• The body’s first line of defense is the mechanical barrier of the skin and mucus membranes. 
• The body’s second line of defense consists of nonspecific cellular and chemical defenses. 
       - Phagocytes engulf and destroy pathogens that break through the first line of defense. 
       - Natural killer cells are lymphocytes that kill virus-infected and malignant cells. 
       - Inflammation is when an area of tissue is fighting off an infection. The blood vessels dilate and become more permeable. It is characterized by redness, heat, swelling, and pain. 
       - Interferons are proteins that prevent viruses from multiplying.
       - Fever increases metabolism so that the defensive and repair functions occur more rapidly. 
• The body’s third line of defense is the immune system. 
       - Antigens are substances that trigger a response from the immune system. 
       - The body creates a certain antibody to fight off a specific antigen.
• Organ transplants may not work because the body may reject the foreign tissue. It recognizes it as “invading” tissue and tries to destroy it. Infections also are common in transplants. Medication is used to help the body to accept new tissue.
• Immune problems
     - Immunodeficiencies - overwhelming infections cause death because the immune system can’t combat them; examples include AIDS and congenital thymic aplasia 
     - Hypersensitivities (allergies) - abnormally intense reaction to an allergen (a non-specific type of antigen) 
     - Autoimmune disease - the body attack its own tissue; examples include rheumatoid arthritis and multiple sclerosis

• The Lymphatic System 
• The lymphatic system is made up of lymphatic vessels, lymph nodes, lymphoid organs, and tissues. It returns fluids (that have leaked from the blood vascular system) back to the blood. It also protects the body by removing foreign matter from the lymph stream. 
vThe lymphatic system forms a one-way network of lymphatic capillaries, collecting vessels, trunks, and ducts. It takes the fluid (lymph) to the heart, entering the blood vascular system at the junction of the internal jugular and subclavian veins in the neck. 
• The flow of lymph is slow. It is controlled by skeletal muscle contractions. There are one-way valves to prevent backflow. 
• The lymphatic capillaries are very permeable, allowing materials to freely enter them from the interstitial space. 
• On the negative side, pathogens and cancer cells may spread through the body via the lymphatic system. Lymphoid cells include lymphocytes (T cells or B cells), plasma cells (which produce antibodies), macrophages (phagocytes that destroy foreign matter), and reticular cells (that form the lymphoid tissue stroma). 
• Lymphoid tissue is made of reticular connective tissue and houses macrophages and lymphocytes. 
• Lymphoid organs are made up of lymphoid tissue. The major ones include lymph nodes, the spleen, thymus, tonsils, and nodule aggregates. 
• Lymph nodes filter the lymph. They are clustered along lymphatic vessels. The spleen destroys old red blood cells and blood-borne pathogens. 
• Tonsils and other nodules remove pathogens in the respiratory & digestive