The lymphatic system and immunity

A circulatory system for fluids

returns fluid to the blood

removes antigens from the body

exposes antigens to the immune system

Formation and composition of interstitial fluid

Blood that enters the arterial end of a capillary is under high pressure. The pressure is sufficient to cause fluid to leak continuously from the blood into the spaces between the cells. This fluid is known as interstitial fluid. It fills the spaces between the cells and constantly bathes the cells.

Interstitial fluid is important to cells because it is through this fluid that the exchange of materials between blood capillaries and cells occurs. Nutrients and oxygen diffuse from the blood through the interstitial fluid into body cells. Waste products and carbon dioxide diffuse from body cells through the interstitial fluid into the blood.

Interstitial fluid Contain = water + dissolve nutrients + hormones + waste product + gases + small protein. * leucocytes

Does not contain = plasma protein albumin + globulin + fibrinogen + erythrocytes + platelets.

85% of fluid leaves the blood at arterial end re-enter at venous end = maintain normal blood volume.

15% ? Equivalent to about 4 litre of fluid return to blood through lymphatic system

How is fluid moved?

Contraction of skeletal muscles against lymphatic vesselsSmooth muscle contractionValves in lymphatic vesselsBreathing

Obstruction of system leads to oedema

Lymph nodes

Grouped together at various parts of the body

Filtration

Immune surveillanceimmune cells are concentrated there(as is antigen)

Lymphocytes develop in lymph nodes (afterthey are formed in the bone marrow)

T cells develop in the thymus and then enterthe circulation

Macrophages and dendritic cells presentantigen in the lymph nodes

What are the major organs/tissues of the lymphatic system?How do the cells get there?

Thymus

T cell development: cells migrate from bonemarrow and differentiate into T cells

T helper cells

Cytotoxic T cells

Thymus gets progressively smaller (and lessactive) through life

Spleen

Filters blood, while lymph nodes filter lymph

White pulp- concentration of lymphocytes (around arteries)

Red pulp- red cells are filtered too

Macrophages are plentiful throughout

B and T lymphocytes confer specific immunity

Body also has non-specific responses toinfection

Barriers- skin, mucosa, chemical barriers

Inflammationredness, swelling, heat, pain

Phagocytes

Fever

Cells of inflammation

Neutrophils- leave blood and enter site ofinjury- kill and phagocytose microbes

Macrophages- also phagocytes

Mast cells- release inflammatory substances

Complement proteins- contribute toinflammation

Lymphocytes may be activated, too

What about specific immunity?

Arises when barriers (first line of defense)and inflammation (second line) do notcontrol the infection

Is directed against specific antigens

What is an antigen?

What are the cells of specific immunity?

B lymphocytes (produce antibodies)

T lymphocytes (helper, cytotoxic)

Helper T cells regulate the immune response

Cytotoxic T cells kill altered cellsinfected with virusestumor cells

What do these cells do, when exposed toantigen?

Proliferate (divide rapidly)

Produce effector molecules

B cells- antibodieshelper T cells- cytokinescytotoxic T cells- cytotoxic granules

Macrophages, dendritic cells- present antigento T cells

What do antibodies do? (five classes)

Ig (immunoglobulin) G- active in blood againstbacteria and viruseshelps activate complementhelps phagocytes eliminate antigens

most common antibody in the blood

IgM- reacts with certain antigens, usually onfirst exposure

IgA- most common in mucosa

IgD and IgE are rare in blood

IgE is involved in allergic reactionssticks to mast cells, which releaseinflammatory substances

IgD is usually found on B cells (not released)may be involved in B cell activation

When the body is exposed to an antigen for thefirst time, antibody production is slow andat low levels. Usually IgM

If exposed to the same antigen again, the antibody response is much more rapidand intense (IgG)

(Most antibody in the blood stream is IgG)

VaccinationExposure to antigen will produce an immune responseRepeated exposure will produce memoryVaccine-produce the memory response without getting the diseaseWhy are vaccine produced to protect against some diseases but not others?

Immune system protects against infection,but also against other antigens

Blood group antigens

Tissue antigens (i.e., graft rejection)

For successful organ graft, immune systemmust be suppressed

Transplanted tissue must be cleared of immunecells, too

What if there is an immune response againstthe wrong antigens?

Allergies- antigen that is otherwise harmless(hypersensitivity)

Immediate type is mediated by IgE

Delayed-type is caused by T cells

Autoimmunity

Normally immune system does NOT react toself antigens

Autoimmunity occurs when it doesdisease can be localized (to kidneys,joints, thyroid, etc.) or can be systemic(lupus)

Treatment usually requires some form ofimmunosuppression

Immune deficiency

Primary- lack of development of all or part ofthe immune systemSCID- severe combined immune deficiency

DiGeorge syndrome- lack of a thymus, etc.

Secondary- due to diseaseAIDScan also be temporary

Summary

The lymphatic system helps maintain homeostasisof fluids, and also helps remove antigen fromthe body

The immune system consists of barriers (physicaland chemical) and specific and nonspecificmechanisms to eliminate antigen

Immune cells are blood cells. Some circulate inthe blood and can then migrate into tissuesat site of injury. These include neutrophilsand macrophages.

All blood cells arise in the bone marrow. B lymphocytes initially develop in the bonemarrow and then migrate to lymphoidtissues (esp. lymph nodes and spleen)T lymphocytes develop in the thymus.

B cells produce antibodies, which interact withantigen to help eliminate it.

Helper T cells regulate the immune response;cytotoxic T cells kill virus-infected cellsand probably tumor cells. (They also areresponsible for transplant rejection.)

B and T cell response is antigen-specific andhas memory (second response is fasterand stronger than the first)

Immune system can be overly responsive toantigens (hypersensitivity/allergy) or canmistakenly be directed against self antigen(autoimmunity)

Immune deficiencies leave people vulnerableto infection

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