Blood and Formed Elements

• Blood consists of formed elements and plasma.
• Formed elements constitute 45% of blood content.
• Plasma constitutes 55% of blood content.

Formed Elements

• Red blood cells (RBCs) are also known as erythrocytes.
• White blood cells (WBCs) are also known as leukocytes.
• Platelets are also known as thrombocytes.
• RBCs appear as biconcave discs with a normal size of 6-9 µm.
• Macrocytic anemia is indicated by RBCs larger than 9µm.
• Microcytic anemia occurs when RBCs are smaller than 6µm.
• Anemia can result from defects in bone marrow, such as aplastic anemia.
• Iron deficiency, hemorrhage (hemorrhagic anemia), and hemolysis (hemolytic anemia) can each cause anemia.
• Polycythemia can result from hemoconcentration, where there is decreased plasma.
• Hemoconcentration is caused by loss of fluids from diarrhea, vomiting, or polyuria.
• Compensatory polycythemia involves increased bone marrow production due to hypoxia.
• Situations causing Compensatory polycythemia includes high altitudes, fetal state, severe muscular exercise, and congenital heart diseases.
• A normal WBC count is 4,000-11,000/mm3.
• Leukocytosis is an abnormal condition with a WBC count exceeding 11,000.
• Physiological causes of leukocytosis include pregnancy and being a newborn infant.
• Pathological leukocytosis can be triggered by acute infections.

Classification of WBCs

• Granular leukocytes include neutrophils, eosinophils, and basophils.
• Non-granular leukocytes include lymphocytes and monocytes.
• This classification depends on the presence or absence of specific granules.

Neutrophils

• Neutrophils make up to 60-75% of total WBCs.
• Neutrophil size is 10-12 µm.
• Neutrophils have a single, segmented nucleus with 2-5 segments.
• Neutrophils contain 3 types of granules in their cytoplasm.
• Specific granules within neutrophils contain bactericidal and bacteriostatic substances.
• Non-specific granules in neutrophils contain hydrolytic enzymes.
• Glycogen is present in neutrophil cytoplasm.
• Neutrophils defend against infection by microorganisms via migration and phagocytosis.
• Neutrophils secrete proteolytic enzymes.

Eosinophils

• Eosinophils account for 2-5% of total WBCs.
• Eosinophil size: 12-15 µm.
• Eosinophils have a bilobed, horseshoe-shaped nucleus.
• Large acidophilic granules are found within Eosinophil cytoplasm.
• Large ellipsoid granules (histaminase & sulphatase) are found in electron microscopy.
• Serve an anti-allergic function
• Act in an anti-bilharzial & antiparasitic manner.
• Eosinophilia occurs when eosinophils are >5%.
• Eosinophilia is linked to allergic and parasitic (bilharziasis) conditions.

Basophils

• Make up about 0.5-1% of total WBCs.
• Size is 10-12 µm.
• The nucleus is large, very irregular and often S-shaped.
• Cytoplasm contains large basophilic granules, similar to mast cells.
• These cytoplasmic granules contain heparin & histamine.
• Basophils secrete heparin and histamine during allergy.

Lymphocytes

• Account for about 25-30% of total WBCs.
• Size varies: Small (4-7µm), Medium (7-10µm), Large (10-15 µm).
• Mainly small & medium lymphocytes in blood.
• All sizes of lymphocytes are found in lymphoid organs.

Small Lymphocytes

• Can be classified as either B-Lymphocytes or T-Lymphocytes.
• B-Lymphocytes development occurs in bone marrow (humans) and Bursa of Fabricius (birds).
• B-Lymphocytes make up 30% of small lymphocytes in the blood stream.
• B-cells initiate a primary immune response via antigen activation.
• B-cells form plasma cells to secrete antibodies, which is Humoral immunity.
• Secondary immune responses occur when some of the active B-cells remain as memory cells.
• Memory cells produce a rapid response when exposed to the same antigen.
• T-Lymphocytes development starts in the bone marrow and completes in the thymus.
• T-Lymphocytes account for approximately 70% of small lymphocytes in the blood stream.
• T-Lymphocytes exist as killer cells, memory cells, helper cells and lymphokine secreting cells.

Monocytes

• Monocytes account for 3-8% of total WBCs.
• Monocyte size is 15-18 µm.
• Consist of large, pale, kidney-shaped nucleus
• Abundant & pale blue cytoplasm.
• Give origin to macrophages.

Lymphatic System

• The cells of the immune system and lymphoid tissue and lymph vessels.
• Lymphoid tissue may be primary or secondary.
• Primary lymphatic organs: thymus gland and bone marrow.
• Secondary lymphatic organs: non-capsulated nodules, lymph nodes, spleen, and tonsils.
• Lymphatic vessels are capillaries, small and large lymph vessels.
• Capsule and trabeculae form the stroma of the lymph node.

Parenchyma of Lymph Node

• The outer cortex contains regular lymphatic follicles, as well as cortical lymphatic sinuses.
• The inner Medulla contains irregular medullary cords and medullary lymphatic sinuses.
• Cortical lymphatic follicles are typical and non-capsulated.
• Medullary cords are similar to lymphatic follicles but contain more plasma cells.
• The paracortex is known as being the thymus-dependent zone.
• The paracortex is the site of the corticomedullary junction.

Components of the paracortex

• More T lymphocytes
• Postcapillary venules
• T-lymphocytes developed in the thymus migrate to the thymus-dependent zone through these venules

Lymph Sinuses of the Lymph Node

• Cells lining the sinuses are endothelial cells with no basement membrane.
• The cells function in filtration of lymph passing from afferent to efferent lymphatic vessels.
• Lymphatic fluid, macrophages and lymphocytes present.
• These sinuses are found in subcapsular (marginal) areas.
• Also found in cortical and subcortical areas.
• Additionally found as medullary structures with wide irregular channels branching and connecting, reaching efferent lymphatics.

Lymph Node

• Responsible for filtration of lymph by macrophages.
• Facilitate cell mediated immunity with T lymphocytes.
• Facilitate Humoral immunity with B lymphocytes.

Tonsils

• Definition of tonsils: Lymphatic tissue masses in the lamina propria of mucous membranes.
• Several types of tonsils include palatine tonsils of the oropharynx.
• Pharyngeal tonsil (adenoid) of the nasopharynx.
• Lingual tonsils found in mucus membrane of the tongue.
• Function in defense against inhaled or eaten agents.
• Palatine tonsils of the oropharynx consist of stratified squamous non-keratinized epithelium.
• Palatine tonsils structure also includes lymphatic tissue and dense connective tissue capsule.

Spleen

• The stroma is covered with mesothelium.
• Dense fibrous connective tissue with smooth muscles forms the capsule.
• Afferent lymphatics do not penetrate the capsule, but septa/trabeculae made dense fibrous connective tissue with smooth muscles.
• Septa divides the region into incomplete lobules with reticular network of cells & fibers.

White Pulp of the Spleen

• Referred to as Malpighian corpuscles.
• Site: scattered all over the spleen.
• Structure: Non-capsulated lymphatic nodule containing germinal center.
• A central artery has an eccentric position.

Red Pulp of the Spleen

• Fills spaces between white pulps.
• Composed of splenic cords and blood sinusoids with longitudinal endothelial cells that are separated by longitudinal spaces.
• Also contains macrophages (Littoral cells).
• Reticulin "hoop" lines the blood sinusoid walls, as well as endothelial cells and macrophages.
• Consists of irregular branching and anastomosing masses of cells known as splenic cords, also includes:
• Diffuse lymphocytes.
• RBCs
• All types of Leucocytes
• Macrophages.
• The periarterial lymphatic sheath (PALS) defines the thymus dependent zone.
• The thymus dependent zone surrounding the central artery consists of reticular fibres and T-lymphocytes.
• It is a bilobed organ in the superior mediastinum.
• T- cell development happens in the thymus, which also acts as an endocrine organ.
• The Thymus is large in infants, decreasing in size at puberty, then involuting.
• Each thymic lobule consists of an outer cortex and an inner medulla.
• The outer cortex is dark basophilic due to numerous lymphocytes.
• The inner medulla stains lighter -fewer lymphocytes & epithelial reticular cells.

Cells of Thymus Cortex

• Epithelial reticular cells (Epitheliocytes) consist of branched cells with paler nuclei.
• Branching processes connect to form a reticulum for support.
• Epithelial reticular cells function in production of thymic hormones.
• Few macrophages present: phagocytic cells.

Cells of the Medulla

• T-lymphocytes are completely developed, appearing less in number compared to the cortex.
• T-lymphocytes enter circulation through postcapillary venules, settling in thymic dependent zones of lymph nodes and spleen.
• Epithelial reticular cells present.
• Few macrophages present.
• Hassall’s corpuscles are acidophilic masses surrounded by epithelial reticular cells.