LYMPHATIC SYSTEM

Lymphatic system: made up of lymphatic vessels, lymph nodes, and accessory structures such as thymus, spleen, Peyer's patches and tonsils.

Important to the function of circulatory, digestive, and immune systems.

3 Main Functions:

1) Transport of fluid.
2) Fat transport.
3) Immune.

Lymph: composite fluid consisting of interstitial fluid, fats, and lymphocytes (relates to 3 functions above).

One-way network - lymph fluid flows toward heart. Vessels are small, thin-walled tubes which have one-way valves. They empty into either the thoracic duct or right lymphatic duct, which drain into the left and right subclavian veins.

There is always leakage of plasma fluid into interstitial space due to pressure imbalance, so fluid must be returned to circulatory system

Lymph nodes: collection points for lymph in regions of the body. Reticular tissue containing phagocytes which help to purify lymph - attack bacteria, cancer cells, etc. Vary in size depending on location in body.

Clinical Importance:

Spleen

Left side near stomach. Largest lymphoid organ. Numerous sinuses.
Removes aged or defective blood cells and returns components to circulatory system.
General blood filter.
Site of lymphocyte proliferation.
If it’s so important, why can we live without it?

Thymus
In thorax. Larger earlier in life.
Secretes hormones (thymopoietins and thymosin) important for T-cell maturation.

Tonsils
Palatine, lingual, and pharyngeal.
Remove some pathogens - combat ear, nose and throat infections.

Peyer's Patches
In wall of ileum of small intestine. Increased number closer to large intestine.
Phagocytes here can combat bacteria.

 

IMMUNE SYSTEM

Immunity: the ability to resist disease or disease causing agents

Non-Specific Defenses

Non-specific Cellular and Chemical Defenses

Phagocytosis by macrophages (connective tissue macrophages, monocytes, or fixed-phagocytes) or neutrophils. Neutrophils are in blood and can leave blood to fight invader. Many macrophages are permanent residents of different tissues - "fixed-phagocytes."

Inflammation:

Stages & Signs of Inflammation

Complement

Fever

Specific Defenses: Humoral and Cellular Immunity.

Either a cell or an antibody recognizes invading pathogen specifically.

Important Characteristics

Antigen: any chemical sequence that can be recognized by antibody or antibody-like protein. Generally part of a protein (6 amino acid sequence) or a carbohydrate.

Exception: some people are allergic to nickel. Immunity is to nickel-protein complex.

Active Humoral Immunity: B-cells secrete antibodies (complex, specialized proteins shaped like a Y) in response to particular antigen. Response is amplified because B-cell is stimulated to divide, forming a "clone" -- each cell now produces same type of antibody.

First exposure is "primary response."
Antibodies are receptors for antigens that circulate in blood or are secreted.

When an antibody attaches to an antigen it can:

When antigen no-longer present, most of the specific B-cells die off, but many are left behind as "memory cells." When antigen is present again, these cells can quickly divide and produce antibodies again. This is the "secondary response." Therefore, the immune response is much swifter the second time one is exposed to an antigen.

Immunizations (form of active immunity) takes advantage of this feature. Body is challenged with foreign antigen. Immune response generated and memory cells produced.

Passive Immunity

Direct transfer of antibodies. Maternal-fetal transfer. Injections of immunoglobins to combat snake bites, botulism, rabies, and tetanus.

Immunoglobin classes
IgG: most abundant. Circulates widely. Secreted in primary and secondary responses.
IgM: huge. Produced in primary response.
IgA: dimer form in mucous membranes.
IgE: attached to mast and basophils cells. When it binds to antigen, histamine released. Inflammatory and allergic responses (asthma).
IgD: important in activation of B cell.

Cell Mediated Immunity - T-cells

T cells are sensitized by binding to an antigen coupled with a major histocompatability complex (MHC) protein which is displayed by macrophage.

A clone is produced with is specific to the antigen. Different T cells produced:
Cytotoxic "killer" T cells - attack antigen directly. Produce perforins. Attack abnormal cells -- cancer, mutants, and tissue grafts from incompatible persons or animals. ***

Helper T cells: facilitate activation of B cells, enabling them to produce antibodies.
Suppressor T cells which inhibit B cells. Somewhat controversial.

Self vs Non-Self - Tissue-typing and Graft rejection
"Self" is recognized by immune system as having the appropriate histocompatibility antigens on cell membrane surface. White blood cells are used to "tissue type" people, so antigens are known as "human leukocyte antigens" or HLA. A person's HLAs are coded by 4 genes (A,B,C,D), but there are many subclasses for each gene/protein. So it is unlikely that two people will have the same combination of genes. The greater the difference between antigens, the greater the chance of graft tissue rejection.

Interleukins: Enhance immune responses

Immunodeficiences

Congenital thymic aplasia: thymus doesn't develop

Severe combined immunodeficiency (SCID) - genetic mutation which results in absence of B and T lymphocytes.


Acquired immunodeficiency syndrome (AIDS) - virus slowly destroys T cells. Virus attaches to specific receptor (CD4) on T cells and enters cell. Cell mediated immunity decreases slowly. Humoral immunity compromised because of lack of Helper T's.

Hypersensitivities & Allergies