The Lymphatic System and Immunity

• The immune system and lymphatic system are closely linked, with the lymphatic system transporting fluids and filtering pathogens.
• Lymph nodes swell during infections, and lymphocytes are transported via lymphatic vessels, highlighting the connection.

Functions of the Lymphatic System

• The lymphatic system drains body fluids and returns them to the bloodstream.
• Blood pressure causes fluid leakage from capillaries into interstitial spaces (spaces between cells).
• 20 liters of plasma are released daily into interstitial spaces due to capillary filtration.
• 17 liters of this interstitial fluid are reabsorbed directly by blood vessels.
• The lymphatic system drains the remaining 3 liters of excess fluid and returns it to the bloodstream.
• Lymph is interstitial fluid that has entered the lymphatic system.
• Damage to the lymphatic system can cause protein-rich interstitial fluid to accumulate, leading to lymphedema. Lymphedema may have serious medical consequences.
• Lymphatic vessels transport immune cells and dietary lipids/fat-soluble vitamins.
• Lymph nodes serve as staging areas for immune responses.
• A lymph node is a small, bean-shaped organ located throughout the lymphatic system.

Structure of the Lymphatic System

• Lymphatic vessels begin as blind-ending capillaries that feed into larger vessels, eventually emptying into the bloodstream.
• Lymph travels through lymph nodes, commonly found near the groin, armpits, neck, chest, and abdomen.
• The human body has about 500–600 lymph nodes.
• Lymphatic vessels in the arms and legs convey lymph to larger vessels in the torso.
• Lymph is not actively pumped by the heart but is forced through vessels by body movements, skeletal muscle contractions, and breathing.
• One-way valves in lymphatic vessels keep lymph moving toward the heart.
• Lymph flows from lymphatic capillaries through lymphatic vessels and into the circulatory system via lymphatic ducts located at the junction of the jugular and subclavian veins in the neck.

Lymphatic Capillaries

• Lymphatic capillaries, or terminal lymphatics, are vessels where interstitial fluid enters the lymphatic system to become lymph fluid.
• Located in almost every tissue except the central nervous system, bone marrow, bones, teeth, and cornea of the eye.
• Interlaced among arterioles and venules of the circulatory system in soft connective tissues.
• Lymphatic capillaries consist of a one-cell-thick layer of endothelial cells with overlapping cells that allow interstitial fluid to flow in.
• Endothelial flaps close when interstitial pressure is low to prevent backflow.
• Collagen filaments anchor capillaries to surrounding structures and pull on endothelial flaps as interstitial pressure increases, allowing easier fluid entry.
• Lacteals, lymphatic capillaries in the small intestine, transport dietary lipids and lipid-soluble vitamins to the bloodstream.
• In the small intestine, dietary triglycerides combine with other lipids/proteins and enter lacteals as chyle.
• Chyle travels through the lymphatic system, eventually entering the bloodstream.

Larger Lymphatic Vessels, Trunks, and Ducts

• Lymphatic capillaries empty into larger lymphatic vessels, similar to veins with a three-tunic structure and valves.
• One-way valves cause bulges in lymphatic vessels, giving them a beaded appearance.
• Superficial and deep lymphatics merge to form larger lymphatic vessels called lymphatic trunks.
• On the right side of the body, the head, thorax, and right upper limb drain lymph fluid into the right subclavian vein via the right lymphatic duct.
• On the left side of the body, the remaining portions drain into the larger thoracic duct, which drains into the left subclavian vein.
• The thoracic duct begins beneath the diaphragm in the cisterna chyli, receiving lymph from the lower abdomen, pelvis, and lower limbs via the lumbar trunks and the intestinal trunk.
• The drainage system is asymmetrical; the right lymphatic duct receives lymph from only the upper right side of the body.
• Lymph from the rest of the body enters the bloodstream through the thoracic duct via the remaining lymphatic trunks.
• Superficial lymphatics follow the same routes as veins, while deep lymphatic vessels generally follow the paths of arteries.

The Organization of Immune Function

• The immune system is a collection of barriers, cells, and soluble proteins with complex interactions.
• The modern model of immune function has three phases based on timing, including barrier defenses, innate immune response, and adaptive immune response.
• Barrier defenses like skin and mucous membranes act instantaneously to prevent pathogen invasion.
• The rapid but nonspecific innate immune response involves specialized cells and soluble factors.
• The slower but specific adaptive immune response involves lymphocytes (white blood cells).
• Blood cells, including those involved in the immune response, arise in the bone marrow from hematopoietic stem cells.
• Hematopoietic stem cells allow for continuous differentiation of blood cells.
• Cells are divided into phagocytic cells, lymphocytes, and cells containing cytoplasmic granules based on function.
• Phagocytic cells ingest pathogens to destroy them.
• Lymphocytes coordinate adaptive immunity activities.
• Cells with cytoplasmic granules mediate immune responses against parasites and intracellular pathogens.

Lymphocytes: B Cells, T Cells, Plasma Cells, and Natural Killer Cells

• Lymphocytes are the primary cells of adaptive immune responses.
• B cells and T cells are morphologically identical, distinguished by surface protein markers and secreted molecules, and both initially develop from bone marrow.
• T cells migrate from bone marrow to the thymus gland for further maturation.
• B and T cells circulate in the blood/lymph and reside in secondary lymphoid organs like the spleen and lymph nodes.
• The human body contains approximately 10^12 lymphocytes.
• B cells are immune cells that function primarily by producing antibodies.
• An antibody is any protein that binds specifically to pathogen-associated molecules called antigens.
• An antigen is a chemical structure on the surface of a pathogen that binds to T or B lymphocyte antigen receptors.
• Once activated by antigen binding, B cells differentiate into plasma cells that secrete soluble antibodies.
• T cells do not secrete antibodies, but perform various functions in the adaptive immune response.
• T cells secrete factors that communicate with other cells or destroy infected cells.
• Plasma cells are B cells that have differentiated in response to antigen binding and secrete soluble antibodies.
• Plasma cells contain a large amount of cytoplasm packed with protein-synthesizing machinery (rough endoplasmic reticulum).
• Natural killer cells (NK cells) participate in the innate immune response.
• NK cells are circulating blood cells with cytotoxic granules that kill cells, mechanism shared with cytotoxic T cells.
• NK cells are among the body’s first lines of defense against viruses and certain types of cancer.

Primary Lymphoid Organs and Lymphocyte Development

• Differentiation and development of B and T cells is critical to the understanding of the adaptive immune response.
• The body learns to destroy only pathogens while leaving the body’s own cells intact.
• The primary lymphoid organs are the bone marrow and thymus gland.
• Lymphoid organs are where lymphocytes mature, proliferate, and are selected, enabling them to attack pathogens without harming the body’s cells.

Bone Marrow

• In the embryo, blood cells are made in the yolk sac, then the spleen, lymph nodes, and liver.
• The bone marrow takes over most hematopoietic functions.
• Red bone marrow is where hematopoiesis occurs.
• Yellow bone marrow is a site of energy storage, consisting largely of fat cells.
• B cells undergo nearly all of their development in the red bone marrow.
• Immature T cells (thymocytes) leave the bone marrow and mature largely in the thymus gland.

Thymus

• The thymus gland is a bilobed organ found between the sternum and the aorta of the heart.
• Connective tissue holds the lobes together but separates them and forms a capsule.
• The connective tissue capsule divides the thymus into lobules via extensions called trabeculae.
• The outer region (cortex) contains large numbers of thymocytes, epithelial cells, macrophages, and dendritic cells.
• The medulla contains a less dense collection of thymocytes, epithelial cells, and dendritic cells.

Secondary Lymphoid Organs and their Roles in Active Immune Responses

• Lymphocytes develop and mature in the primary lymphoid organs, but they mount immune responses from the secondary lymphoid organs.
• A naïve lymphocyte is one that has left the primary organ and entered a secondary lymphoid organ.
• Naïve lymphocytes are fully functional immunologically but have yet to encounter an antigen.
• Lymphocytes concentrate in secondary lymphoid organs, including lymph nodes, spleen, and lymphoid nodules.
• Common features of these tissues include lymphoid follicles (sites of lymphocyte formation), reticular fibers with macrophages, germinal centers (sites of dividing B lymphocytes), and high endothelial venules that allow blood cells to enter tissues.

Lymph Nodes

• Lymph nodes remove debris and pathogens from the lymph ("filters of the lymph").
• Bacteria infecting interstitial fluid are taken up by lymphatic capillaries and transported to regional lymph nodes.
• Dendritic cells and macrophages internalize and kill pathogens.
• Lymph nodes are the site of adaptive immune responses by T cells, B cells, and accessory cells.
• Lymph nodes are surrounded by a connective tissue capsule and separated into compartments by trabeculae.
• Structural support is provided by reticular fibers.
• Afferent lymphatic vessels are the major routes into the lymph node.
• Efferent lymphatic vessels allow cells and lymph to leave the lymph node.
• Lymph enters the lymph node via the subcapsular sinus, which is occupied by dendritic cells, macrophages, and reticular fibers.
• Lymphoid follicles in the cortex consist of germinal centers of dividing B cells surrounded by T cells and accessory cells.
• The medulla consists of medullary cords of B cells and plasma cells, and the medullary sinuses where lymph collects before leaving the node.

Spleen

• The spleen is a major secondary lymphoid organ attached to the stomach.
• It is fragile and dark red due to extensive vascularization.
• The spleen filters the blood, removing microbes and other materials, including dying red blood cells ("filter of the blood").
• Functions as the location of immune responses to blood-borne pathogens.
• Divided by trabeculae of connective tissue, with each splenic nodule containing red pulp (mostly red blood cells) and white pulp (lymphoid follicles).
• Splenic artery splits into arterioles (surrounded by white pulp) and sinusoids.
• Blood collects in venous sinuses and leaves via the splenic vein.
• Red pulp consists of reticular fibers with macrophages, free macrophages, and blood cells.
• White pulp surrounds a central arteriole and consists of germinal centers of dividing B cells surrounded by T cells and accessory cells.
• Red pulp primarily functions as a filtration system using nonspecific immune response cells.
• White pulp is where adaptive T and B cell responses are mounted.

Lymphoid Nodules

• Lymphoid nodules have a simpler architecture than the spleen and lymph nodes.
• They consist of a dense cluster of lymphocytes without a surrounding fibrous capsule.
• Located in the respiratory and digestive tracts, areas exposed to environmental pathogens.
• Tonsils are lymphoid nodules along the inner surface of the pharynx, developing immunity to oral pathogens.
• The pharyngeal tonsil is sometimes called the adenoid when swollen.
• Tonsils lack a complete capsule, and the epithelial layer invaginates to form tonsillar crypts.
• Accumulate materials taken into the body, encouraging pathogens to penetrate deep into the tonsillar tissues where they are acted upon by lymphoid follicles.
• Tonsils help children recognize, destroy, and develop immunity to common environmental pathogens.
• Mucosa-associated lymphoid tissue (MALT) consists of lymphoid follicles associated with mucous membrane epithelia.
• MALT makes up dome-shaped structures underlying the mucosa of the gastrointestinal tract, breast tissue, lungs, and eyes.
• Peyer’s patches, a type of MALT in the small intestine, are important for immune responses against ingested substances.
• Peyer’s patches contain M cells that sample material from the intestinal lumen and transport it to nearby follicles.
• Blockage of the lumen in the appendix triggers cells to elicit an inflammatory response, leading to appendicitis.
• Bronchus-associated lymphoid tissue (BALT) consists of lymphoid follicular structures with an overlying epithelial layer found along the bifurcations of the bronchi.
• These tissues, in addition to the tonsils, are effective against inhaled pathogens.

Barrier Defenses and the Innate Immune Response

• The immune system is divided into the innate immune response (rapid, nonspecific) and the adaptive immune response (slower, specific).
• The innate immune response is not always effective.
• Barrier defenses prevent pathogens from entering the body.
• Barrier defenses are continuously working, part of the body’s most basic defense mechanisms, but occur independently of infection.
• Barrier defenses are associated with external surfaces of the body.
• The skin is the primary barrier, covered with a layer of dead, keratinized epithelium that is too dry for bacteria to grow.
• Skin cells are continuously sloughed off, carrying bacteria and pathogens away.
• Sweat and other skin secretions lower pH, contain toxic lipids, and physically wash microbes away.
• Saliva in the mouth is rich in lysozyme, destroying bacteria by digesting cell walls.
• The acidic environment of the stomach is fatal to many pathogens.
• The mucus layer of the gastrointestinal tract, respiratory tract, reproductive tract, eyes, ears, and nose traps microbes and debris.
• Ciliated epithelial cells in the upper respiratory tract move contaminated mucus upwards to the mouth, swallowed into the digestive tract.

Cells of the Innate Immune Response

• A phagocyte surrounds and engulfs a particle or cell (phagocytosis).
• Phagocytes engulf particles/cells to clear debris/old cells or kill pathogenic organisms.
• Phagocytes are the body’s fast-acting, first line of immunological defense against organisms that have breached barrier defenses.
• Phagocytosis is an important mechanism of destroying pathogens during innate immune responses.
• The phagocyte takes the organism inside itself as a phagosome, which fuses with a lysosome and its digestive enzymes, effectively killing many pathogens.
• Some bacteria, including Mycobacteria tuberculosis, are resistant to these enzymes.
• Macrophages, neutrophils, and dendritic cells are the major phagocytes of the immune system.

Phagocytes: Macrophages and Neutrophils

• A macrophage is an irregularly shaped phagocyte that moves through tissues and capillary walls using pseudopodia.
• Macrophages participate in innate and adaptive immune responses.
• Macrophages exist in many tissues of the body, either roaming or fixed to reticular fibers.
• Macrophages are the first line of defense when pathogens breach barrier defenses.
• Different names based on tissue like Kupffer cells in the liver, histiocytes in connective tissue, and alveolar macrophages in the lungs.
• A neutrophil is a phagocytic cell attracted via chemotaxis from the bloodstream to infected tissues.
• Neutrophils are granulocytes containing cytoplasmic granules, which contain vasoactive mediators like histamine.
• Macrophages are agranulocytes (few/no cytoplasmic granules).
• Macrophages act like sentries, and neutrophils are reinforcements called into battle.
• Neutrophils play a role in the adaptive immune response as well.
• A monocyte is a circulating precursor cell that differentiates into a macrophage or dendritic cell.

Natural Killer Cells

• NK cells induce apoptosis (programmed cell death) in cells infected with intracellular pathogens.
• NK cells recognize these cells by mechanisms that involve surface receptors.
• NK cells induce apoptosis by:
  • Responding to chemical signals and expressing the fas ligand, which binds to the fas molecule on the infected cell, sending it apoptotic signals killing both the cell and the pathogen within.
  • Releasing perforins and granzymes.
    • A perforin is a protein that forms pores in the membranes of infected cells.
    • A granzyme is a protein-digesting enzyme that enters the cell via the perforin pores and triggers apoptosis intracellularly.
• Both mechanisms are effective against virally infected cells.

Recognition of Pathogens

• Phagocytic cells and cytotoxic NK cells recognize patterns of pathogen-specific molecules using pattern recognition receptors.
• A pattern recognition receptor (PRR) is a membrane-bound receptor that recognizes characteristic features of a pathogen and molecules released by stressed/damaged cells.
• Present on the cell surface whether they are needed or not.
• The variety is limited by the number of genes and the surface area of the cell membrane.
• Innate immune system uses a limited number of receptors active against a wide variety of pathogens.
• Adaptive immune system uses large numbers of different receptors, each specific to a particular pathogen.
• Cells bind to and initiate phagocytosis or cellular apoptosis of recognized pathogens.
• Receptors vary somewhat according to cell type, but usually include receptors for bacterial components and complement.

Soluble Mediators of the Innate Immune Response

• Chemical signals induce cells to change physiological characteristics.
• These soluble factors are secreted during innate or early induced responses, and later during adaptive immune responses.

Cytokines and Chemokines

• A cytokine is a signaling molecule that allows cells to communicate over short distances.
• A chemokine is a soluble chemical mediator similar to cytokines except that its function is to attract cells (chemotaxis) over longer distances.

Early induced Proteins

• Not constitutively present in the body, but are made as they are needed early during the innate immune response.
• Interferons are an example: Cells infected with viruses secrete interferons that travel to adjacent cells and induce them to make antiviral proteins.
• Mannose-binding protein and C-reactive protein, made in the liver, bind specifically to polysaccharide components of the bacterial cell wall.
• Phagocytes have receptors for these proteins, which enhances phagocytosis of the bacterium by opsonization.
• Opsonization is the tagging of a pathogen for phagocytosis by the binding of an antibody or an antimicrobial protein.

Complement System

• A series of proteins constitutively found in the blood plasma made in the liver, not considered part of the early induced immune response.
• Functions in the innate immune response (alternate pathway) and the adaptive immune response (classical pathway).
• The system consists of several proteins that enzymatically alter and fragment later proteins in a cascade.
• Once activated, the series of reactions is irreversible, and releases fragments that:
  • Bind to the cell membrane of the pathogen that activates it, labeling it for phagocytosis (opsonization).
  • Diffuse away from the pathogen and act as chemotactic agents to attract phagocytic cells to the site of inflammation.
  • Form damaging pores in the plasma membrane of the pathogen.
• The classical pathway requires antibodies of the adaptive immune response.
• The alternate pathway does not require an antibody to become activated.
• The splitting of the C3 protein is the common step to both pathways.
• In the alternate pathway, C3 is activated spontaneously and splits after reacting with molecules such as factor P, factor B, and factor D.
• The larger fragment, C3b, binds to the surface of the pathogen, while the smaller fragment, C3a, diffuses outward and attracts phagocytes to the site of infection.
• Surface-bound C3b then activates the rest of the cascade, with the last five proteins, C5–C9, forming the membrane-attack complex (MAC).
• The MAC can kill certain pathogens by disrupting their osmotic balance.
• The classical pathway is similar, except the early stages of activation require the presence of antibody bound to antigen.
• Phagocytic cells are attracted to an infection site by chemotactic attraction to smaller complement fragments.
• Receptors for surface-bound C3b opsonize the pathogen for phagocytosis and destruction.

Inflammatory Response

• The hallmark of the innate immune response.
• Four characteristics: heat, redness, pain, and swelling.
• Can be initiated by infection or tissue injuries.
• The release of damaged cellular contents is enough to stimulate the response.
• Brings in phagocytic cells to clear cellular debris and set the stage for wound repair.
• Brings in cells of the innate immune system to get rid of possible infection.
• Helps to isolate the site, limiting the spread of the pathogen. Acute inflammation is a short-term inflammatory response to an insult to the body.
• Chronic inflammation is ongoing inflammation associated with tissue destruction and fibrosis, caused by foreign bodies, persistent pathogens, and autoimmune diseases.
• There are four important parts to the inflammatory response:
  • Tissue Injury.
    • The released contents of injured cells stimulate the release of mast cell granules and their potent inflammatory mediators such as histamine, leukotrienes, and prostaglandins.
    • Histamine increases the diameter of local blood vessels (vasodilation), causing an increase in blood flow.
    • Histamine also increases the permeability of local capillaries, causing plasma to leak out and form interstitial fluid, causing swelling.
    • Injured cells, phagocytes, and basophils are sources of inflammatory mediators, including prostaglandins and leukotrienes.
    • Leukotrienes attract neutrophils and increase vascular permeability.
    • Prostaglandins cause vasodilation and pain.
  • Vasodilation.
    • Inflammatory mediators increase the diameters of local capillaries, increasing blood flow and causing heat and redness.
  • Increased Vascular Permeability.
    • Inflammatory mediators increase the permeability of the local vasculature, causing leakage of fluid into the interstitial space, resulting in swelling.
  • Recruitment of Phagocytes.
    • Leukotrienes are particularly good at attracting neutrophils.
    • Macrophages are recruited to clean up the debris.
    • Neutrophils are attracted to the sites of infections in large numbers, and their accumulated cellular remains are visible as pus at the infection site.
• Inflammation is valuable for many reasons.
• Pathogens are killed and debris is removed.
• The increase in vascular permeability encourages the entry of clotting factors.
• Facilitates the transport of antigen to lymph nodes for the development of the adaptive immune response.

The Adaptive Immune Response: T lymphocytes and Their Functional Types

• Innate immune responses and early induced responses are in many cases ineffective at completely controlling pathogen growth, but they slow pathogen growth.
• The innate immune system sends signals to the cells of the adaptive immune system, guiding them in how to attack the pathogen.

The Benefits of the Adaptive Immune Response

• Specificity is the great strength.
• Antigens are the chemical groups recognized by receptors on the surface of B and T lymphocytes.
• The adaptive immune response has a unique way to develop as many as 1011, or 100 trillion, different receptors to recognize nearly every conceivable pathogen.

Primary Disease and Immunological Memory

• The immune system’s first exposure to a pathogen is called a primary adaptive response.
• Symptoms of a first infection, called primary disease, are always relatively severe because it takes time for an initial adaptive immune response to a pathogen to become effective.
• Upon re-exposure to the same pathogen, a secondary adaptive immune response is generated, which is stronger and faster that the primary response.
• The secondary adaptive response often eliminates a pathogen before it can cause significant tissue damage or any symptoms.
• This secondary response is the basis of immunological memory, which protects us from getting diseases repeatedly from the same pathogen.

Self Recognition

• A third important feature of the adaptive immune response is its ability to distinguish between self-antigens and foreign antigens.
• As T and B cells mature, there are mechanisms in place that prevent them from recognizing self-antigen, preventing a damaging immune response against the body.

T Cell-Mediated Immune Responses

• The primary cells that control the adaptive immune response are the lymphocytes, the T and B cells.
• T cells are particularly important, as they not only control a multitude of immune responses directly, but also control B cell immune responses in many cases as well.
• T lymphocytes recognize antigens based on a two-chain protein receptor.
• The most common and important of these are the alpha-beta T cell receptors.
• There are two chains in the T cell receptor, and each chain consists of two domains.
• The variable region domain is furthest away from the T cell membrane and is so named because its amino acid sequence varies between receptors.
• The constant region domain has less variation.
• The differences in the amino acid sequences of the variable domains are the molecular basis of the diversity of antigens the receptor can recognize.
• Each T cell produces only one type of receptor and thus is specific for a single particular antigen.

Antigens

• Antigens on pathogens are usually large and complex, and consist of many antigenic determinants.
• An antigenic determinant (epitope) is one of the small regions within an antigen to which a receptor can bind.
• They usually consist of six or fewer amino acid residues in a protein or one or two sugar moieties in a carbohydrate antigen.
• Carbohydrate antigens are found on bacterial cell walls and on red blood cells (the ABO blood group antigens).
• Protein antigens are complex because of the variety of three-dimensional shapes that proteins can assume, are especially important for the immune responses to viruses and worm parasites.
• It is the interaction of the shape of the antigen and the complementary shape of the amino acids of the antigen-binding site that accounts for the chemical basis of specificity.

Antigen Processing and Presentation

• T cells do not recognize free-floating or cell-bound antigens as they appear on the surface of the pathogen.
• They only recognize antigen on the surface of specialized cells called antigen-presenting cells.
• Antigens are internalized by these cells.
• Antigen processing is a mechanism that enzymatically cleaves the antigen into smaller pieces.
• The antigen fragments are then brought to the cell’s surface and associated with a specialized type of antigen-presenting protein known as a major histocompatibility complex (MHC) molecule.
• The MHC is the cluster of genes that encode these antigen-presenting molecules.
• The association of the antigen fragments with an MHC molecule, known as antigen presentation, results in the recognition of antigen by a T cell.
• The peptide-binding cleft is a small indentation at the end of the MHC molecule where the processed fragment of antigen sits.
• It is the combination of the MHC molecule and the fragment of the original peptide or carbohydrate that is actually physically recognized by the T cell receptor.
• Two distinct types of MHC molecules, MHC class I and MHC class II, play roles in antigen presentation.
• Both bring processed antigen to the surface of the cell via a transport vesicle and present the antigen to the T cell and its receptor.
• Antigens from different classes of pathogens use different MHC classes and take different routes through the cell to get to the surface for presentation.
• Intracellular antigens are processed in the cytosol by an enzyme complex known as the proteasome and are then brought into the endoplasmic reticulum by the transporter associated with antigen processing (TAP) system.
• Extracellular antigens are brought into the endomembrane system of the cell by receptor-mediated endocytosis.

Professional Antigen-presenting Cells

• Many cell types express class I molecules for the presentation of intracellular antigens.
• These MHC molecules may then stimulate a cytotoxic T cell immune response.
• Class II MHC molecules are expressed only on the cells of the immune system (professional antigen-presenting cells).
• The three types of professional antigen presenters are macrophages, dendritic cells, and B cells.
• Macrophages stimulate T cells to release cytokines that enhance phagocytosis.
• Dendritic cells also kill pathogens by phagocytosis, but their major function is to bring antigens to regional draining lymph nodes.
• B cells may also present antigens to T cells, which are necessary for certain types of antibody responses.