unit 4, lesson 1, part 4. Immune System Organs and Functions

• The immune system consists of organs, cells, and proteins.

Lymphoid Organs

• Composed of tissues and cells throughout the body.

Primary Lymphoid Tissue

• Bone marrow: Pluripotent hematopoietic stem cells are a source of immune cells.
• Thymus: T cells produced in the bone marrow must travel to the thymus to complete their maturation.

Secondary Lymphoid Tissue

• Lymph node - MALT
• Spleen

Bone Marrow

• Sponge-like tissues that are found inside the bones.
• Immune system cells are produced in the bone marrow and multiply.
• These cells move to other organs and tissues throughout the blood.
• At birth, many bones contain red bone marrow, which actively creates immune system cells.
• In adulthood, some bones still contain red bone marrow. e.g. ribs, breastbone, pelvis
• Over time, more and more red bone marrow turns into fatty tissue.

Thymus

• Located behind the breastbone above the heart.
• Gland-like organ
• Reaches full maturity only in children, and is then slowly transformed to be primarily fatty tissue.
• Special types of immune system cells called T cell lymphocytes (T cells) mature in the thymus.
• Thymus encapsulated lymphoid organ where T cell maturation and differentiation occurs.
• Its cortex is rich with immature T cells, while its medulla contains sparse mature T cells.

Immune Tissues

• Lymph nodes
• Spleen
• Mucosa-associated lymphoid tissue (MALT)
• Sites important for adaptive immune response and contain lymphocytes
• Lymphatic system – a system of vessels draining fluids from body tissues
• Lymph nodes monitor lymph for signs of infection
• Mucosa-associated lymphoid tissues are important for mucosal immune response in the gut and airways.
• The spleen serves as a 'lymph node' for the blood.

Lymph Node Immune Function

• Found along lymphatic vessels.
• They act as filters.
• Immune cells trap pathogens and activate the immune system.
• Serves as a storage and activation centre for B and T lymphoid cells, and as a centre for antibody production.
• The lymph node is covered by a capsule of dense connective tissue, with capsular extensions called trabeculae which support blood vessels entering into the nodes entering into the nodes.
• Lymph circulates throughout the body through lymphatic vessels.
• Vessels drain into and from lymph nodes, Afferent vessels drain into nodes.

Components of Lymph Node

• Follicle: Site of B cell storage and proliferation
• Paracortex: Location of T cells
• Medulla: Composed of medullary cords and sinuses acting as a filter.

Spleen

• Stores various immune system cells. _ Scavenger cells (phagocytes) act as a filter for pathogens.
• Breaks down red blood cells (erythrocytes)
• Stores and breaks down platelets (thrombocytes)

Anatomic structures

• Sinusoids are composed of elongated channels with fenestrated membrane and adjacent macrophages to filter..
• Periarterial lymphatic sheath (PALS): red pulp houses T cells.
• There is always a lot of blood flowing through the spleen tissue.
• A severe injury may cause the spleen to rupture easily.
• Surgery is usually necessary because otherwise there is a danger of bleeding to death, if the spleen needs to be removed completely (splenectomy), other immune system organs can carry out its roles.
• Infections, mostly by encapsulated organisms, are the most well-known complications following splenectomy.
• pneumococcal infections
• Vaccines are recommended for individuals with asplenia, splenic dysfunction, or complement disorders.
  • Pneumococcal polysaccharide conjugate vaccine
  • Meningococcal groups A with C and B vaccine
  • Influenza vaccine

Tonsils

• Part of the immune system
• Located at the throat and palate
• Stop pathogens from entering the body through the mouth or the nose are stopped here.
• Contain a lot of white blood cells
• There are different types: palatine tonsils, adenoids and the lingual tonsil.
• Lymphatic tissue on the side of the throat can perform the functions of the palatine tonsils if they are removed.

Mucus Membranes

• Gut plays a central role in defending the body against pathogens
• More than half of the body's cells that lead to production of antibodies (B cell derived plasma cells) are found in the bowel wall, in the last part of the small intestine and in the appendix.
• Large bowel also contains harmless bacteria called gastrointestinal or gut flora or gut microbiome, makes it difficult for pathogenic microorganisms to spread.
• Mucous membranes support the immune system in other parts of the body, such as the respiratory and urinary tracts, and the lining of the vagina.
• The immune system cells are directly beneath the mucous membranes, and prevent bacteria and viruses from attaching.

Problem Based Learning

• Sam (33) is due to have an operation to remove his spleen, known as a splenectomy, as it's not working properly
• Splenectomy weakens immune system and more likely to get an infection

Vaccinations

• Living without a spleen
• Antibiotics
• Signs of infection
• The spleen fights invading pathogens in the blood containing white blood cells, B cells and T cells.
• It controls blood cells (white and red blood cells and platelets)
• It filters blood and removes old/damaged red blood cells.
• If spleen needs to be removed, other organs, like the liver can take over, and you'll still cope with most infections.
• Small risk a serious infection my develop quickly for the rest of your life.
• Young children have higher risk.
• Risk is increased if have a health condition (sickle cell anaemia, coeliac disease, or a health condition that affects your immune system, like HIV).

Check Vaccinations

• Needed with your GP
• Pneumococcal infections, such as pneumonia, with regular boosters at least every 5 years _ Flu (get the flu jab every autumn)
• Haemophilus influenzae type b and meningitis C (Hib/MenC)
• MenACWY

Antibiotics

• May need to take low-dose for some infections and is particularly important for
  • children under the age of 16
  • the first 2 years after your spleen is removed
  • if your immune system does not work properly

Need to see a GP as soon as possible if you get signs of an infection

• Signs include
  • a high temperature
  • a sore throat
  • a cough
  • a severe headache
  • a headache with drowsiness or a rash
  • abdominal pain
  • redness and swelling around the surgical wound