Immunology and Immune System

Immunity and Immunology

• Immune system: cells, tissues, and molecules that mediate resistance to infections
• Immunology: study of structure and function of the immune system
• Immunity: resistance of a host to pathogens and their toxic effects
• Immune response: collective and coordinated response to the introduction of foreign substances in an individual mediated by the cells and molecules of the immune system

Role of the Immune System

• Defense against microbes
• Defense against the growth of tumor cells
• Homeostasis: destruction of abnormal or dead cells (e.g. dead red or white blood cells, antigen-antibody complex)

Immune System Components

• Organs: tonsils, adenoids, thymus, lymph nodes, spleen, Payer's patches, appendix, lymphatic vessels, bone marrow
• Cells: lymphocytes (T-lymphocytes, B-lymphocytes, plasma cells, natural killer lymphocytes), monocytes, macrophages, granulocytes (neutrophils, eosinophils, basophils)
• Molecules: antibodies, complement, cytokines, interleukins, interferons

Types of Immunity

• Innate (non-adaptive) immunity: first line of immune response, relies on mechanisms that exist before infection, rapid response, non-specific, no memory
• Acquired (adaptive) immunity: second line of response, relies on mechanisms that adapt after infection, handled by T- and B- lymphocytes, specific, has memory

Innate Immunity Mechanisms

• Mechanical barriers/surface secretion: skin, acidic pH in stomach, cilia
• Humoral mechanisms: lysozymes, basic proteins, complement, interferons
• Cellular defense mechanisms: natural killer cells, neutrophils, macrophages, mast cells, basophils, eosinophils

Adaptive Immunity

• Active and passive immunity
• Active immunity: natural (clinical or sub-clinical infection), artificial (vaccination: live, killed, purified antigen vaccine)
• Passive immunity: via breast milk, placenta, immune serum, immune cells

Adaptive Immunity Mechanisms

• Cell-mediated immune response (CMIR): T-lymphocytes eliminate intracellular microbes that survive within phagocytes or other infected cells
• Humoral immune response (HIR): B-lymphocytes mediate antibody production to eliminate extracellular microbes and their toxins

Cell-Mediated Immune Response

• T-cells recognize peptide antigen on macrophage in association with major histocompatibility complex (MHC) class, identifies molecules on cell surfaces, helps body distinguish self from non-self
• T-cells differentiate into effector cells that kill infected cells

T Lymphocytes

• Helper T-lymphocytes (CD4+): activate phagocytes to kill microbes
• Cytolytic T-lymphocytes (CD8+): destroy infected cells containing microbes or microbial proteins

Cell-Mediated Immune Response Phases

• Primary response: production of specific clones of effector T cells and memory clones, develops in several days, does not limit the infection

• Secondary response: more pronounced, faster, and more effective at limiting the infection

• Examples: cytotoxic reactions against intracellular parasites, delayed hypersensitivity (e.g., Tuberculin test), and allograft rejection### Immune Response

• Some B lymphocytes evolve into the resting state, known as memory cells.

Antibodies (Immunoglobulins)

• Belong to the gamma-globulin fraction of serum proteins
• Y-shaped or T-shaped polypeptides consisting of 2 identical heavy chains and 2 identical light chains
• Not all immunoglobulins are antibodies
• There are five kinds of antibodies: IgG, IgM, IgA, IgD, and IgE

IgG

• Accounts for 70-75% of total immunoglobulin
• Secreted in high quantities in secondary exposures
• Can cross the placenta
• Indicates active infection with a 4-fold rise or fall
• Major functions: neutralize microbes and toxins, opsonize antigens for phagocytosis, and activate the complement
• Protects the newborn

IgM

• Secreted initially during primary infection
• Cannot cross the placenta
• Presence in newborn indicates infection
• Major functions: activates the complement, used as a marker of recent infection
• Secreted first during primary exposure, used to detect early phase of infection

IgA

• Monomeric in serum, dimeric with secretory component in the lumen of the gastro-intestinal tract and in the respiratory tract
• Major function: neutralizes microbes and toxins
• Used in sero-diagnosis of tuberculosis and synactical respiratory virus tests

IgD

• Monomeric
• Major functions: present on the surface of B lymphocytes, functions as membrane receptor, and has a role in antigen-stimulated lymphocyte differentiation

IgE

• Mediates type I hypersensitivity
• Monomeric
• Major functions: associated with anaphylaxis, plays a role in immunity to helminthic parasites, and used in sero-diagnosis of infectious and non-infectious allergies

Sequential IgM-IgG Humoral Response

• IgM produced as a first response to many antigens, levels remain high transiently
• IgG produced after IgM, higher levels persist in small amounts throughout life, produced in large amounts during secondary response
• Persistence of antigen-sensitive ‘memory cells’ after primary response

Failure of Immune Response

• Immune response helps individuals defend against microbes and some cancers
• Immune response can fail, resulting in hypersensitivity reactions, immunodeficiency, and others

Hypersensitivity Reactions

• Cause cell damage through excessive immune response to antigens
• Types: hypersensitivity, allergy, and autoimmunity

Immunodeficiency

• Loss or inadequate function of various components of the immune system
• Can occur in any part or state of the immune system
• The immuno-compromised host has an impaired function of immune system and is at high risk of infection

Types of Immunodeficiency

• Congenital (primary) immunodeficiency: genetic abnormality, defect in lymphocyte maturation
• Acquired (secondary) immunodeficiency: results from infections, nutritional deficiencies, or treatments, such as AIDS or chronic leukemia

Altered Immunity: Immuno-compromised

• Altered anatomic barrier: reduction in IgA, elevated pH, and change in flora
• Immune system disorders: reduction of complement, neutropenia, monocytopenia, reduction of T cells, and hypo-gammaglobulinemia

Summary (1)

• Innate immunity relies on mechanisms already existing before microbe infects host
• Is the first line of defense
• Has no memory for subsequent exposure
• Relies on non-specific mechanisms

Summary (2)

• Adaptive immunity develops following entry of microbe into the host
• Comes into action after innate immunity fails to get rid of microbe
• Has memory to deal with subsequent exposure
• Happens through specific cells: T cells (cell-mediated) and B cells (antibody-mediated)

Summary (3)

• Primary immune response: short-lasting, smaller in magnitude
• Secondary immune response: longer in duration, larger in magnitude, develops ‘memory cells’ following primary response
• Failure of immune response can result in hypersensitivity, immunodeficiency, and others

Learn about the immune system, immunology, and immunity, including cells, tissues, and molecules that mediate resistance to infections and the collective response to foreign substances.