Human Health and Diseases

Your body is under constant attack — bacteria, viruses, fungi, parasites. So why aren't you sick all the time? Because you have an immune system that's essentially a secret army patrolling your body 24/7. This chapter covers how that army works, what happens when it fails, and the diseases that threaten health. Immunity comes in two flavors. Innate immunity is what you're born with — physical barriers (skin, mucus membranes), chemical defenses (stomach acid, lysozyme in tears), and cellular defenses (phagocytes like neutrophils and macrophages). It's non-specific and immediate but has no memory. Adaptive immunity is specific — it targets particular pathogens and remembers them. That's why you only get chickenpox once. Adaptive immunity involves lymphocytes — B cells (produce antibodies) and T cells (kill infected cells and coordinate the immune response). The immune response begins when an antigen (a molecule on a pathogen) is recognized. Macrophages engulf pathogens and present their antigens to helper T cells (CD4+). These activate B cells, which differentiate into plasma cells (mass-produce antibodies) and memory B cells (provide long-term immunity). Antibodies (immunoglobulins) are Y-shaped proteins. The tips of the Y bind to antigens; the stem signals other immune cells. There are five classes: IgG (most abundant, long-term immunity), IgA (mucous membranes), IgM (first response), IgE (allergies, parasitic worms), and IgD (B cell receptor). Vaccines work by exposing the body to weakened or inactivated pathogens, triggering adaptive immunity without causing disease. When the real pathogen appears, memory cells mount a rapid response. Active immunity (your body produces antibodies) lasts longer than passive immunity (antibodies transferred from another source — like mother to baby or antivenom injections). Common diseases include typhoid (Salmonella typhi — prolonged fever, spread through contaminated food), pneumonia (Streptococcus pneumoniae — lungs fill with fluid), malaria (Plasmodium — spread by Anopheles mosquito, causes fever cycles), amoebiasis (Entamoeba histolytica — intestinal infection), and ringworm (fungal skin infection). The malaria life cycle is especially important: the mosquito injects sporozoites, which infect the liver, multiply, then infect red blood cells, causing periodic fevers when they burst out. Cancer is when cells divide uncontrollably. They form tumors, invade nearby tissues, and spread (metastasize). Causes include tobacco, radiation, certain viruses (HPV causes cervical cancer, hepatitis B causes liver cancer), genetics, and environmental factors. Treatment includes surgery, radiation, chemotherapy, and immunotherapy. AIDS is caused by HIV, which attacks helper T cells (CD4+). Without these generals, the immune army can't fight off even minor infections. AIDS is the final stage when T cell count drops below 200. Transmission: unprotected sex, infected blood, contaminated needles, and mother to child. Not curable but manageable with antiretroviral therapy (ART). Diagnosis uses ELISA (screening) and Western blot (confirmation). Drug abuse is a major health problem. Opioids (heroin) bind to pain receptors producing euphoria then addiction. Cannabinoids (marijuana) affect memory and coordination. Cocaine blocks dopamine reuptake — intense high then severe crash. Alcohol depresses the CNS. All affect the brain's reward pathway, making addiction a chronic brain disorder, not a moral failing.