Blood consists of red blood cells (carrying oxygen via hemoglobin, vastly outnumbering white blood cells at about 700:1), white blood cells (fighting infection), platelets (enabling clotting), and plasma (the liquid carrying cells, nutrients, hormones, and waste). A normal complete blood count shows WBC 4,500–11,000/μL, hemoglobin 13.5–17.5 g/dL in men and 12.0–16.0 g/dL in women, and platelets 150,000–400,000/μL. Hematologic disorders include anemia (low healthy red blood cells, most commonly from iron deficiency), sickle cell disease (abnormal crescent-shaped hemoglobin causing pain crises and organ damage), hemophilia (genetic clotting factor deficiency), von Willebrand disease (the most common inherited bleeding disorder), thalassemia, leukemia (cancer of blood-forming tissues), lymphoma (cancer of the lymphatic system), and multiple myeloma (plasma cell cancer producing bone lesions and kidney damage). Treatments range from iron supplementation and blood transfusions to bone marrow transplantation and targeted therapies.
The immune system defends against pathogens through two branches: innate immunity (immediate, non-specific defenses including skin barriers, neutrophils, macrophages, and inflammation) and adaptive immunity (delayed but specific, involving T cells and B cells that produce antibodies and generate memory). Antibodies (immunoglobulins) include IgG (most abundant), IgA (mucosal surfaces), IgM (first response), IgE (allergies), and IgD (B cell activation). The four types of hypersensitivity reactions range from immediate IgE-mediated anaphylaxis to delayed T-cell-mediated contact dermatitis. Autoimmune diseases occur when the immune system attacks the body's own tissues, including Type 1 diabetes, rheumatoid arthritis, lupus (SLE), multiple sclerosis, and Hashimoto's thyroiditis.
Infectious diseases are caused by bacteria (single-celled organisms treated with antibiotics), viruses (requiring host cells, treated with antivirals when available), fungi (treated with antifungals), and parasites (treated with antiparasitics). The immune system mounts inflammation, characterized by redness, heat, swelling, pain, and loss of function. Bacterial infections are treated with antibiotics such as penicillins, cephalosporins, macrolides, fluoroquinolones, and tetracyclines; bacterial versus viral infections are distinguished clinically because antibiotics do not work against viruses. Antimicrobial stewardship addresses the growing crisis of antibiotic resistance, in which bacteria like MRSA and multi-drug resistant tuberculosis have evolved to survive standard treatments. Vaccines introduce harmless forms of a pathogen (weakened, inactivated, or a piece of it) to stimulate antibody and memory cell production, allowing the body to fight off the real pathogen quickly upon exposure. Herd immunity protects vulnerable individuals when a sufficient portion of the population is immune, with required thresholds varying by disease (about 95% for measles, 80% for polio).