Blood is a connective tissue consisting of plasma (~55%) and formed elements (~45%). Plasma: water (91.5%), proteins (albumin, globulin, fibrinogen), glucose, electrolytes, hormones, dissolved gases. Formed elements: RBCs (erythrocytes, ~5 million/cu.mm), WBCs (leukocytes, 6000-8000/cu.mm), platelets (thrombocytes, 1.5-3.5 lakh/cu.mm). Functions of blood: transport of O2, CO2, nutrients, hormones, waste products. Regulation of pH (7.35-7.45), temperature, osmotic pressure. Defence (WBCs, antibodies). Haemostasis (clotting).

Differential WBC count shows the percentage of each WBC type. Normal values: Neutrophils: 60-65% (most abundant). Multi-lobed nucleus (2-5 lobes). First responders to bacterial infection. Lymphocytes: 20-25%. Includes B cells, T cells, NK cells. Key cells of adaptive immunity. Monocytes: 6-8%. Largest WBCs. Differentiate into macrophages in tissues. Eosinophils: 2-3%. Bilobed nucleus. Granules with toxic proteins. Active against parasites and in allergic reactions. Basophils: 0.5-1%. Least numerous. Release histamine and heparin from granules. Role in allergic reactions and inflammation. Mnemonic: Never Let Monkeys Eat Bananas.

Granulocytes: WBCs with visible cytoplasmic granules and lobed nuclei. Neutrophils: fine granules, multi-lobed nucleus. Phagocytose bacteria. Eosinophils: large red/orange granules (stain with eosin). Bilobed nucleus. Anti-parasitic, anti-allergic. Basophils: large dark purple granules (stain with basophilic dyes). Bilobed nucleus. Release histamine (inflammation), heparin (anticoagulant). Agranulocytes: no visible granules, non-lobed nucleus. Lymphocytes: small round nucleus, scant cytoplasm. B cells, T cells, NK cells. Monocytes: kidney-shaped or horseshoe-shaped nucleus. Become macrophages (phagocytes) in tissues.

Innate immunity: non-specific, first line of defence. Physical barriers: skin, mucus, cilia. Cellular: neutrophils and macrophages (phagocytosis), NK cells (kill virus-infected and cancer cells), mast cells (release histamine), dendritic cells (antigen-presenting cells). Chemical: complement system, interferons, acute phase proteins, lysozyme. Adaptive immunity: specific, memory-forming. Humoral immunity: B lymphocytes → plasma cells → antibodies (IgG, IgM, IgA, IgE, IgD). Cell-mediated immunity: T lymphocytes. CD4+ helper T cells: coordinate immune response. CD8+ cytotoxic T cells: kill virus-infected cells directly. Memory cells: long-lived B and T memory cells — basis of vaccine protection.

Eosinophils (2-3% of WBCs): named for their affinity for eosin dye (red/orange granules). Granule contents: major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase — all toxic to parasites. Anti-parasitic function: in parasitic infections (helminth worms like Ascaris, Toxocara, Trichinella), eosinophils bind to the parasite surface and release toxic granule contents → parasite death. Role in allergy: eosinophilia is a hallmark of allergic diseases (asthma, hay fever, eczema, food allergy). IL-5 from T helper 2 (Th2) cells stimulates eosinophil production and activation. Eosinophilia causes: parasitic infections, allergic disorders, drug hypersensitivity, Addison disease, tropical pulmonary eosinophilia. Eosinopenia (low eosinophils): stress, Cushing syndrome (excess cortisol).

Lymphocytes (20-25% of WBCs): key cells of adaptive immunity. Origin: all from pluripotent stem cells in bone marrow. B lymphocytes (B cells): mature in Bone marrow. Function: antibody production (humoral immunity). Activated by antigen + T helper cell help → differentiate into plasma cells (antibody factories) + memory B cells. Antibodies: IgG (most abundant in blood, crosses placenta), IgM (first in immune response, pentamer), IgA (in secretions — saliva, breast milk, tears), IgE (allergic reactions, anti-parasitic), IgD (B cell surface receptor). T lymphocytes (T cells): mature in Thymus. Types: CD4+ T helper (Th) cells: coordinate both humoral and cell-mediated immunity. Th1 → macrophage activation. Th2 → antibody production. CD8+ cytotoxic T lymphocytes (CTLs): kill infected cells directly using perforin/granzyme. Regulatory T cells (Tregs): suppress immune response, prevent autoimmunity. Memory T cells: long-lived, rapid response to re-exposure.

Neutrophils (60-65% of WBCs, most abundant): first cells to arrive at site of bacterial infection. Multi-lobed nucleus (2-5 lobes) — also called polymorphonuclear leukocytes (PMNs). Life span: 6-12 hours in blood, few days in tissues. Mechanism of killing bacteria: Phagocytosis: engulf bacteria into phagosome → fuses with lysosomes (containing myeloperoxidase, defensins, elastase) → respiratory burst (oxidative killing using reactive oxygen species — superoxide, H2O2, hypochlorous acid). Degranulation: release granule contents extracellularly. Neutrophil Extracellular Traps (NETs): chromatin + antimicrobial proteins released to trap and kill bacteria extracellularly. Neutrophilia: high neutrophil count = bacterial infection, stress, corticosteroids. Neutropenia (low neutrophils): chemotherapy, aplastic anaemia, drug side effects → severe risk of bacterial infection.

Platelets (thrombocytes): small anucleate cell fragments derived from megakaryocytes in bone marrow. Count: 1.5-3.5 lakh (150,000-350,000)/cu.mm. Lifespan: ~7-10 days. Functions: Primary haemostasis: platelets adhere to damaged vessel wall (von Willebrand factor bridge) → activate → aggregate → form platelet plug. Secondary haemostasis (clotting cascade): platelets provide phospholipid surface for clotting factors. Release ADP, thromboxane A2 (platelet activators), serotonin (vasoconstriction). Coagulation cascade: intrinsic pathway (factor XII activated by exposed collagen) → extrinsic pathway (tissue factor/factor III) → common pathway → thrombin → fibrin → stable clot. Clot retraction: platelets pull fibrin strands, sealing the wound. Thrombocytopenia: low platelets → bleeding tendency. Causes: ITP, dengue fever, aplastic anaemia, chemotherapy.