Immune System Cells and Organs Lecture Notes

Epigenetic Changes

• Cells of the innate immune system can be trained by past infections, vaccines (e.g., Bacillus Calmette-Guérin - BCG), or microbial components (e.g., LPS).

• This training enhances their response to the original or another trigger.

• The trained state results from epigenetic reprogramming of transcriptional pathways, not gene recombination.

• While monocytes and neutrophils have short lifespans, marrow hematopoietic stem cells (HSCs) conserve epigenetic memory and maintain lifelong production of innate immune cells due to their self-renewal properties.

Cells of the Innate Immune System

• A core group of cells plays a primary role in innate immunity.

• "Professional" phagocytes (neutrophils, monocytes, macrophages, and eosinophils) represent the critical effector component.

• Dendritic cells express pattern recognition receptors (PRRs) and originate as undifferentiated, innate cell types.

• As principal antigen-presenting cells, dendritic cells are the key link between innate and adaptive immunity.

Phagocytes

• Phagocytes, including neutrophils and macrophages, ingest and destroy microbes and remove damaged tissues.

  • Recruitment to infection sites.

  • Recognition of and activation by microbes.

  • Ingestion of microbes via phagocytosis.

  • Destruction of ingested microbes.

• Phagocytes communicate with other cells through direct contact and cytokine secretion to promote or regulate immune responses.

Distinguishing Properties of Neutrophils and Macrophages

Neutrophils

• Circulating phagocytes with a short lifespan and rapid, non-prolonged defense.

Monocytes/Macrophages

• Monocytes circulate in the blood and become macrophages in tissues, providing a prolonged defense.

• They produce cytokines that initiate and regulate inflammation, phagocytose pathogens, clear dead tissue, and initiate tissue repair.

• Macrophages develop along two different pathways.

Neutrophils Details

• Neutrophils are the most abundant circulating white blood cells, and the principal cell type in acute inflammatory reactions.

• Cytoplasm contains two types of membrane-bound granules:

  • Specific granules: filled with enzymes like lysozyme, collagenase, and elastase.

  • Azurophilic granules: contain enzymes (e.g., myeloperoxidase) and microbicidal substances (defensins and cathelicidins).

• Production stimulated by:

  • Granulocyte colony-stimulating factor (G-CSF).

  • Granulocyte-macrophage colony-stimulating factor (GM-CSF).

• Neutrophils migrate rapidly to infection sites.

• After entering tissues, they function for 1-2 days and then die.

Neutrophil Chemotactic Factors

• Neutrophils are attracted by four major chemotactic factors:

  • N-formyl bacterial oligopeptide

  • Complement-derived C5a

  • Leukotriene B4 (secreted by numerous immune cells)

  • Interleukin (IL) 8, the neutrophil chemokine secreted by activated innate immune cells and epithelial cells

Leukocyte Migration

• The image describes the steps involved in leukocyte migration from the bloodstream into tissues, including rolling, integrin activation by chemokines, stable adhesion, and migration through the endothelium.

Neutrophil Function

• Phagocytosis of microbes, especially opsonized microbes and products of necrotic cells.

• Secretion of granule contents.

• Extrusion of nuclear contents, forming neutrophil extracellular traps (NETs), which immobilize and kill extracellular microbes but may also damage healthy tissues.

Mononuclear Phagocytes

• The mononuclear phagocyte system includes:

  • Circulating bone marrow-derived cells called monocytes.

  • Macrophages that differentiate from monocytes after migrating into tissues.

  • Tissue-resident macrophages, derived from yolk sac or hematopoietic precursors during fetal life.

Development of Macrophages and Monocytes

• After birth, cells of the monocyte-macrophage lineage arise from committed precursor cells in the bone marrow, driven by monocyte (or macrophage) colony-stimulating factor (M-CSF).

• These precursors mature into monocytes, which circulate in the blood for 1-7 days.

• Most macrophages at inflammation sites are monocyte-derived.

• Long-lived tissue-resident macrophages are derived from yolk sac or fetal liver precursors during fetal development and have self-renewal capacity.

Monocyte-Derived Macrophages in Inflammation and Tissue Resident Macrophages in Homeostasis

• The image illustrates the differentiation pathways of macrophages, distinguishing between monocyte-derived macrophages in inflammation and tissue-resident macrophages in homeostasis.

Macrophage Tissue Specificity

Monocytes

• 10 to 15 \mum in diameter with bean-shaped nuclei and finely granular cytoplasm containing lysosomes, phagocytic vacuoles, and cytoskeletal filaments.

• All human monocytes express class II major histocompatibility complex (MHC) molecules.

Macrophage Polarization

Macrophage Functions

• Sacrificial death to alert immune system.

  • Salmonella infects a macrophage.

  • Macrophage tries phagocytosis.

  • Salmonella escapes the phagosome.

  • Inflammasome activated.

Macrophages and Tissue Repair

• Macrophages promote the repair of damaged tissues by stimulating new blood vessel growth (angiogenesis) and synthesis of collagen-rich extracellular matrix (fibrosis, thickening or scarring of the tissue).

• These functions are mediated by cytokines secreted by macrophages that act on various tissue cells.

Granulocytes

• Neutrophil

• Mast cell

• Basophil

• Eosinophil

Mast Cell

• Releases granules filled with chemicals that cause inflammation, such as histamine.

• Inflammation allows more immune cells and helpful particles in the blood to reach a site of infection or injury more easily.

• Disease: Inflammatory chemicals can cause allergy symptoms when the immune system reacts inappropriately to harmless substances.

• Too many mast cells can cause persistent problems with inflammation in a rare condition called mastocytosis.

• Location: Reside outside the bloodstream in tissues, especially in skin, lung tissue, lymph nodes, the liver, and the spleen. Basophils, which also play a large role in allergies, are located in the blood.

Allergies

• Special B cells recognize allergens, activate, and produce IgE antibodies.

• Mast cells capture IgE antibodies.

• When IgE antibodies on mast cells connect to the allergen again, the mast cell releases chemicals, especially histamine.

Basophil

• Loaded with granules that can be released by various inflammatory, infectious, and allergic triggers.

• Granules are filled with various proteins and chemicals, including histamine, which increases blood flow during an inflammatory response.

• May play a role in control of parasites and insect vectors of disease.

• Disease: An otherwise harmless substance can make basophils release chemicals and proteins, causing allergy symptoms.

• Severe allergies can cause airway tightening and low blood pressure, a life-threatening condition known as anaphylaxis.

• Location: Make up less than 1 percent of the immune cells in the blood. Mast cells, another immune cell type that plays a large role in allergic responses, are located in the body's tissues.

Eosinophil

• Contain granules full of molecules that kill cells the immune system has marked for destruction.

• Help clear parasitic infections and mediate inflammation.

• Disease: Several immune disorders involve the presence of too many or overactive eosinophils, including eosinophilic esophagitis and asthma.

• Location: Circulate in the blood and then migrate to tissues that interact with the outside environment, like the lungs and the lining of the gastrointestinal tract.

Antibody-Dependent Cellular Cytotoxicity (ADCC)

• A method of killing that depends on the ability of the immune cell to recognize specific antibody bound to a cell and trigger the death of that cell without the use of complement.

Dendritic Cells (DCs)

• Tissue-resident and circulating cells that detect the presence of microbes and initiate innate immune defense reactions.

• Capture microbial proteins for display to T cells to initiate adaptive immune responses.

• Subsets are defined based on different cell surface markers, transcription factors, development from different precursor cells, tissue localization, and functions.

Dendritic Cell Subsets

• Classical DCs (cDCs) are the major type of DC involved in capturing protein antigens of microbes that enter through epithelial barriers and presenting them to T cells.

• Plasmacytoid DCs (pDCs) produce the antiviral cytokine type I interferon (IFN) in response to viruses and may capture blood-borne microbes and carry their antigens to the spleen for presentation to T cells.

• Monocyte-derived DCs (MoDCs) have functions similar to those of cDCs but are derived from monocytes recruited into tissue inflammatory sites.

• Langerhans cells are DCs found in the epidermis that share functions with cDCs but are developmentally related to tissue-resident macrophages, arising from embryonic fetal liver and yolk sac precursors.

Birbeck Granules

• Structures found in Langerhans cells.

Follicular Dendritic Cells (FDCs)

• Have a dendritic morphology but are not derived from bone marrow precursors.

• Do not present protein antigens to T cells.

• Involved in B cell activation in the germinal centers of secondary lymphoid organs.

Lymphocytes

• Cells of the adaptive immune system.

Maturation of Lymphocytes

• Process of lymphocyte development.

Life of Lymphocytes

• Naive lymphocytes typically live for 1 to 3 months.

• Their survival requires signals from antigen receptors and cytokines.

Lymphocyte Effector Functions

• B lymphocyte:

  • Antigen recognition

  • Effector function: Antibody, Neutralization of microbe, phagocytosis, complement activation

• Helper T lymphocyte:

  • Microbial antigen presented by antigen-presenting cell

  • Effector Function: Cytokines, Activation of macrophages, Inflammation, Activation (proliferation and differentiation) of T and B lymphocytes

• Cytotoxic T lymphocyte (CTL):

  • Infected cell expressing microbial antigen

  • Effector Function: Killing of infected cell

• Regulatory T lymphocyte

  • Responding T lymphocyte

  • Efefctor function: Suppression of other lymphocytes

Antigen Recognition Molecules of Lymphocytes

• Each cell of the lymphoid lineage is clinically identified by characteristic surface molecules.

• Mature, naive B lymphocytes express two isotypes of antibody or immunoglobulin (IgM and IgD) within their surface membrane.

• Mature, naive T cells express a single genetically related molecule, called the T-cell receptor (TCR), on their surface.

Antigen Receptors of Mature Lymphocytes

• Mature B Lymphocyte:

  • IgM

  • IgD

• Mature T Lymphocyte:

  • Alpha Chain

  • Beta Chain

B-Lymphocyte Antigen Recognition Molecule (Membrane-Bound Immunoglobulin)

• The antigen receptor of the B lymphocyte, or membrane-bound immunoglobulin, is a 4-chain glycoprotein molecule that serves as the basic monomeric unit for each of the distinct antibody molecules destined to circulate freely in the serum.

• This monomer has 2 identical halves, each composed of a heavy chain and a light chain.

• Flexibility of movement is permitted between the halves by disulfide bonds forming a hinge region.

• Includes:

  • Antigen-binding site

  • Light chain

  • Heavy chain

  • Hinge

  • C_{H1}

  • C_{H2}

  • C_{H3}

  • Determines idiotype

  • Determines isotype

Idiotype and Isotype

• The unique structure of the antigen-binding site is called the idiotype of the molecule.

• Although two classes (isotypes) of membrane immunoglobulin (IgM and IgD) are co-expressed on the surface of a mature, naive B lymphocyte, only one idiotype or antigenic specificity is expressed per cell (although in multiple copies).

• Each individual can produce hundreds of millions of unique idiotypes.

T-Lymphocyte Antigen Receptor

• The antigen receptor of the T lymphocyte is composed of 2 glycoprotein chains, a beta and alpha chain that are similar in length.

• The antigen-binding site is formed between the 2 chains, whose 3-dimensional shape will accommodate the binding of a small antigenic peptide complexed to an MHC molecule presented on the surface of an antigen-presenting cell.

• There is no hinge region present in this molecule, and thus its conformation is quite rigid.

B- versus T-Lymphocyte Antigen Receptors

• The membrane receptors of B lymphocytes are designed to bind unprocessed antigens of almost any chemical composition, i.e., polysaccharides, proteins, lipids, whereas the TCR is designed to bind only peptides complexed to MHC.

• Also, although the B-cell receptor is ultimately modified to be secreted antibody, the TCR is never released from its membrane-bound location.

Antigen Binding

• When a lymphocyte binds to an antigen complementary to its idiotype, a cascade of messages transferred through its signal transduction complex will culminate in intracytoplasmic phosphorylation events leading to activation of the cell.

B- versus T-Lymphocyte Antigen Receptors Properties

Natural Killer Cells (NK Cells)

• NK cells are an innate immune cell type with unique features.

• They are lymphoid cells that do not express antigen-specific receptors derived from exposure to specific antigens, such as T cell receptors or surface immunoglobulin on B cells.

• NK cells can alter their behavior based on prior exposure to particular antigens, including after viral infection.

• NK cells play an important role in controlling infection by herpesviruses and flaviviruses (e.g., Dengue and Zika), and influenza, hepatitis C, human immunodeficiency virus 1 (HIV-1), and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viruses.

NK Cell Receptors and Function

• NK cells express a broad array of activating and inhibitory receptors, including TLRs 2, 3, 4, 5, 7, and 8, and they recognize and respond to the respective TLR ligands directly.

• Activating receptors are central to the "killer" function, responding to viral infections and malignant tumors by recognizing damaged or "stressed" host cells for elimination.

• NK cells have granules with perforins and granzymes that, upon activation, are released into the interface between target and effector NK cells, disrupting target cell membranes and inducing apoptosis.

• NK cells distinguish and avoid healthy host cells through receptors that recognize major histocompatibility complex (MHC) class I molecules expressed on all normal healthy cells.

• Binding of these receptors inhibits NK cell-mediated lysis and cytokine secretion, whereas deficiency or absence of surface MHC I will target that cell for attack.

• Virus-infected and malignant cells often downregulate MHC class I molecules, making them susceptible to attack by NK cells.

• The inhibitory receptors on NK cells are counterbalanced by activating receptors that recognize "stress" ligands expressed on cell surfaces in response to intracellular DNA damage.

• NK cells play a critically important role in maintaining the balance between protecting the placenta against infection without rejecting the half-foreign fetus.

NK Cell Killing Mechanism

• Normal Cell:

  • Inhibitory Receptor binds to Class I MHC leading to No killing

  • Activation signal

• Virus-infected cell:

  • Activation signal leads to killing

NK Cells and Disease

• In the give and take between the host and an infecting virus, NK cells play a central role, as shown by the serious herpesvirus infections sustained by patients lacking functional NK cells.

• NK cells are reduced in number, metabolically stressed, and functionally deficient in children with obesity, which could relate to the increased rates of cancer in this population.

• The "immunosenescence" that occurs with aging is regularly associated with a decrease in the number and function of NK cells.

• Exercise has been experimentally shown to increase the number and cytolytic capacity of NK cells in this population.

Innate Lymphoid Cells (ILCs)

• Innate lymphoid cells (ILCs; groups 1 to 3) are a diverse group of lymphocytes found in many tissues, particularly the skin and the mucosal barriers of the lungs and gastrointestinal tract.

• They develop from the common lymphoid progenitor but do not express antigen receptors or expand into a clone when stimulated.

• Instead, they react quickly to signals from infected or injured tissues by releasing an array of cytokines, including IFN-gamma, IL-5, and IL-17, which direct the immune response to the source of ILC stimulation.

• Some have cytolytic potential and can act directly to kill tumor cells.

• They limit T cell adaptive responses to intestinal commensal bacteria.

• Continuously produce IL-5, which regulates eosinophil homeostasis.

• Promote glycosylation of the intestinal epithelial cell surface, which is required to allow survival of the gut microflora but prevent their invasion.

• Genetic impairment of ILC3 function may be involved in the pathogenesis of Crohn disease.

Lymphatic System

• Includes Lymph Nodes and Spleen, Bone Marrow, Lymph Vessels, and Thymus.

Lymph

• Interstitial fluid from tissues

• Drains into lymphatic system

• Eventually drains into subclavian veins.

Lymphoid Organs

• Primary lymphoid organs:

  • Sites of lymphocyte formation (Bone marrow, Thymus)

  • Create B and T cells

• Secondary lymphoid organs:

  • B cells and T cells proliferate (Lymph nodes, Spleen, Peyer’s patches, Tonsils)

Lymph Nodes

• Lymph fluid drains from the site of infection.

• Dendritic cells are activated and express MHC I, MHC II, and B7.

• Dendritic cells enter the lymph, carrying processed antigens.

• Free antigens are also carried with lymph.

• Lymph enters nodes.

• Many B and T cells wait for a matching antigen.

• Dendritic cells present to T cells; APCs in lymph nodes process antigen.

• B cells react to antigen.

• Result: Generation of adaptive immune response.

Lymph Node Structure

• Follicle

• Cortex

• Paracortex

• Efferent Lymph Vessel

• Artery/Vein

• Medulla (sinus)

• Medulla (cords)

• Afferent Lymph Vessel

Lymph Node Regions

• Cortex:

  • Primary follicle (B-cell-rich)

• Paracortex:

  • (T-cell-rich)

• Medulla

• Afferent lymphatic (Ag enters)

• Efferent lymphatic (memory cells exit)
  Germinal center of follicle (clones dividing)

Lymphoid Follicles

• Found in the cortex of lymph nodes.

• Site of B-cell activation.

• Contain follicular dendritic cells (FDCs).

  • Different from tissue dendritic cells

  • Permanent cells of lymph nodes

  • Surface receptors bind complement-antigen complexes

  • Allows easy crosslinking of B cell receptors

• FDCs are an important reservoir for HIV. Early after infection, large amounts HIV particles are found in FDCs.

Lymphoid Follicles Types

• Primary follicles:

  • Inactive follicles

  • Follicular dendritic cells and B cells

• Secondary follicles:

  • “Germinal center”

  • B cell growth and differentiation, class switching

  • Nearby helper T cells can bind → more growth

Lymph Node Regions Cont.

• Lymphoid follicles.

• Adipose tissue.

• Capsule

• Germinal center

Paracortex

• Two key features:

  • #1: Contain T cells activated by dendritic cells and antigen

  • #2: Contain high endothelial venules

    • Vessels that allow B/T cell entry into the node

• Engorged in immune response (swollen nodes).

• Underdeveloped in rare T-cell deficiency disorders

  • E.G: DiGeorge syndrome

Medulla

• Medullary sinuses (cavities):

  • Contain macrophages

  • Filters lymph → phagocytosis

• Medullary chords (tissue between cavities):

  • Contain plasma cells secreting antibodies

Spleen

• Filters blood (no lymph).

• All blood elements can enter.

• No high endothelial venules, no selective entry T and B cells

Spleen Structure

• Marginal Zone

• Sinusoids

• PALS (Periarteriolar Lymphocyte Sheath)

• Follicle

• Artery

Spleen - White and Red Pulp

• White pulp:

  • Exposure to B and T cells

  • Exposure to macrophages

• Red pulp:

  • Filters blood in sinusoids

  • Removes old RBCs (red)

  • Stores many platelets

White Pulp Details

• Marginal zone:

  • Macrophages remove debris; Dendritic cells process antigen

• Follicles:

  • B cells

• Periarteriolar lymphocyte sheath (PALS):

  • T cells

Sinusoids of Spleen

• Red pulp lined by vascular “sinusoids.”

• Open endothelium → cells pass in/out.

• Exit into splenic cords.

• Cords contain macrophages (filtration).

Splenic Dysfunction

• Increased risk from encapsulated organisms.

• Loss of marginal zone macrophages → ↓ phagocytosis.

• Also loss of opsonization:

  • ↓ IgM and IgG against capsules (splenic B cells)

  • Loss of IgG opsonization

  • ↓ complement against encapsulated bacteria

  • ↓ C3b opsonization

Sepsis and Splenic Dysfunction

• Strep pneumo is predominant pathogen for sepsis.

  • Death in >50% of patients.

• Others: H. flu (Hib), Neisseria meningitidis.

• Less common: Strep pyogenes, E coli, Salmonella

• Also malaria and babesia (RBC infections)

Howell-Jolly Body

• Nuclear remnants in RBCs.

Target Cells

• Too much membrane for amount of hemoglobin inside.

• Too little hemoglobin for the size of the cell wall.