• Immunity
  • Immunity is the ability of a person’s body to fight against pathogens by producing specific antibodies.
  • After the first infection, a portion of the lymphocytes is stored in the body as memory cells.
  • Memory cells remain in the body for many years, the body is said to be immune to the disease.
• Antigen
  • Antigens are foreign protein molecules in the human body that stimulate the body's immune system to produce antibodies against the antigen.
• Antibody
  • Antibodies are protein molecules produced by the body's immune system in response to the presence of an antigen.
  • Antibodies destroy or weaken as well as neutralize the toxins.

First-line defense

• Not specific.
• Prevent the entry of pathogens into the body via physical and chemical.

1) Physical

• Skin epidermis; the outermost layer of skin that is permeable to microorganisms.
• Blood clots; formed on wounds that help to prevent pathogens from entering the body.
• Mucus membrane; lines the respiratory tract, digestive tract, urinary tract, and reproductive tract. 
• Cilium; moves the trapped microorganisms in the respiratory tract to the pharynx to be swallowed or spit.

2) Chemical

• Mucus; secreted by mucus membrane to trap dirt particles and bacteria. 
• Lysozyme enzyme; found in tears and saliva that can kill the bacteria by digesting the cell wall of bacteria.
• Hydrochloric acid; found in gastric juice that can kill pathogens.
• Ear wax; antibacterial and antifungal, helps to trap dirt particles.
• Sweat; contains lysozyme to kill pathogens.
• Sebum; secreted by the sebaceous gland; protect the skin by killing pathogens and fungus.

Second-line defense

• Acted after microorganisms successfully passed the first-line defense.
• Non-specific defense mechanisms.
• Fever, inflammation, and phagocytosis are parts of the second-line defense.
• Fever is an increase in the body temperature; increases the metabolic rate in body cells; increases the phagocytic rate towards pathogens. 
  • The high temperature helps to disintegrate the proteins of pathogens and makes their survival becomes harder. 
  • The high temperature also helps in the healing process.
• Inflammation is the condition of the cells that swell due to injuries/infection by pathogens. 
  • Swollen tissues help to isolate pathogens at certain locations. 
  • Swollen tissues release chemical substances; attract the phagocytic cells to move to the tissues and swallow the pathogens. 
  • Puss will form.
• Phagocytosis involves two types of phagocytes; neutrophils and monocytes (macrophages). 

Third-line defense

• Involved immune response and depends on the ability of the antibodies to recognize the specific pathogens and protect the body from that pathogens.
• Antibodies produced are specific to kill specific pathogens.
• Two groups of leucocytes; lymphocytes and macrophages.
  • Two types of lymphocytes; lymphocyte T and lymphocyte B. 
  • Lymphocyte T; attack the infected cells and produces chemical substances that trigger the lymphocyte B.
  • Lymphocyte B divides and forms the plasma cells and memory B cells.
  • Plasma cells produce antibodies that are compatible with the antigens. 
  • Memory B cells produced by the lymphocyte B function to store the information about the pathogens that infect the body. 
  • Plasma cells get activated to produce antibodies rapidly when the body is exposed to the same pathogens.