Micro Lecture Notes: Chap 18

Immunity :: the ability to resist organisms/toxins that tend to damage to cells and tissues.

Immune System :: Overview

Properties :

Protects the body from pathogens

Removes dead or damaged tissue and cells

Recognizes and removes abnormal cells

Anatomy:

Primary and secondary lymphoid tissues

Primary lymphoid tissue = bone marrow and thymus

Secondary lymphoid tissue =

Encapsulated: LN, spleen

Nonencapsulated: Tonsils, Galt, Malt

Immune cells : leukocytes

Immune Responses:

1) Detection and Identification

2) Communication (Antibodies and Hormones called Cytokines)

3) Recruitment and Coordination

4) Destruction and suppression

5) Tolerance

Note: Substances that trigger an immune response are called Antigens

Divisions:

General defense process --> Nonspecific defenses, inherited, innate, native protection

Specific defense process --> must be exposed to antigen before it can respond/protect

with specific defenses, acquire specificity

Two parts: Cell mediated and Humoral (AB) mediated

Immune System:: Detail

I. Nonspecific defenses:: first and second line of Body defenses

Does NOT require previous exposure to an antigen

Ranges from mechanical barriers to cells (WBC, compliment)
Species Immunity from innate or genetic
Behavioral immunity

A. Mechanical barriers --> first line of defense

Cutaneous membrane = Skin : keratinized epithelium + dermis

Mucus membranes that line all body cavities that opens to the exterior

Secretions: enzymes/chemicals ::

Common Portals of Entry for Microorgansims: Skin, GI, Respiratory and they enter by physical contact, ingestion, and breathing. All three portals are lined with a continuous epithelial membrane that physically interfers with the entry portal. Epithelial cells also produce peptide antibiotics that kill bacteria. Intra epithelial T cells (those expressing gamma - delta receptors) recognize microbial lipids and other similar structures and help serve as sentinels against agents who attempt to breach the barriers.

SPECIFIC IMMUNITY :: 3rd line of body defenses

(1) Destroys specific foreign antigens

(2) has systemic (body wide) effects

(3) creates memory (daughter) cells

(4) tolerant of normal antigen

Review of Lympocyte Origins:

Recall that lymphocytes develop from a red bone marrow stem cell called the hematocytoblast or pluripotent hematopoietic stem cell (PHSC) via the lymphoid cell line to become lymphoblasts -> prolymphocytes-> lymphocytes. These lymphocytes then have one of three pathways to become EITHER a Natural Killer cell (NK), or a Thymic Lymphocyte (T cell) or a Bursal lymphocyte (B cell).

A. Innate Immunity :: genetically determined, no prior exposure

Involves the NK cells ~10% of circulating large granular lymphocytes and part of native immunity. Natural killer cells are preprogrammed with the genetic, innate information about body defenses. They do not undergo specialization because they are already programmed to recognize the correct numbers of the self antigen found on all normal cells. If a cell has an abnormal number (high or low) this triggers a response. Those cells NOT expressing normal numbers of self antigen (such as in viral / cancerous infected cells) will be killed via a perforin protein that disrupts the plasma membrane to cause cell lysis.

B. Acquired Immunity ::

Antigen specific immunity: Antigens may be proteins, lipids, or carbohydrates

May require 1-2 weeks to be fully active after the FIRST exposure to an antigen

1. Humoral-Antibody Mediated Immunity

2. Cell-Mediated Immunity

Specific Immune system cells the Lymphocytes and how MO tie nonspecific with specific

10% B- lymphocytes (B-cells) are for humoral immunity, secrete antibody

80% T- lymphocytes for cell-mediated immunity. Various subpopulations

T-Helper Cells (Th) 55% secrete cytokines

T-Cytoxic Cells (Tc) 25% kill cells expressing foreign antigen

Macrophages for help in phagocytosis and PRESENTING antigen to Th cells.
Dendritic cells also phagocytose and present antigen to Th cells

Macrophages can also present antigen to B cells

Bacterial Antigens

If microbial antigen is captured in the epithelial or connective tissue it is taken by the macrophage to the LN to present it to the T and B cells that reside in the cortex.
If a microbial antigens that enter into the blood stream are captured by the APCs in the spleen and then displayed to the T and B cells that reside in the white pulp.

Specific lymphocytes will have their own unique receptor (TCR or BCR), a group of Cluster determinate proteins (CD) and the normal self antigen protein MHC-HLA.

1. T- Lymphocytes ....start as immature lymphocytes from the bone marrow and then migrate to the thymus. Will divide and mature under hormonal influence (Thymosins) and have self-tolerance and I.D. foreign antigen.

When mature, they will have T-cell receptors (TCR that consists of two protein chains either alpha/beta or gamma delta) on their membrane

T cells have other protein markers (i.e. CD 4 or CD8) to help with binding to MHC regions to provide a second costimulatory signal and to provide stabilization.

********T cells ONLY recognize processed (digested) antigen presented (shown) to them on the MHC-HLA antigen regions. Usually antigen presenting cells such as dendritic cells, macrophages, and even B cells are involved with the processing of antigen for the T-cells.

2. B-Lymphocytes...arise from fetal liver and bone marrow. Will mature in bone marrow and have a B-cell receptor (IgD) on its membrane surface to bindto specific antigen in its native (non-digested/processed) form. Recognition of foreign antigen is done by forming noncovalent bonds (hydrogen bonds) of high affinity to portions of the molecule. This bond is a close one and the distribution of electrostatic charges (+/-) will also help in bond formation.

The lymphocyte receptors responsible for bond formation are coded by the DNA and there are many diverse receptor genes due to various genetic rearrangement, nucleotide changes, errors in replication or repair, and a combinational diversity as the receptor chains are assembled and folded. All receptors have various segments : V, D, J, C. and each of these has various versions.

Specific Lymphocytes are selected by the hormones in the primary lymphoid tissues:

the Thymus or Bone Marrow

1. Hormones promote the expression of functional receptors: IgD or IgM [monomer] for B-cells, TCR for T cells

2. Lymphocytes reactive against self (fetal) antigens are eliminated

Remember, in the fetus all antigens are considered self during this immune selection and discrimination process and results in TOLERANCE.

3. CD proteins are then expressed (examples are CD4, CD8, CD20, etc)

4. Lymphocytes are then allowed to mature and tested to see if they can recognize foreign antigen (by binding to it with their receptors and CD proteins)

After preprocessing for immunocompetence, the lymphocytes will go to secondary lymphoid organs (lymph node, spleen, tonsils, etc.). 75 % of the T cells will be in the blood circulation and are normally the lymphocytes seen on a stained blood smear.

Resting (naïve) specific lymphocytes must receive 2 signals:

a) binding of the receptor (TCR or BCR + CD protein) to an antigen = sensitized

b) Costimulatory signal (cytokines, B-7 protein) received via other CD proteins = activated

Summary of Stimulatory Signals for Lymphocytes ::

Primary signal is antigen binding by B and T cell receptors and specific CD proteins

Secondary signal is a costimulatory signal through the cell membrane.

a. Interleukins

b. other CD proteins involved with binding to MHC-HLA proteins

Because macrophages are an important connection between the specific and nonspecific immune divisions and are very important in helping to sensitize and activate T lymphocytes, it is important to know where they are found. Macrophages will be distributed to lymphoid organs and other areas such as::

Connective tissue................histiocytes

Liver...................................Kupffer cells

Brain...................................microglia

Bone...................................osteoclasts

Macrophage main function is to engulf foreign particles and clean up debris/ dead /damaged cells.

This helps control inflammation and promote healing of tissues.

They also PRESENT ANTIGEN, & secrete proteins (Interleukins) that help activate T cells, which in turn help activate the B-cells.

SO WHY do we have a specific immune system ???? :: For antigen recognition !!!!

1. Precise to allow rapid / potent killing mechanisms

2. Avoid normal cells and tissues --> TOLERANCE

3. Specificity allows for precise binding of receptors to antigen to RECOGNIZE

4. memory cells --> REMEMBER

ANTIGEN ::

(Ag) are large, rigid, and complex substances that can be easily degraded. Most are proteins which provide the most reactivity. Lipids and carbohydrates can also be antigens.

Degredation then makes this substance capable of promoting an immune response.

There are two types of antigen :: complete antigens or incomplete antigens.

1) Complete antigens will stimulate antibody production & react with antibodies and therefore considered to be antigenic. These are large substances with at least two sites for antibodies to bind. They can also be degraded by APCs and presented to T lymphocytes.

2) Incomplete antigens called HAPTANS are low molecular weight and only have one site for antibodies to bind. Therefore, they are NOT antigenic

(capable of stimulating antibody production).

Examples of haptans are ::

drugs, and allergy related substances --> pollen, dust, mold, dander etc

If bound to body's protein carriers, will cause an antibody production and response because the binding sites have increased and it will be recognized as a foreign substance

To summarize :: certain portions on the antigen will become the antigenic determinants-->

the sites where antibodies will bind. Can have several sites with several different structures and therefore provoke a response from many different lymphocytes.

The number of the antigenic determinants on an antigen is called the VALENCE.

The actual binding site of the antigen that is recognized by the lymphocytes is called the EPITOPE. Remember that strength of binding is determined by electrostatic charges and hydrogen bonds.

IMMUNOLOGICAL MEMORY -->

a. Primary Immune Response ::

cells proliferate and differentiate.

Takes 5-7 days for onset, lasts about 10 days.

Weak potency, short duration.

b. Second Immune Response ::

faster 3-5 days

more prolonged

more potent.

System already primed with sensitized memory cells.

Occurs for HUMORAL and CELL-MEDIATED Immunity.

***************** Humoral response ***********************************

Antibodies (Ab) produced by B-lymphocytes response to antigen. The antibodies then help promote a fluid (humoral) defense by:

1. Marking the antigen for destruction

2. Inactivating the pathogen by blocking toxin secretion

3. Attracting other components of the immune system to help in elimination.

Process :: B cells are sensitized when antigens bind to specific antibody (Ig D oe IgM) receptors. Bcells are able to recognize the shapes or confirmation of native macromolecules, including proteins, lipids, carbohydrates, and nucleic acids, as well as simple small chemical groups and parts of macromolecules.

Most B cells (T cell dependent B cells) need helper T cells to activate them via cytokines to cause a stimulation of B cell growth and division into clone cells (B cells with the same antigenic receptor complexes). Clone cells can become plasma cells that secrete the specific Ab. These plasma cells live about 5 days.

The Ab circulates in various body fluids and will bind with its specific antigen to MARK THE CELL FOR DESTRUCTION.

Clone cells that do not become plasma cells become MEMORY B CELLS.

If exposed again to that specific antigen, will have a quicker response since they already were sensitized.

***************ANTIBODIES***(Ab)*******************************************

Immunoglobulins --> abbreviated Ig........are gamma globulins of the blood protein

(about 20% of plasma proteins)

These are the soluble proteins secreted by activated B cells and plasma cells in response to antigen. The secreted antibody will bind to that antigen marking it for destruction.

Immunoglobulins are grouped into 5 classes that have slightly different structure

Basic Structure::

4 polypeptide chains linked by disulfide bonds (S-S)

Two chains are heavy chains (400 amino acids) and are identical

Two chains are light chains (2 as long as the heavy)and are identical to one another.

The heavy chains have a flexible hinge in the middle so that their shape takes on a

"Y" appearance. This Y shaped molecule is called an antibody monomer.

On the arms of the monomer, there are two regions.

One constant region and one variable region for both the heavy and light chains

In antibodies of a given class, (its istoype) the constant region has the same amino acid sequence.

This determines characteristics related to membrane solubility, & adherence to antibody & compliment complex

Variable regions:: the sequence of amino acids is variable in this section of the protein. The variable light chain and the variable heavy chain form antigen binding sites for each arm. These two sites fit specific antigen binding sites and will be different for each antibody. The variable region determines the Ab's antigen specificity.

Antibody Classes: five classes based on constant regions in their heavy chains ::

IgDelta, IgMu, IgGamma, IgAlpha, IgEpsilon.

Monomers --> Ig D, Ig G, Ig E

Dinomers --> IgA called secretory IgA (can also be a monomer if in serum, called serum IgA)

Pentomer --> IgM

Each class plays a different role in the immune response.

IgE :: 0.002%, involved with allergies/parasites. Secreted by plasma cells in the skin, gi, tonsils, respiratory. IgE will bind to mast cells and basophils to cause degranulation an release of histamine. (a very potent vasodilator).

IgG :: most abundant (75% - 80%), crosses placental barrier, and fixes compliment. Four subclasses, IgG-1, IgG-2, IgG-3, IgG-4

IgA :: 10%-15%, found in mucus and glandular secretions such as tears, saliva, milk. Prevents pathogens from entering. Two subclasses, IgA-1, IgA-2

IgM :: 5% -10%, first antibody released by plasma cells. Can be free in plasma as a pentamer or membrane bound as a monomer to form a Bcell receptor. Fixes compliment.

IgD :: .0.2%, attached to Bcell to function as a receptor.

Gene segments code for Heavy and Light chains. Located on the same gene, but separate so that the two chains are separate. The mixing of these segments creates the diversity and there can be mutations in the gene to cause a hyper-variable region.

ANTIBODY FUNCTION :: reversibley bind to the epitope of the antigen by a noncovalent reaction (hydrogen bonds and charge interactions) to inactivate antigen and target pathogen for destruction.

Ab+Ag ==> forms Ag|Ab immune complexes to cause::

1) neutralization / inhibition......blocks toxic sites on virus/bacteria/toxin can't bind to tissues to cause injury.

2) Agglutination ..... cross-linked clumping of cell bound antigen of cells. (seen with RBC)

3) Precipitation....cross-linked Ag|Ab are formed into large complexes and settle out of solution. Immune complexes result when antigen is soluble and therefore can be brought out of solution. Antigen is dissolved in solution.

4) FIXES AND ACTIVATES COMPLIMENT.....the chief way used.

Antigen binding to antibody changes shape and exposes compliment C1 fixation to antigenic cell surface to cause cell lysis.

Promotes phagocytosis by opsonization and causes inflammation .

Monoclonal AB :: Ab prepared for research, treatment, tests.

Descendants of a single cell that is pure and specific.

Done by exposing lymphocytes to tumor cells to create hybridoma cells.

Cells that produce the desirable Ab are then cultured.

Recognition and Activation of T cells occurs by binding of antigen to lymphocyte surface of

T cell receptors :: TCR (looks like lower 2 of AB) and are composed with an alpha and beta chain that has a variable and constant region. The TCR is also associated with the Cluster determinate (CD-3). TCR with their CD and other signaling proteins participate in specific recognition of MHC molecules with bound peptides. Some Tcell's TCR have a gamma and delta chains that have a different specificity for lips and other nonpeptide antigens. These gamma-delta T-cells are present in high numbers in epithelia.

To recognize self, there are membrane glycoproteins on most all cell surfaces that are coded for by genes in a region of the DNA called the MAJOR HISTOCOMPATABILITY COMPLEX (MHC).

Two types of glycoprotein classes are Class I MHC and Class II MHC marker proteins

1. Class I MHC are glycoproteins found on the surface of all nucleated cells and are called HLA (Human Leukocyte Antigen). They are composed of several protein chains: alpha -1, alpha -2, alpha -3, and beta -2. Between the alpha 1 and alpha 2 there is a peptide binding cleft or groove used to accommodate 8-11 amino acids. The alpha-3 site is used as the binding site for the CD8 co-receptor. There are three polymorphic class I genes and each person inherits one set of these from each parent.
The polymorphic class I proteins are HLA-A, HLA-B, HLA-C.

2. Class II MHC are glycoproteins found on the surface of dendritic cells, macrophages, T and B lymphocytes. Therefore, they can recognize one another. MHC class II HLA proteins are comprised of an alpha -1, alpha -2, beta -1 and beta -2. The peptide binding cleft is formed between the alpha 1 and beta -1 protein chains and can accommodate 10-30 amino acid residues. The beta -2 portion of this molecule provides the binding site for the CD 4 co-receptor. These marker proteins are also called HLA (Human Leukocyte Antigen) and come from three sets of polymorphic genes also inherited from each parent. They are called: HLA-DR, HLA-DQ, and HLA-DP.

The MHC regions of the DNA code for these proteins that are specific for each species: