White Blood Cells (Leukocytes)

Overview and Development

White blood cells (leukocytes) are essential components of the immune system, responsible for protecting the body against disease. They develop from hematopoietic stem cells in the bone marrow, stimulated by hormones such as interleukins and colony-stimulating factors.

• Function: Protect against disease by identifying and eliminating pathogens.

• Transport: Move to sites of infection via the bloodstream.

• Diapedesis: Ability to squeeze between endothelial cells to exit blood vessels and enter tissues.

• Motility: Self-propel through tissue to reach infection sites.

Types of White Blood Cells

White blood cells differ by size, cytoplasm, nuclear shape, and staining characteristics. They are classified as granular or agranular based on the presence of cytoplasmic granules.

• Granular Leukocytes: Have visible granules when stained, lobed nuclei, are about twice the size of red blood cells (RBCs), and have a lifespan of about 12 hours.

• Agranular Leukocytes: Lack distinct granules and generally have a longer lifespan.

Granulocytes

Neutrophils

• Granules stain light pink to purple.

• Nucleus has 2-5 segments.

• Most numerous (50-70% of WBCs).

• First to arrive at infection sites.

• Kill bacteria by releasing toxic chemicals and phagocytizing dead bacteria.

Eosinophils

• Granules stain deep red.

• Nucleus has 2 lobes.

• 1-3% of circulating WBCs.

• Release toxic chemicals to kill parasites (e.g., tapeworms, roundworms).

• Phagocytize antigen-antibody complexes and help regulate inflammatory responses.

Basophils

• Similar in size and nuclear shape to eosinophils.

• Fewer, irregularly shaped granules that stain blue.

• Rare (

  • Migrate to damaged tissues and release heparin (anticoagulant) and histamine (promotes inflammation via vasodilation).

  • Play a role in allergic reactions.

Agranulocytes

Monocytes

• Nucleus can be spherical, kidney-shaped, oval, or lobed.

• 3-9% of circulating WBCs.

• Lifespan: several weeks to months.

• Leave bloodstream, migrate to tissues, and become macrophages (highly phagocytic, can engulf larger molecules than neutrophils).

• Number increases with viral infection and inflammation.

Lymphocytes

• Smallest WBCs, only slightly larger than RBCs.

• Large, spherical nucleus occupies most of the cell.

• 25-33% of circulating WBCs.

• Can live for years.

• T cells: Directly attack microorganisms, tumor cells, and transplanted cells.

• B cells: Produce antibodies (immunoglobulins) that mark foreign molecules for destruction.

Platelets (Thrombocytes)

Structure and Function

Platelets are not complete cells but fragments of megakaryocytes found in red bone marrow. They play a crucial role in blood clotting and wound repair.

• Develop in response to thrombopoietin.

• Less than half the size of RBCs.

• Contain lysosomes, mitochondria, cytoskeleton, and certain chemicals, but no nucleus.

• Lifespan: ~10 days.

• Essential for the stoppage of bleeding (hemostasis).

Platelet Disorders

• Thrombocytosis: Too many platelets, increasing risk of blood clots and bleeding.

• Thrombocytopenia: Too few platelets, leading to easy bleeding and bruising.

Plasma

Composition and Function

Plasma is the clear, straw-colored liquid portion of blood, making up about 55% of total blood volume.

• Composed of 92% water.

• Contains dissolved substances, primarily proteins important for blood colloid osmotic pressure.

Hemostasis

Mechanisms of Stoppage of Bleeding

Hemostasis is the process that prevents and stops bleeding, involving several steps:

• Vascular Spasm: Rapid contraction of smooth muscle in vessel walls, lasting about 30 minutes, slows bleeding. Platelets release serotonin, a vasoconstrictor.

• Platelet Plug Formation: Platelets adhere to damaged endothelium, become sticky, and release chemicals (e.g., thromboxane A2) to attract more platelets. In small vessels, this may be sufficient to stop bleeding.

• Blood Coagulation: Balance between coagulants and anticoagulants. Damage increases coagulant concentration, leading to conversion of fibrinogen to fibrin, reinforcing the plug and forming a clot. The clot retracts and eventually dissolves.

Clotting Disorders

• Thrombus: Clot formed in a blood vessel.

• Thrombosis: Blockage of blood flow by a thrombus.

• Infarction: Obstruction of blood supply to an organ.

• Embolus: Clot or fragment dislodged and carried in blood, causing embolism when lodged in smaller vessels.

• Atherosclerosis: Accumulation of fatty deposits in arterial lining, common cause of thrombosis in medium vessels.

Blood Types

ABO Blood Group System

Blood type is determined by the presence of specific antigens on the surface of RBCs. These antigens are used as cell recognition markers and can stimulate an immune response.

• Antigen: Molecule on the cell surface capable of stimulating an immune response.

• Antibody: Protein (immunoglobulin) produced against foreign antigens.

• ABO blood type is based on the type of antigen present on RBCs. The host does not produce antibodies against its own antigens but will attack foreign antigens.

A and B alleles are codominant; both are dominant over i (no antigen).

Transfusion Reactions

• Antibodies attack foreign blood cells, causing agglutination and hemolysis.

• Can result in clots, heart attack, stroke, or other complications.

Rhesus (Rh) Factor

• Rh antigen may be present (+) or absent (-).

• Rh+ individuals carry the antigen and do not have Rh antibodies.

• Rh- individuals do not carry the antigen and do not naturally have the antibody, but may develop it after exposure.

• Hemolytic disease can occur if an Rh- mother is exposed to Rh+ fetal blood.

Additional Info

• Anemia: Abnormal reduction in the number of RBCs or amount of hemoglobin, leading to decreased oxygen-carrying capacity.

• Damaged RBCs: Rupture as they pass through the spleen and liver, then are phagocytized by macrophages.