The Immune System

Purpose of the immune system

• The immune system is a host defense system that is comprised of a network of cells, tissues, and organs that work together to protect the body against foreign pathogens.
• The immune system must detect and eliminate a wide variety of agents including viruses, bacteria, parasitic worms, fungi, and cancerous/ defective body cells that can cause disease.

Components of the immune system

• Leukocytes
• Lymph Vessels and Nodes
• Spleen
• Thymus
• Tonsils
• Peyer’s patch

LeukoCytes

• 5 main types of Leukocytes that play a major role in the Immune System
• Leukocytes
• Neutrophils
• Eosinophils
• Basophils
• Monocytes

Lymphocytes

• Small WBCs made in red bone marrow involved in fighting foreign substances/ antigens
• Specialize into T-lymphocytes and B-Lymphocytes

T- Lymphocytes

• Main purpose is to destroy infected or cancerous cells (Including: Toxins, bacteria, viruses, cancer cells, etc.)
• Killer/CytotoxicT-cells:
• - Directly destroy virus-invaded cells and mutant cell
• - Secrete potent chemicals to cause targeted cell to undergo cell apoptosis
• Helper T-cells:
• - Help determine which immune responses the body makes to particular pathogen (stimulate production of more Helper T- cells and B - Cells)
• - Support Macrophages and other cells that are fighting the pathogens (keep them alive)

B-LymphoCytes

• Main purpose is to make antibodies that attack foreign bacteria and toxins.
• - Antibodies attach to bacteria/pathogens
• - Flags pathogen to be destroyed
• - Disable pathogens ability to bind to other body cells (immobilize)

Neutrophils

• Most common type of leukocyte in the body
• First cells to reach an area where a breach in the body has been made
• Phagocytic cells that engulf and destroy unwanted materials.
• - Can also release a burst of super oxides that have the ability to kill many bacteria at the same time

Eosinophils

• Secrete toxins from granulocytes that destroy parasitic worms/ pathogens.
• Eosinophils are also greatly involved in allergic reactions.

Basophils

• Have the ability to secrete anticoagulants and antibodies that have function against hypersensitivity reactions in the bloodstream.
• Release histamine and heparin- dilates the vessels to bring more immune cells to the area of injury
• Also are involved in allergic reactions.

Monocytes

• Transform into macrophages when they exit the bloodstream.
• - large, tissue-bound phagocytic specialists
• - Ability to digest large foreign particles (ex: somatic cell, dead neutrophil..)

Lymphatic vessels and Lymph nodes

• Both made up of Lymphatic tissue
• - Composed of reticular connective tissue
• - Houses and provides lymphocytes grounds for rapid replication
• - Gives vantage points for lymphocytes and macrophages to launch counter attacks on foreign invaders.

Lymphatic vessels

• Collect interstitial fluid (lymph) that leaks into your tissues from your bloodstream
• Returns it to your circulatory system
• - Only flows toward the heart

Lymph Nodes (structure)

• Structure:
• - Small bean/oval shaped structure
• - Clustered in connective tissue along Lymph vessels
• - Separated into outer cortex and inner medulla

Outer Cortex

• Contains B cells arranged in follicles (develop germinal center when challenged with antigen)
• May also contain T-cells in deeper paracortex

Inner Medulla

• Contains large blood vessels, sinuses, and medullary cords
• - Containing antibody secreting plasma cells

Lymph Nodes (Function)

• Lymph, nutrients, and waste material flow through the Lymph Vessels and Lymph Nodes from the body tissues and the bloodstream
• Lymph Nodes filters lymph (Inner Medulla)
• Macrophages in the medullary cords of the inner medulla support in removing and destroying microorganisms
• Activate immune system
• - B and T cells recognise bacteria and pathogens which have entered your lymph via your bloodstream
• - When foreign material is detected, other dedicated immune cells are recruited to the node to deal with the infection
• - This helps in preventing the infection from spreading throughout your body.

Spleen (Structure/Location)

• Structure:
• Blood rich organ the size of a clenched fist
• Surrounded by fibrous capsule and trabeculae extend inward (contain lymphocytes and macrophages)
• white pulp and red pulp
• - White pulp- areas where lymphocytes are attached to reticular fibers
• - Red pulp is made up of blood-filled cavities (venous sinuses) and splenic cords
• Location:
• Left side of the abdominal cavity beneath the diaphragm
• Curves around stomach

Spleen (function)

• Exchange lymphocytes with the blood (remove, store, produce, and add them)
• Lymphocytes the spleen stores produce antibodies and activated T cells which are released into the blood
• Store breakdown substances (hemoglobin) from RBCs for reuse
• Blood Cleaning:
• - Store macrophages that remove debris and foreign matter
• - Extract old and defective red blood cells and Platelets
• - Store platelets

Thymus

• Overlies the heart
• Site of the maturation process for T lymphocytes
• Secretes the hormone thymosin
• - Stimulates the production of T-cells
• Degenerates after puberty; is eventually replaced by fibrous and fatty tissue

Tonsils (structure)

• Simplest lymphoid tissue
• Ring of lymphoid tissue around entrance to pharynx
• - Palatine – either side of posterior oral cavity (Become infected most often)
• - Lingual – base of tongue
• - Pharyngeal – (adenoids); posterior wall of nasopharynx
• - Tubal – around opening of auditory tubes into pharynx

Tonsils (Function)

• Immune System's first line of defense against inhaled foreign pathogens.
• Tonsils capture the antigens produced by pathogens with the M cells on their surface (antigen capture cells).
• M cells alert the underlying B and T cells in the tonsil that the a pathogen is present and stimulate antibody production.

Peyer’s Patches (structure/Location)

• Egg-shaped lymphatic tissue nodules that are similar to lymph nodes in structure, except that they are not surrounded by a connective tissue capsule
• Clusters of lymphoid follicles in the distal portions of the small intestine
• Also heavily concentrated in the appendix

Peyer' Patches (Function)

• monitoring intestinal bacteria populations
• preventing the growth of pathogenic bacteria in the intestines
• Generate memory cells (lymphocytes) for long term immunity

Innate Immunity

• Refers to the group of nonspecific defense processes that occur immediately or within a few hours of the appearance of an unknown antigen.
• Activated by the chemical signals of the antigen
• Include anatomical barriers and humoral barriers

Anatomical Barriers to infection

• Preventative measures of the human anatomy that primarily prevent growth or penetration of bacteria into the human body
• Consist of Mechanical factors, chemical factors, and biological factors

Mechanical Factors

• Consists of the skin, respiratory tract, bodily fluid, and the gastrointestinal tract.
• Skin, respiratory tract, gastrointestinal tract, and other epithelial tissue acts as a barrier to keep pathogens out of the body system
• Tears and saliva wash and flush out the eyes and mouth of infectious pathogens.
• Mucous is used to trap masses of pathogens and dispel them through the nasal or oral cavities, or swallowed and destroyed in stomach acid.

Chemical Factors

• Includes bodily fluids such as sweat, saliva, and tears as well as mucous, bile, and gastric acid.
• Sweat and bodily oils on the surface of the skin have an acidic pH of 3-5 caused by fatty acid secretion that prevents bacteria from growing on the skin
• Saliva and tears contain lysozymes that destroy pathogens that enter the eyes and mouth
• Gastric acid - destroys pathogens swallowed with food or piled into mucous.
• Bile manifests in the colon as a preventative measure to prevent the growth of harmful bacteria
• Antimicrobial Proteins called Defensins found in the lungs and gastrointestinal tract deter pathogens from these areas

Inflammation response

• Inflammation response is the process that occurs when pathogens and foreign bodies bypass the anatomical barriers of the body.
• The inflammatory response consists of the complement system and the coagulation system

The Complement System

• The complement system is the non-specific defense mechanism of the humoral barrier
• Activated by various proteins in the blood, and have various effects
• --Opsonize bacteria to make them easier to destroy through phagocytosis
• --Recruit a variety of immune cells such as phagocytic cells and macrophages
• --Clean up the remains of immune system activity and apoptotic cells
• The complement system can have negative effects on a host
• --Causes inflammation and tissue damage
• Can cause anaphylaxis which results in acute allergic response locally or throughout the body

Coagulation System

• System that restores integrity of skin and epithelial tissue upon injury.
• Activated depending on severity of injury and the release of Tissue Factor
• This system mediates the clotting of blood while serving as an assistant to immune responses
• --Proteins produced by platelets and other blood cells during coagulation serve directly as antimicrobial agents
• Also increases vascular permeability, allowing more red and white blood cells to flow to injured areas (inflammation)

Other Innate Immune Proteins

• Lactoferrin and transferrin - Proteins that impede cell growth
• Interferons - Proteins that impede the replication of viruses in body cells
• Lysozyme - Proteins that break down the cell walls of bacteria
• Interleukin-1 - Main protein that induces fever

Adaptive immunity

• The Adaptive immune system is needed when the innate immune system is unable to clear the body of pathogens.
• --When the components of the adaptive are activated they adapt to the infectious pathogens by creating/activating potent mechanisms that are able to destroy microbes.
• --The two responses that the adaptive immune system has are humoral immunity and cell mediated immunity

Humoral Immunity

• Portion of the adaptive immune system that focuses on B cells and their products.
• Humoral response
• --starts with the recognition of foreign antigens by unspecialized B cells
• --Unspecialized B cells then matures into an antibody secreting plasma cell through clonal expansion and then differentiation.
• --These specialized antibody proteins attack and bind onto pathogen’s antibodies disable them
• --Even after pathogen is cleared from the body, specialized plasma cells remain in the body in the form of memory cells
• ----This allows the body to respond much faster to the same pathogen in the event of a second invasion

B-cell differentiation

Cell Mediated Immunity

• Portion of adaptive immune system that deals with T cells
• --Helper T cells secrete chemical mediators that aid other T cells, B cells, and phagocytes respond to pathogens, essentially marking pathogens for destruction
• ----Respond to MHC 2 molecules
• --Killer (or cytotoxic) T cells directly destroy infected body cells by forcing them to undergo apoptosis
• Both T cells specialize to fight infections
• --Similar to B cell specialization
• --First, recognition of infected MHC proteins on infected body cells are identified
• --Clonal expansion and differentiation then occur to create specialized cells that have adapted to destroy specific pathogen - helpers turn into either Th1, Th2, Th17, or Tfh
• --Body cells are then marked by T helper cells or killed by T killer cells
• --After onslaught, some of the specialized T cells are kept as memory cells

Differentiated Helper T cell

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