response in future infections. 00:02:24.01 Cytotoxic T cells recognize antigen, 00:02:26.19 or small pathogen-derived particles, 00:02:28.28 on infected cells and kill them in a pathogen-specific manner. 00:02:32.25 Lastly, upon activation by specialized cells in the body, 00:02:37.18 helper T cells secrete cytokines that boost the adaptive immune response. 00:02:41.18 For example, helper T cells play an important role in B cell activation. 00:02:47.28 Now let’s take a look at the myeloid lineage. 00:02:52.01 The myeloid lineage produces most cells of the innate immune system 00:02:54.29 as well as important antigen-presenting cells that prime the adaptive immune response. 00:03:00.25 The myeloid progenitor gives rise to neutrophils, 00:03:03.22 which are innate immune cells that specialize in the capture and killing of microorganisms 00:03:07.27 throughout the body; 00:03:09.20 eosinophils, which are a type of granulocyte 00:03:12.10 that releases cytokines to defend against parasites; 00:03:15.24 and monocytes, which further differentiate into dendritic cells and macrophages. 00:03:21.02 Dendritic cells are a specialized type of phagocytic cell 00:03:24.10 that bridges innate and adaptive immunity. 00:03:27.14 Macrophages are tissue resident phagocytic cells. 00:03:30.21 They patrol the body and assist in cleaning up infection 00:03:33.12 and activating other immune cells. 00:03:37.08 Other cells that arise from the myeloid progenitor 00:03:40.06 include mast cells, another type of granulocyte that are implicated in allergy; 00:03:45.02 as well as basophils, 00:03:47.11 which are a less well-understood cell type that is involved in the immune response to parasites. 00:03:54.06 So how are immune cells activated? 00:03:56.28 Here, we’ll briefly cover the molecular paradigms 00:03:59.29 for innate and adaptive immune activation. 00:04:02.16 Cells of the innate immune system 00:04:05.13 express molecules known as pattern recognition receptors at their surface. 00:04:09.07 These receptors bind pathogens or parts of pathogens, 00:04:12.04 which induce intracellular signals 00:04:14.29 that activate an innate immune response. 00:04:18.17 The particles recognized by these receptors 00:04:20.20 are common amongst pathogens. 00:04:23.14 To activate adaptive immunity, 00:04:25.12 cells present antigen 00:04:28.06 -- small peptide fragments of pathogens -- 00:04:30.01 to T cells to inform a specific immune response. 00:04:33.01 Antigen is presented by two types of surface molecules: 00:04:36.22 MHC class I and MHC class II. 00:04:39.27 MHC class I molecules are expressed by all cells in the body 00:04:43.24 and are used in defense against intracellular pathogens such as viruses. 00:04:49.00 They do this by presenting endogenous, or intracellular, antigens 00:04:53.18 to cytotoxic T cells. 00:04:56.02 MHC class II molecules present exogenous antigen, 00:04:59.02 which is antigen found on pathogens outside of cells, 00:05:02.09 and activate helper T cells. 00:05:05.21 MHC class II molecules are expressed 00:05:07.14 by what are known as professional antigen-presenting cells, 00:05:11.27 which include dendritic cells, macrophages, and B cells. 00:05:15.02 There are safeguards in place to ensure that the immune system 00:05:18.01 isn’t activated against self antigens. 00:05:23.15 The mammalian immune system 00:05:25.09 is comprised of a complex set of cells 00:05:27.04 that contribute to innate and adaptive immunity. 00:05:30.16 The foundational understanding you’ve gained from this video 00:05:32.26 serves as a starting point for a deeper look at this topic.
