· An individual' s genetically predetermined resistance to certain diseases is called innate
    resistance.
· Individual resistance is affected by gender, age, nutritional status, and general health.

Immunity
· Immunity is the ability of the body to specifically counteract foreign organisms or substances
    called antigens.
· Immunity results from the production of specialized lymphocytes and antibodies.

Types of Acquired Immunity
· Acquired immunity is specific resistance to infection developed during the life of the
    individual.
· A person may develop or acquire immunity after birth.

I. Naturally Acquired Immunity
· Immunity resulting from infection is called naturally acquired active immunity; this type of
    immunity may be long-lasting.
· Antibodies transferred from a mother to a fetus (transplacental transfer) or to a newborn in
    colostrum results in naturally acquired passive immunity in the newborn; this type of
    immunity can last up to a few months.

II. Artificially Acquired Immunity
· Immunity resulting from vaccination is called artificially acquired active immunity and can be
    long-lasting.
· Vaccines can be prepared from attenuated, inactivated, or killed microorganisms and toxoids.
· Artificially acquired passive immunity refers to humoral antibodies acquired by injection; this
    type of immunity can last for a few weeks.
· Antibodies made by a human or other mammal may be injected into a susceptible individual.
· Serum containing antibodies is often called antiserum.
· When serum is separated by gel electrophoresis, antibodies are found in the gamma fraction of
    the serum and are termed immune serum globulin, or gamma globulin.
 

III. The Duality of the Immune System
· Humoral immunity is in body fluids.
· Cell-mediated immunity is due to certain types of lymphocytes.

The Humoral (Antibody-Mediated) Immune System
· The humoral immune system involves antibodies produced by B cells in response to a specific
    antigen.
· Antibodies primarily defend against bacteria, viruses, and toxins in blood plasma and lymph.

The Cell-Mediated Immune System
· The cell-mediated immune system depends on T cells and does not involve antibody
    production.
· Cellular immunity is primarily a response to intracellular bacteria and viruses, multicellular
    parasites, transplanted tissue, and cancer cells.
 

IV. Antigens and Antibodies
The Nature of Antigens
· An antigen (or immunogen) is a chemical substance that causes the body to produce specific
    antibodies or sensitized T cells.
· As a rule, antigens are foreign substances; they are not part of the body' s chemistry.
· Most antigens are components of invading microbes: proteins, nucleoproteins, lipoproteins,
    glycoproteins, or large polysaccharides with a molecular weight greater than 6,000.
· Antibodies are formed against specific regions on the surface of an antigen called epitopes or
    antigenic determinants.
· Most antigens have many different determinants.
· A hapten is a low-molecular-weight substance that cannot cause the formation of antibodies
    unless combined with a carrier molecule.

IV. The Nature of Antibodies
· An antibody, or immunoglobulin, is a protein produced by B cells in response to the presence
    of an antigen and capable of combining specifically with that antigen.
· An antibody has at least two identical antigen-binding (valence) sites.

V. Antibody Structure
· A single bivalent antibody unit is a monomer.
· Most antibody monomers consist of four polypeptide chains. Two are heavy chains, and two
    are light chains.
· Within each chain is a variable (V) region, where antigen binding occurs, and a constant (C)
    region, which serves as a basis for distinguishing the classes of antibodies.
· An antibody monomer is Y- or T-shaped; the variable regions form the tips, and the constant
    regions form the base and Fc (stem) region.
· The Fc region can attach to a host cell or complement.

VI. Immunoglobulin Classes
· IgG antibodies are the most prevalent in serum; they provide naturally acquired passive
    immunity, neutralize bacterial toxins, participate in complement fixation, and enhance
    phagocytosis.
· IgM antibodies consist of five monomers held by a joining chain; they are involved in
    agglutination and complement fixation.
· Serum IgA antibodies are monomers; secretory IgA antibodies are dimers that protect mucosal
    surfaces from invasion by pathogens.
· IgD antibodies are antigen receptors on B cells.
· IgE antibodies bind to mast cells and basophils and are involved in allergic reactions.
 

VI. B Cells and Humoral Immunity
· Humoral immunity involves antibodies that are produced by B cells.
· Bone marrow stem cells give rise to B cells.
· Mature B cells migrate to lymphoid organs.
· A mature B cell recognizes an antigen with antigen receptors.

Apoptosis
· Lymphocytes that are not needed undergo apoptosis, or programmed cell death, and are
    destroyed by phagocytes.

VII. Activation of Antibody-Producing Cells by Clonal Selection
· According to the clonal selection theory, a pre-existing B cell becomes activated when an
    antigen reacts with antigen receptors on its surface.
· Recombination events in the gene coding for the variable region result in the ability to produce
    huge numbers of different antibody molecules.
· The activated B cell produces a clone of plasma cells and memory cells.
· Plasma cells secrete antibodies. Memory cells recognize pathogens from previous encounters.
· T cells and B cells that react with self antigens are destroyed during fetal development; this is
    called clonal deletion.

VIII. Antigen- Antibody Binding and Its Results
· An antigen binds to the antigen-binding site (variable region) of an antibody to form an antigen-
    antibody complex.
· IgG antibodies inactivate viruses and neutralize bacterial toxins.
· Agglutination of cellular antigens occurs when an IgG or IgM antibody combines with two
    cells.
· Antigen- antibody complexes involving IgG and IgM antibodies can fix complement, resulting
    in the lysis of a bacterial (antigenic) cell.

IX. Immunological Memory
· The amount of antibody in serum is called the antibody titer.
· The response of the body to the first contact with an antigen is called the primary response. It
    is characterized by the appearance of IgM followed by IgG.
· Subsequent contact with the same antigen results in a very high antibody titer and is called the
    secondary, anamnestic, or memory response. The antibodies are primarily IgG.

X. T Cells and Cell-Mediated Immunity
· Cell-mediated immunity involves specialized lymphocytes, primarily T cells, that respond to
    intracellular antigens.
· Chemical Messengers of Immune Cells: Cytokines
· Cells of the immune system communicate with each other by means of chemicals called
    cytokines.

Interleukins (IL) are cytokines that serve as communicators between leukocytes.
Chemokines cause leukocytes to move to the site of infection.
Cytokines may be useful in treating tumors.

Cellular Components of Immunity
· T cells are responsible for cell-mediated immunity.
· After differentiation in the thymus gland, T cells migrate to lymphoid tissue.
· T cells differentiate into effector T cells when they are stimulated by an antigen.
· Some effector T cells become memory cells.

Types of T Cells
· T cells are classified according to their functions and cell-surface receptors called CDs.
· The antigen must be processed by an antigen-presenting cell (APC) and positioned on the
    surface of the APC.
· The major histocompatibility complex (MHC) consists of cell-surface proteins that are unique
    to each individual and provide self molecules.
· A T cell recognizes antigens in association with MHC on an APC, causing the APC to release
    IL-1.
· After binding to an APC, helper T (TH) or CD4 cells secrete IL-2 to activate other TH cells
    specific for that antigen.
· Cytotoxic T (TC) or CD8 cells release perforin to lyse cells carrying the target antigen and
    MHC.
· Delayed hypersensitivity T (TD) cells are associated with certain types of allergic reactions
    and transplant rejection.
· Suppressor T (TS) cells appear to regulate the immune response.

Nonspecific Cellular Components
· Macrophages that are stimulated by ingesting an antigen or by cytokines become activated to
    have enhanced phagocytic ability.
· Natural killer (NK) cells lyse virus-infected and tumor cells. They are not T cells and are not
    antigenically specific.
 

The Interrelationship of Cell-Mediatedand Humoral Immunity
· TH cells activate B cells to produce antibodies against T-dependent antigens.
· Antigens that directly activate B cells are called T-independent antigens.
· In antibody-dependent cell-mediated cytoxicity (ADCC), NK cells, macrophages, and other
    leukocytes lyse antibody-coated cells.
· ADCC