Introduction to Immunology

• Immunity involves processes that defend the body against foreign organisms or molecules.
• Inflammation, complement activation, phagocytosis, antibody synthesis, and effector T lymphocytes are all part of immunity.

Types of Immunity

• Innate immunity is nonspecific and natural, present at birth without prior exposure; its effectiveness varies with age. Physical and chemical barriers are the first line of defense.
• Physical barriers include epithelial cells and mucus, while chemical barriers include acidic pH, complement, interferons, and lysozymes.
• Phagocytosis, inflammation, and the complement system are the second line of defense.
• Enhancing phagocytosis, stimulating inflammatory response, and lysing foreign cells are the functions of the complement system.
• Adaptive immunity is specific and acquired after encountering a challenge and responds specifically to it.
• Humoral-mediated immunity (HMI), involving antibody production by plasma cells, protects against extracellular pathogens.
• Cell-mediated immunity (CMI) is crucial for intracellular pathogen defense and involves natural killer cells, T helper cells, cytotoxic T lymphocytes (CTLs), and cytotoxins.
• Natural active immunity occurs after exposure to a foreign immunogen through infection, prompting immune cells to produce products to eliminate it.
• Artificial active immunity is vaccination, where the immune system responds to an altered organism; generally endures for life.
• Natural passive immunity involves maternal antibodies crossing the placenta to protect the infant.
• Artificial passive immunity involves injecting immune products from another animal into the host, offering short-term protection without memory cells.
• An immunogen can induce an immune response.
• An antigen specifically interacts with cells or substances of the immune system; immunogens are also antigens, but not all antigens produce an immune response.
• The epitope is the portion of an antigen that binds to an antibody or T cell receptor.
• Thymic-dependent immunogens require T helper cells to stimulate antibody formation.
• Thymic-independent immunogens initiate antibody production without T helper cells.
• For immunogenicity, a substance must be recognized as "nonself," be larger than 10 kilodaltons, and have proteins/carbohydrates as the most immunogenic components.
• Immunogenicity increases with molecular complexity, host entry route, immunogen dose, and degradability (to be presented to immune cells).
• A hapten, a low-molecular-weight molecule, can stimulate an immune response when combined with another molecule.
• An adjuvant enhances an immune response but cannot induce one alone.
• An antibody (immunoglobulin [Ig] or gamma globulin) is a protein with five classes: IgG, IgM, IgE, IgA, and IgD; mainly migrates in the beta and gamma regions.
• Antibodies consist of two heavy and two light polypeptide chains (kappa or lambda), with each chain having a variable and constant domain.
• The variable domain defines antibody specificity and is referred to as the fragment of antigen binding (Fab).
• The crystalline fragment (Fc), located at the carboxy-terminus, is responsible for the molecule's biological activity, including activating complement.
• Isotypes are variations between light and heavy chains, defined by constant regions. Allotypes are species-specific variations in constant domains, involving different alleles.
• Idiotypes are variations in the variable region; a single clone of cells produces one.
• A J (joining) chain links multiple monomers of IgM and IgA, with one J chain needed for each molecule that is linked together.

Antibody Classes

• IgG: Predominant serum antibody (75%), with subclasses like IgG1, IgG2, IgG3, and IgG4; it's the only immunoglobulin that crosses the placenta. Produced in the secondary antibody response, IgG1, IgG2, and IgG3 activates the classical complement pathway.
• IgM: Made of five monomers linked by a J chain and interchain disulfide bonds; 10% of serum immunoglobulins. It is the first antibody produced against an immunogen, is produced in both primary and secondary immune responses, and best activates the classical complement pathway.
• IgA: Exists in serum and secretory forms with two subclasses(IgA1 and IgA2), accounting for 15-20% of total serum antibody. Serum IgA functions in antigen clearance and immune regulation, while IgA in mucous membranes blocks viruses, bacteria, and toxins from attaching to host cells.
• IgD: Primarily a cell membrane surface component of B lymphocytes with a short half-life (2-3 days).
• IgE: Responsible for allergic (type I hypersensitivity) reactions; its Fc portion binds to receptors on mast cells and basophils. It triggers degranulation and releases mediators like histamine and leukotrienes upon allergen binding.
• Monoclonal antibodies are identical, produced from a single plasma cell clone, and are found in individuals with multiple myeloma, these are also produced in industry by fusing an antigen-sensitized B lymphocyte with a nonsecreting myeloma cell, therefore creating an immortal cell line that secretes an antibody of an idiotype.
• Quantification of antibodies provides information about an individual's functional immune status.
• IgG, IgM, and IgA are quantified using radial immunodiffusion, nephelometry, or turbidimetry.

The Immune System

• Myeloid cells are responsible for nonspecific responses.
• As monocytes in the blood, macrophages in tissues (alveolar, Küppfer, astrocytes, microglia) perform phagocytosis of invaders, present immunogens to T helper cells, and release cytokines.
• Macrophages have MHC class II, complement, and antibody Fc receptors.
• Neutrophils (60-70% of WBCs) perform phagocytosis and contribute to inflammatory response.
• Eosinophils (1-3% of WBCs) mediate IgE allergic response.
• Basophils (0-1.0% of WBCs) have IgE receptors and granules responsible for allergic reactions.

Lymphocytes

• Twenty to forty percent of WBCs.
• B lymphocytes (20%) mature after birth in the bone marrow and express surface molecules like CD19 and CD20 and can differentiate into plasma cells or memory B cells.
• T lymphocytes (80%) express surface molecules like CD2 and CD3; CTLs lyse virus-infected and tumor cells, produce lymphokines, and stimulate or suppress other cells.
• Pre-T cells mature in the thymus.
• NK cells contain cytoplasmic granules and are slightly larger than T or B cells.

Immune Response Cells

• Dendritic cells present antigen to T cells.
• Langerhans cells are dendritic cells in the dermis and squamous epithelia.
• Mast cells resemble basophils and contain chemicals affecting immune response.
• Cytokines are soluble proteins secreted by one cell type to affect other cells.
• Interferons alpha (INF-α) and beta (INF-ẞ) inhibit viral replication and activate NK cells; produced by virus-infected cells.
• INF-γ has antiviral effects, activates macrophages and NK cells, and stimulates B cells; produced by TH 1 cells.
• Produced by macrophages, lymphocytes, and NK cells, tumor necrosis factor-alpha (TNFα) encounters bacteria, viruses, tumor cells, toxins, and complement protein C5a
• TNFB: Produced by CD4 and CD8 positive cells after exposure to a specific antigen
• Interleukin 1 (IL-1) is produced by macrophages, B cells, and other cell types while IL-2 is produced by T helper cells.
• Activating T helper cells, increases the number of B cells, activates vascular endothelium, causing fever and acute-phase protein synthesis, and induce T cells to produce lymphokines is the functions of Interleukin 1(IL-1).
• IL-2 causes proliferation of activated T and B cells, IL-3 increases mast cells in skin, spleen, and liver, and IL-4 induces T cell proliferation and class switching from IgM to IgG1 and IgE.

Organs and Tissues of the Immune Cells

• The bone marrow is the tissues where Pre-B lymphocytes develop into mature B cells and Pre-T lymphocytes develop into mature T cells at the Thymus
• B cells migrate to the cortex and T cells to the paracortex on Lymph nodes, many small B cells in Primary follicles and After stimulation, primary follicle becomes a secondary follicle.
• Contains both T & B cells with the Purpose is to filter blood from the system,
• Mucosal-associated lymphoid tissue (MALT) is Found in submucosa in gastrointestinal tract, respiratory tract, and urogenital tract.
• HLA antigens are cell surface markers that allow immune cells to distinguish "self" from "nonself" and coded for by genes in the MHC located on chromosome six..

Three Classes of MHC Products

• Class I loci: Molecules are found on nearly every nucleated cell surface antigen-presenting cells with MHC I molecules present antigens to CTLs, including HLA-A, HLA-B, HLA-C, HLA-E, HLA-F, HLA-G, and HLA-J.
• Class II: molecules are located on the surface of monocytes, macrophages, B cells, activated T cells, dendritic cells, Langerhans' cells, and some epithelial cells, including include HLA-DM, HLA-DO, HLA-DP, HLA-DQ, and HLA-DR while Antigen-presenting cells with MHC II molecules present antigens to T helper cells.
• Class III products :These include Complement proteins, TNFα and ẞ, and other proteins (e.g., heat shock protein) not associated with cell membrane surfaces

HLA & Inheritance

• Haplotype is the Combination of inherited HLA alleles while Two haplotypes (one from each parent) are a genotype.
• HLA inheritance patterns can exclude fathers with approximately 99% accuracy in Paternity testing while Not all individuals who have a particular HLA antigen have a disease, HLA B-27 is associated with ankylosing spondylitis.

Nonspecific Immune Response: Cellular Mechanisms

• Skin and mucous membranes are the Barrier and first line of defense
• Involved in nonspecific response by attachment to damaged epithelium, migration into tissues, chemotaxis, phagocytosis and digestion of target cells
• Defect in oxidative pathway (respiratory pathway) phagocytes use to create hydrogen peroxide: Chronic granulomatous disease while Impaired production of toxic oxygen molecules (decrease respiratory burst) used by phagocytes to kill ingested bacteria: Myeloperoxidase (MPO) deficiency:
• Granules contain acid phosphatase, peroxidase, histamines, and several other types of molecules known as Eosinophils

Mediators and Basophil Function

• Mast cells, basophils, and platelets release substances that mediate immune reactions.
• Amplify the reactions that start with the mast cell
• Alveolar macrophages, splenic macrophages, Kupffer cells of the liver, etc. known as the mononuclear phagocyte system
• Sequenced events following tissue damage that protect the host from foreign invaders and attempt to minimize tissue damage is known as Inflammation

Inflammation

• Capillaries, arterioles, and venules are dilated to increase blood flow to the site of the injury.
• Chemotaxins and endothelial activating factors is released after the injury.
• PMNs adhere to activated endothelial cells and move between the endothelial cells to the site of tissue damage by diapedesis and macrophages release IL-1.
• Macrophages release IL-1, attracts monocytes, macrophages, and lymphocytes to the injury
• Cellular proliferation and repair happens with Fibroblasts help repair the damage and return the injury site to normal.

Chemical Mechanisms of the Nonspecific Immune Response

• Complement system: Collection of serum proteins involved in lysis of cell membranes, mediation of inflammation, enhancement of phagocytosis, metabolism of immune complexes, synthesis of Components occurs in the liver, except C1, which is synthesized in the epithelial cells of the intestine and Approximately 20 proteins are involved in three separate pathways of activation, Five proteins unique to classical pathway: C1q, Clr, Cls, C4, and C2 Three proteins unique to alternative pathway: factor B, factor D, and properidin Six proteins common to both pathways: C3, C5, C6, C7, C8, and C9

Outcome of complement activation

• C4a, C3a, and C5a cause basophils and mast cells to cause the release of histamine, Anaphylatoxins also cause smooth muscle contraction and increased vascular permeability
• C3b adheres to immune complexes and surfaces to facilitate clearing. (Immune adherence).
• If C3b is attached to a cell, phagocytosis is enhanced., is known as Opsonization.

Control mechanisms

C1 inhibitor combines with Clr and Cls to block C1. - Hereditary angioedema, is an autosomal dominant disease results from deficiency in C1INH, swelling of mucous membranes in airways, and unregulated classical pathway activation . This C5b-7 compound protein Anaphylatoxin inactivator, removes a single amino acid from C4a, C3a, and C5a, removing them as anaphylatoxins

• Complement can be consumed in infections and collagen vascular diseases, C3 and C4 are measured to indicate its consumption.

Adaptive Immune Response

• The Total functional complement assay (CH50) is used to measure the activity of the classical pathway. a. C-reactive protein (CRP) concentration increases several hundred times after injury and can activate the classical pathway, stimulating NK cells and monocytes to target tumor cells.
• Phagocytic cells process antigen and express it on the cell surface associated with MHC I or II molecules like (Monocytes/macrophages and Dendritic) while Nonphagocytic cells attach to antigens in their native form, process antigens, and express B-cell receptors. The T-cell receptor consists of two nonidentical peptides and CD3 that allow T cells recognize antigens that are processed by other cells.

Mediated Immunity

• Is Primarily mediated by TH 1 cells, a subset of T helper cells, that secrete cytokines that activate other cells
• Stimulated by cytokines from TH 1 cells, inflammatory reaction cells CTLs destroy targets by cell-to-cell contact and activated by cytokines from TH 1 cells. NK cells kill target cells without being previously sensitized.

Humoral-Mediated Immunity

• B cell activation begins when the antigen binds to the antibody on B cell surface and antigen is internalized
• The cytokines stimulate the B cell to divide and differentiate into a memory B cell or a plasma cell that will synthesize antibody due to T and B cell interactions
• IgG antibodies persist longer in circulation than IgM due to Secondary (anamnestic) response: Produced after the host has previously been exposed to an antigen

Autoimmune Disease

• A autoimmune disease occurs when an individual produces antibodies or a The presence of certain HLA types has been correlated with specific diseases. Self-recognition error leads autoantibodies to be produced and burnet-postulated known as "Forbidden Clone theory"

Non-Organ Specific Autoimmune Diseases

• Systemic lupus erythematosus ( SLE), mixed connective tissue disease (MCTD), and rheumatoid arthritis associated with Antinuclear antibodies (ANAs)
• Rheumatoid factor (RF) is an anti-antibody, typically IgM, that binds to the Fc portion of abnormal IgG
• Reversibly precipitate at 4°C in Cryoglobulins & The disease is more likely to occur in women than men and in blacks than whites with Systemic lupus erythematosus

HLA Type Autoimmune Diease

• Graves Disease & Type 1 Diabetes is associated with HLA-8 SLE is associated with HLA-DR2
• Rheumatoid arthritis, Type 1 diabetes, and Pemphigus vulgaris is associated with HLA-DR4

Hypersensitivity

• The disease that tissue injury is caused by autoantibodies and immune complexes deposited in the tissues.
• Humoral- and cell-mediated responses involve in Hypersensitivity reaction
• Molecule that triggers a hypersensitivity reaction Allergen
• It can be life-threatening, anaphylaxis causes shock/ edema of the upper of respiratory system caused by Type 1 Hypersensitivity Reaction

Hypersensitivity Mediator & Treatment

• Histamine: Causes contraction of bronchioles and smooth muscle of blood vessels, increases capillary permeability & mucus secretion
• Prostaglandins cause vasodilation and increased vascular permeability while Leukotrienes cause erythema and wheal formation and they have 30-1000 abilities to cause bronchospasms & stimulates mucus secretion:
• In In the competitive assay,IgE in the patient sample competes with labeled IgE for binding to anti-IgE attached to a solid surface
• IgG or IgM is directed against cell surface antigens during Type II hypersensitivity (cytotoxic)reaction

Immune Deficiency

• Cytokines activate complement and tissue damage in The large complexes cause cell lysis (innocent bystanders) by mononuclear phogocytes There is no antibody and complement involved with in type IV Hypersensitivity

• Marked deficiency of all the classes of immunoglobulins is detected after 6 months to a year is known as Bruton X-linked agammaglobulinemia

• Patients present with small amounts or absence of serum & secretory IgA and can anaphylaxis is caused by Selective IgA deficiency

Transplant Immunology & Tumor Immunology

• Transfer of tissue from one of site’s to another within a individual is referred to as Autograft: -Transfer of tissue between genetically identical individuals is referred as Isograft -The tumor resulting for uncontrolled growths of Normal tissue is referred to as Neoplasm and A benign tumor is a mild and non-progressive tumor the pushes tissue aside and does not invade as it expands

Tumor- Associated Antigens

• Tumor specific peptides are intracellular proteins are expressed on the surface of tumor by interactions with MHC class one and two

• The method the requires more than one serum sample with the diagnosis that is made if a fourfold increase in titer is detected between that contains Borrelia Burgdorferi antigens.

• Treponema Pallidum infection causes the host to produce nonspecific: This nonspecific anti body is described as Reagins & can only be screen.

Detection of Disease

• Anti DNA antibodies are used in what method to detect Syphilis?
• Patients are mixed with serial dilutions of antibodies but not in an electricity field, this reaction has occurred on what specific test method? Precipitin reaction