Patho Lecture 3 (Immune System II)

What are the 2 types of Adaptive Immunity?

> Humoral Immunity

> Cell-Mediated Immunity

Humoral Immunity

Immunity mediated by B lymphocytes, which produce antibodies or immunoglobulins (Ig) in response to antigen challenge

Antibodies

Glycoproteins; selective, highly specific; found in gamma-globulin fraction of serum (humoral=blood; reside in the blood)

What are the differences between the Five classes of Antibodies?

- Physical

- Chemical

- Antigenic

5 classes of Antibodies: IgM

- Primary immune response (7%);

- Type III hypersensitivity reaction;

- Immune complexes

- B cell receptor

5 classes of Antibodies: IgG

- Secondary Immune response

- B memory cells (70%)

5 classes of Antibodies: IgA

- External secretions, produced locally against bacteria and viruses (15%)

5 classes of Antibodies: IgE

- Type I hypersensitivity reactions, minute amounts

5 classes of Antibodies: IgD

- Umbilical cord blood

- Primitive recognition or regulation

- B cell receptor

Antibody Structure

- Y-shaped, consisting of two identical heavy chains and two identical light chains linked by disulfide bonds to form a heterotetrameric molecule

- At the top of the "Y" are two antigen-binding sites (Fab regions) formed by variable domains

- The stem of the "Y" is the constant region (Fc fragment); cellular binding site

In the Antibody structure, what does IgM act as?

Pentamer

In the Antibody structure, what does IgA act as?

Monomeric; becomes multimeric in the endothelium; (acquires a glycoprotein that protects it against digestion)

Kinetics of Antibody Production: What class of antibody is responsible for the Primary Immune Response?

IgM

Kinetics of Antibody Production: What classes of antibody are responsible for the Secondary Immune Response?

IgM and IgG

Kinetics of Antibody Production: What are the characteristics of the Secondary Immune Response?

- Shorter lag

- Higher levels of specific IgG produced

- Steady state level persists longer

- IgG predominates

- Adjuvants

Adjuvants

IgG is produced after the primary response; the secondary IgG response is sustained

What is the quantitative difference between primary and secondary immune responses?

The quantitative difference is due to an increase in the number of potentially reactive B cells

What is the Monoclonal Antibody technique developed by Miller and Kohler?

Technique that revolutionized immunology; involves the development of a single antibody-secreting immortalized cell

What are the steps of the Miller-Kohler technique?

- Inject mice with antigen (2 x)

- After 2 weeks, collect spleen cells (B cells); HGPRT+, Ig+, G-

- Fuse with mouse myeloma (HGPRT-, Ig-, G+) in PEG and HAT (hypoxanthine, aminopterin, thymidine) medium

- Collect and clone hybridoma cells (HGPRT+, Ig+, G+)

- Select for clones that produce a specific antibody

What are the 4 types of Monoclonal Antibodies?

- Monospecific: normally, antibodies are extremely polymorphic even when directed against a single antigen; normal antisera heterogeneity, therefore, no two antisera are identical

- Reproducible: identical antibody, more specific and reliable

- Unlimited quantities: permanent cell line, grows indefinitely, and produces very large amounts of antibody

- Antigen need not be pure or characterized: antibody reacts with only one determinant

What are the Potential uses for Monoclonal Antibodies?

- Reverse biological effects of certain agents (digitalis, morphine, kepone, histamine)

- Drug scavengers: low levels of drugs and chemicals (dioxin)

- Drug targeting: cancer treatment.

- Tumor targeting

- Identification and purification of proteins

What is the problem with Antisera and Monoclonal Antibodies?

They are NOT human!!

Antigen

Substance recognized as "foreign"

Hapten

Small molecule that binds to an antibody; does NOT elicit an immune response

Immunogen

Foreign substance that binds to an antigen and DOES elicit an immune response

Carrier

Large protein

What does Hapten + Carrier equal to?

Hapten + Carrier = Immunogen

Epitope

part of the antigen that is recognized by an antibody

Immunodominant site

Epitopes that are more highly charged or more accessible

What are the 2 types of Antigens?

> T-independent

> T-dependent

Antigens: T-independent antigens

- Complex carbohydrates

- Do not require processing

- Can directly interact with B cells

- No memory

Antigens: T-dependent antigens

- Require macrophages or other AP

- Require T-helper cells

- Require major histocompatibility antigens

- Mostly proteins

What are the characteristics of Antibody-Antigen Interactions?

- Binding of antigen to antibody

- Occurs in the variable region of the antibody molecule

- Instantaneous

- Exothermic

- Cytotoxicity mediated by complement(lysis) or phagocytosis by macrophages

- May form large complexes (IgM) and pathology

Cell-Mediated Immunity

Mediated by T lymphocytes, which release soluble mediators (lymphokines)

- Important in host defense against viruses, certain bacteria, fungi, transplant rejection, and tumor surveillance; Type IV (delayed type) hypersensitivity (DTH) reactions

T cells

- Cells derived from precursor cells in bone marrow;

- Mature in the thymus; become educated

What are the 4 types of T cells?

> T-helper cells: CD4+ (Th1, Th2)

> T regulatory cells: CD4+/CD25+/intracellular FoxP3+

> Cytotoxic (killer) T cells: CD8+

> Th17 cells: autoimmunity; extracellular bacteria

What are the characteristics of the types of T cells?

- Subsets are identified by CD markers (monoclonal antibodies)

- Work together to modulate the immune response

- Actions mediated by lymphokines

T cell Education

*T cell learning to distinguish between self and non-self

>Controlled by MHC (self-marker) proteins

> Selection of subpopulations of immature T cells through their interactions with MHC proteins displayed to them in thymus

> Immature T cells recognize MHC alone; as they mature, they recognize MHC+antigen

> Maturation-dependent genetic alteration in the T-cell receptor

> Uneducated T cells are destroyed in the thymus

Antigen Recognition by T cells

> Specific T cell receptor

> T cells are MHC restricted; only recognize antigen together with the major histocompatibility (MHC) protein

> T-helper cells recognize processed antigen and MHC II (self)

> Cytotoxic T cells recognize processed antigen and MHC I (non-self or altered self)

What does a successful immune response also require?

Costimulatory and/or Coinhibitory molecules: Immune Checkpoints

Immune checkpoints

*Molecules that either stimulate or inhibit T cells

- Critical for maintaining self-tolerance and modulating the duration and amplitude of immune responses in order to minimize collateral damage.

Characteristics of Immune Checkpoints

> Multiple simultaneous signals required for immune response by T cells

> Ensures specificity and tolerance

> Controls the extent of immune response

Signals of Immune Checkpoints

Signal 1: MHC antigen: TCR interaction

Signal 2: Co-stimulatory molecules; required for T cell proliferation, differentiation, activation, and survival. Activation of T cells without co-stimulation can lead to T cell anergy, T cell deletion, or the development of immune tolerance

Signal 2: T cell activation is also controlled by co-inhibitory molecules that suppress T cell responses

Major Histocompatibility Complex (MHC)

*Large cluster of genes coding for proteins essential for the regulation of immune cell function; destruction of non-self

- System for distinguishing between self and non-self

- Mechanism for destroying altered cells, virally infected cells, and tumor cells

- Highly conserved evolution

- Direct T cell responsiveness

MHC antigens

Proteins are present on the surface of every somatic cell in the body (MHC I)

Extreme Polymorphism

Millions of alleles or variants exist

Immune response genes

Control individual responsiveness to a given antigen (mouse chrom 6; human chrom 17); individuals inherit haplotypes

Class I MHC

MHC expressed on all somatic cells; classic "transplantation antigens

Class II MHC

MHC expressed on immune cells (B, T, macrophages, thymus epithelium); immune-associated (IA) antigens; important in immune regulation, cell-cell communication

Class III MHC

MHC complement

T cell Lymphokines

- Macrophage activity factor (IFN-γ)

- Macrophage inhibitory factor (MIF)

- Interferon (IFNa and IFNB)

- Lymphotoxic factor

- Interleukin-2 (IL-2, T cell growth factor)

- Interleukin-3 (IL-3, B cell growth factor)