Ch22 Pt 5

B Cells and Immunity

• B Cells

  • Involved in the humoral response.

  • Produce specific antibodies.

  • There are millions of B cell types in the body, each expressing different antibody molecules.

  • B cells undergo sensitization, which precedes their activation.

B Cell Sensitization

• During sensitization:

  • Antigens are taken into the B cell.

  • Antigens are broken down.

  • Resultant fragments are presented on the cell surface, bound to class II MHC proteins.

• The sensitized B cell awaits a signal from T helper cells before activation.

B Cell Activation

• Helper T cell binds to MHC II-Ag complex.

  • Secretes cytokines that promote B cell activation.

Activated B Cells

• After activation, a B cell divides into:

  • Plasma cells: produce antibodies.

  • Memory B cells: remain inactive but can respond to future infections.

Antibodies

• Structure:

  • Soluble proteins composed of two pairs of polypeptide chains:

    • Heavy chains

    • Light chains

  • Each chain contains:

    • Constant segments: Determine properties of antibodies; include 5 types.

    • Variable segments: Form the Antigen Binding Site.

Antibody Types

• The constant segments of heavy chains determine the classes of antibodies (immunoglobulins):

  • IgG

  • IgE

  • IgD

  • IgM

  • IgA

Antibodies - IgG

• IgG

  • Largest and most diverse class of antibodies (80%).

  • Provides resistance against many pathogens.

  • Can cross the placenta, potentially causing Hemolytic Disease of Newborn (HDN).

Antibodies - IgE

• IgE

  • Binds to surfaces of basophils and mast cells.

  • Accelerates inflammation upon antigen binding; critical in allergic responses.

Antibodies - IgD

• IgD

  • Present on surfaces of B cells; can bind antigens in extracellular fluid.

  • Binding assists in B cell sensitization.

Antibodies - IgM

• IgM

  • First antibody class secreted after encountering an antigen.

  • Decreases as IgG production rises.

  • Effective in blood typing as it can polymerize.

Antibodies - IgA

• IgA

  • Found in glandular secretions; protects against pathogens before they enter tissues.

  • Circulates in pairs or individually.

Antigen-Antibody Complex

• An antibody bound to an antigen.

• Complete antigen: has at least two antigenic determinant sites; binds to variable segments of antibodies.

• Exposure to a complete antigen leads to B cell sensitization and an immune response.

Haptens

• Haptens: Partial antigens that require a carrier to induce B cell activation.

• Antibodies will attack both the hapten and carrier molecule; can cause issues if the carrier is normal tissue (e.g., penicillin allergy).

Antibody Actions

• Primary functions of antibodies:

  • Neutralization of pathogens.

  • Precipitation and agglutination of antigens.

  • Activation of complement system.

  • Attraction of phagocytes.

  • Opsonization.

  • Stimulation of inflammation.

  • Prevention of bacterial and viral adhesion.

Antigen Response

• First Exposure: Initiates a primary response.

• Subsequent Exposure: Triggers a more extensive and prolonged secondary response due to existing memory cells.

Antigen Response - Primary Response

• Takes about 1-2 weeks to develop:

  • B cells activated by antigens ➜ Plasma cells differentiate ➜ Gradual rise in antibodies, primarily IgM.

Antigen Response - Secondary Response

• Activates memory B cells at lower antigen concentrations:

  • Secretes antibodies quickly and in larger quantities (mainly IgG).

  • IgM production also accelerates.

Immunocompetence

• Ability to produce an immune response post-exposure to an antigen:

  • Various immune responses kick in such as B cells activating, plasma cells increasing antibody levels, and the engagement of phagocytes.

Body Response to Pathogen

• Response begins within peripheral tissues upon bacterial entry.

Immunocompetence Development

• A fetus can produce immune responses around 3-4 months but relies on maternal IgG antibodies, then IgA from breast milk.

• As infants encounter antigens, IgG levels steadily increase to reach adult levels.

Stress and Immune Response

• Glucocorticoids:

  • Secreted to limit immune response during chronic stress.

  • Impacts inflammation, phagocyte activity, and lymphocyte response leading to lowered immunity.

Immune Disorders

• Three major types:

  • Allergies: Excessive responses to antigens.

  • Autoimmune disorders: Misidentification of self-antigens.

  • Immunodeficiency diseases: System failures and blockages.

Allergies (Hypersensitivities)

• Excessive immune responses categorized into:

  • Immediate hypersensitivity (Type I)

  • Cytotoxic reactions (Type II)

  • Immune complex disorders (Type III)

  • Delayed hypersensitivity (Type IV)

Immediate Hypersensitivity (Type I)

• Characterized by rapid and severe responses:

  • Common allergic reactions like hay fever.

  • Initial exposure produces IgE that binds to mast cells; subsequent exposures result in massive inflammation.

Anaphylaxis

• A severe allergic reaction that can be life-threatening:

  • Involves widespread activation of mast cells, capillary permeability increases causing swelling, and respiratory obstruction occurs.

Autoimmune Disorders

• Result from improper immune responses, where antibodies target body cells:

  • Examples include Thyroiditis, Rheumatoid arthritis, and Type 1 diabetes.

Immunodeficiency Diseases

• Results from developmental issues or infections leading to conditions like SCID or AIDS:

  • Immunosuppressive treatments may also result in immune failures.

Effects of Aging on Immune Response

• As age increases, the immune system's efficiency declines, increasing susceptibility to infections and cancers:

  • Reduced thymic hormone production and T cell responsiveness, decreasing overall immunity.