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.