Chapter 22 notes (Lymphoid and immune systems)
From Biol557
An introduction to the lymphoid system and immunity
Anatomical barriers and defense mechanisms constitute nonspecific defense, and lymphocytes provide specific defense
- There are lots of non-living things that can hurt us like UV light, bumps, temperatures, etc.
- There are lots of living things that can hurt us (pathogens) like viruses, bacteria, fungi, and parasites.
- Viruses are usually within cells and rupture them.
- Bacteria are usually in the ECF and produce toxic proteins.
- Parasites can burrow through entire internal organ systems.
- "The lymphoid system includes the cells, tissues, and organs responsible for defending the body against both environmental hazards, such as various pathogens, and internal threats, such as cancer cells."
- Lymphocytes respond to abnormal body cells, invading pathogens, and foreign proteins.
- The body has several nonspecific defenses which serve to block entry to the body or to attack anything that is foreign.
- Lymphocytes, however, are specific in their attacks.
- Specific defenses are considered an immune response.
- The immune system refers to not only the lymphocytes, but all the organs and tissues that are associated with specific immune response.
- These include the integumentary, cardiovascular, respiratory, digestive, and other systems.
Lymphatic vessels, lymphocytes, lymphoid tissues, and lymphoid organs function in body defenses
- The lymphatic system is made up of:
- lymph (which is much like ECF but with fewer proteins),
- lymphatic vessels (which run from peripheral tissues to veins),
- some lymphoid tissues and organs (found throughout the body, think tonsils, spleen, thymus, etc.),
- lymphocytes and some phagocytes and several other cell types.
Functions of the lymphoid system
- The primary function of the lymphoid system is to produce and distribute lymphocytes.
- Lymphocytes are primarily generated in lymphatic tissues (like the tonsils) and organs (like the spleen and thymus) but are also generated in red bone marrow.
- Red bone marrow also generates other defense-related cells like monocytes and macrophages.
- The lymphocytes are carried throughout the body via the blood, interstitial fluid, and the lymphatic system.
- The lymphatic system helps circulate interstitial fluid and thus allows for elimination of local abnormalities of interstitial fluid and keeps nutrients moving.
Lymphatic vessels
Lymphatics capillaries
- Lymphatic capillaries run through peripheral tissue, draining interstitial fluid (including proteins, viruses, bacteria, and cellular debris) through endothelial cells that overlap, like shingles.
- Lymphatic capillaries differ from cardiovascular capillaries in four ways:
- they originate as pockets as opposed to tubes,
- they have larger diamters,
- they have thinner walls,
- they have flattened or irregular outlines in sectional views.
- Lymphatic capillaries are found throughout the body:
- They are not found where there are no capillaries; for example, in the cornea.
- They are not found in the central nervous system.
- They are particularly important in the digestive tract for transporting lipids (via lacteals).
Small lymphatic vessels
- Lymphatic capillaries turn into lymphatic vessels as they progress toward the trunk and large veins.
- These larger vessels often appear bulbous because of valves that are placed rather close together.
- The valves are much like those in veins and are important for maintaining proper flow.
- Lymphatic vessels often occur in conjunction with veins.
- In living tissue, arteries are bright red, veins are dark red (illustrated as blue), and lymphatic vessels are pale golden.
- There are usually more lymphatic vessels than veins, but they are smaller, too.
Major lymph-collecting vessels
- Superficial and deep lymphatics collect lymph and deliver it to the even deeper lymphatic trunks.
- Deep lymphatics accompany deep arteries and veins to muscles, organs of the neck, limbs, trunk, and to the walls of visceral organs.
- Superficial lymphatics go everywhere else.
- The lymphatic trunks empty into two large ducts: the thoracic duct which collects from the left side of the body and everything below the diaphragm, and the right duct which drains from the right side of the body above the diaphragm.
- The thoracic duct has a saclike structure called the cisternae chyli at the base.
- The thoracic duct dumps into the left subclavian vein near the left internal jugular vein.
- The right lymphatic duct dumps into the right subclavian vein.
- Blockage of a lymph vessel can cause lymphedema--swelling associated with lack of interstitial fluid buildup.
- This can result in permanent swelling if connective tissue has a chance to loose it's elasticity.
- This can lead to bad things by way of toxin buildup.
Lymphocytes
- Lymphocytes account for 20-30% of all leukocytes.
- Lymphocytes in circulation are only a small fraction of the total lymphocytes in the body--massing up to a kilogram!
Types of lymphocytes
- There are lots of types of lymphocytes:
- Thymus-dependent (T) cells:
- Make up 80% of circulating lymphocytes.
- There are sub-types of T cells:
- Cytotoxic T cells: attack infected cells, usually through direct contact. These are the primary cells involve din the production of cell-mediated immunity or cellular immunity.
- Helper T cells: stimulate and activate both T and B cells.
- Suppressor T cells: inhibit activation of both T and B cells.
- Suppressor / inducor T cells: suppress B cells while activating T cells.
- Inflammatory T cells: stimulate regional inflammation and local defenses up injury.
- Suppressor and helper T cells are considered regulatory T cells.
- Bone marrow-derived (B) cells:
- B cells are the primary cells involved in humoral (liquid, antibodies occur in body fluids) immunity.
- B cells make up 10-15% of lymphocytes in the blood.
- B cells can convert into plasma cells and then produce antibodies.
- Antibodies are the same thing as immunoglobulins.
- Antibodies are generally proteins, but nucleic acids, lipids, and polysaccharides can also function to activate destruction of an antigen (target).
- Natural killer (NK) cells:
- These cells maintain immunological surveillance.
- NK cells are the same thing as large granular lymphocytes.
- These cells attack foreign cells, infected cells, and cancer cells.
- Thymus-dependent (T) cells:
Life span and circulation of lymphocytes
- The ratio of B to T cells is not ubiquitous throughout the body, it differs between the blood, thymus, spleen, etc.
- T cells move faster than B cells (think: 30 minutes in the blood, 15-20 hours in a lymph node).
- Most lymphocyte live for at least 4 years, some live for 20 years. We make more as needed to maintain a proper balance.
Lymphocyte production
- Production of lymphocytes (lymphopoiesis) occurs in the red bone marrow, the thymus, and the peripheral lymphoid tissues.
- Hemocytoblasts (lymphocyte progenitors) are generated in the red bone marrow.
- B cells and NK cells are generated in the bone marrow.
- B cells develop through intimate contact with large stroma cells which secrete interleukin 7 to cause differentiation into B cells.
- B cells move into lymph nodes, the spleen, and other lymphoid tissues.
- The NK cells move through peripheral tissues in search of abnormal cells.
- Some lymphatic stem cells migrate (undifferentiated) to the thymus.
- In the thymus, stem cells are protected by the blood-thymus barrier.
- These stem cells divide under the control of (at least) 7 thymus hormones to generate the various kinds of T cells.
- T cells reenter the bloodstream and return to the bone marrow and also travel to peripheral tissues, including lymphoid tissues and organs, such as the spleen.
- Recall that not all T and B cells enter the blood stream or otherwise move on from the marrow or thymus.
- T cells and B cells that do move on retain the ability to divide (because they leave before their differentiation is complete) and this is crucial for proper immune response.
