Immunity and Lymphocytes

• Immunity is the ability to resist and defend against infectious organisms and damaging substances.
• Resistance is the body's ability to maintain immunity.
• Immune response is the body's reaction to infectious and abnormal agents.

Types of Immunity

• Innate immunity always works the same way against any invading agent.
• Adaptive immunity protects against specific pathogens and depends on lymphocyte activities, developing after exposure to environmental hazards.

Lymphocytes

• Main cells of immunity, including B cells, T cells, and NK cells.
• Tissues maintain different T and B cell populations, with lymphocytes wandering through tissues.
• Lymphocytes enter blood vessels or lymphatics for transport and can survive many years.
• Lymphocyte production, or lymphocytopoiesis, happens in bone marrow, thymus, and peripheral lymphoid tissues.
• Hemocytoblasts in bone marrow divide into two types of lymphoid stem cells.

Lymphoid Stem Cells: Group 1

• Remain in bone marrow, developed by stromal cells, and produce B cells and NK cells.
• B cells differentiate with exposure to interleukin-7.

Lymphoid Stem Cells: Group 2

• Migrate to the thymus and develop in an environment isolated from blood.
• T cells differentiate with exposure to hormones.
• T and B cells migrate throughout the body to defend peripheral tissues and retain their ability to divide, which is essential to immune system function.

Innate Defenses

• Respond the same, regardless of the invader, blocking or attacking any foreign substance or pathogen to deny entry or limit spread.
• Innate defenses cannot distinguish one pathogen from another.
• Physical barriers include skin, hair, mucous membranes, epithelia, and secretions.
• Phagocytes attack and remove dangerous microorganisms.

Microphages

• Neutrophils and eosinophils enter peripheral tissues to fight infections.

Macrophages

• Large phagocytic cells derived from monocytes.
• Distributed throughout the body and make up the monocyte-macrophage system (reticuloendothelial system).
• Activated macrophages respond to pathogens by engulfing and destroying them with lysosomal enzymes, binding to pathogens so other cells can destroy them, and destroying pathogens by releasing toxic chemicals into interstitial fluid.
• Free macrophages and microphages move through capillary walls (emigration) and are attracted or repelled by chemicals in surrounding fluids (chemotaxis).
• Phagocytosis begins when a phagocyte attaches to a target (adhesion) and surrounds it with a vesicle.

Immune Surveillance

• Performed by natural killer (NK) cells.
• Activated NK cells identify and adhere to abnormal cells.
• The Golgi apparatus produces vesicles containing perforins, which are released by exocytosis.
• Perforins form pores in abnormal cell's plasma membrane, causing lysis.

Tumor-Specific Antigens

• On plasma membranes of cancer cells, are identified as abnormal by NK cells.
• Some cancer cells avoid detection through immunological escape.
• Cells infected with viruses present abnormal proteins on plasma membranes, allowing NK cells to identify and destroy them.

Interferons (IFNs)

• Small proteins released by activated lymphocytes and macrophages that trigger the production of antiviral proteins.
• Antiviral proteins do not kill viruses but block viral replication in the cell and are one type of cytokine.
• Cytokines are chemical messengers released by tissue cells and are important to the immune response.

Complement

• More than 30 special complement proteins in plasma that assist antibodies in the destruction of pathogens.
• Complement proteins work together in cascades through three routes of activation: the classical pathway, lectin pathway, and alternative pathway.

Effects of Complement Activation

• Killing of pathogens by cell lysis using proteins to form a membrane attack complex (MAC).
• Enhanced phagocytosis (opsonization) and inflammation (histamine release).

Inflammation

• Localized tissue response to injury, with cardinal signs and symptoms of redness, swelling, heat, and pain.
• Fever is a body temperature above 99°F (37.2°C) to inhibit pathogens and accelerate metabolism; pyrogens released by pathogens can cause a raised body temperature.

Adaptive Immunity

• Responds to particular threats and are either cell-mediated (T cells) or antibody-mediated (B cells).
• Antigens are chemical targets that stimulate an immune response.
• When a lymphocyte contacts an appropriate antigen, it becomes activated and divides to produce a clone.
• Clonal selection (aka clonal expansion) is the process of an antigen "selecting" lymphocytes for cloning.

Primary Types of T Cells

• Cytotoxic T cells attack antigens physically and chemically.
• Helper T cells stimulate responses of T cells and B cells.
• Regulatory T cells moderate the immune response.
• Memory T cells respond to antigens previously encountered.

Forms of Immunity

• Innate immunity is present at birth.
• Adaptive immunity is acquired after birth.
• Active immunity develops after exposure to an antigen.
• Naturally acquired active immunity comes through environmental exposure to pathogens.
• Artificially acquired active immunity is achieved through vaccines.
• Passive immunity is produced by transferring antibodies from another source.
• Naturally acquired passive immunity involves antibodies acquired from the mother.
• Artificially acquired passive immunity is achieved through an injection of antibodies.

Four Properties of Adaptive Immunity

• Specificity: Each T or B cell responds only to a specific antigen and ignores all others.
• Versatility: The body produces many types of lymphocytes, each fighting a different type of antigen. An activated lymphocyte clones itself to fight a specific antigen.
• Memory: Some inactive lymphocytes (memory cells) stay in circulation and provide immunity against new exposure.
• Tolerance: The immune system ignores "normal" antigens (self-antigens).