MP Board · Class 12 · Biology · Human Health and DiseasesWhat is immunity? Explain in detail the two main types of immunity present in the human body: Innate Immunity (including its four barriers) and Acquired Immunity. Also, briefly differentiate between Humoral Immune Response and Cell-Mediated Immunity.

Immunity\nImmunity refers to the overall ability of a host organism to resist and fight off disease-causing pathogens, mediated by its immune system.

\nImmunity is broadly categorized into two types:

1. Innate Immunity (Non-Specific Immunity)\nInnate immunity is a non-specific type of defense mechanism that is present at the time of birth. It acts as the first line of defense against pathogens through four types of barriers:

• Physical Barriers: Prevent the entry of microorganisms into the body.
  • Examples: Skin on our body, mucus coating of the epithelium lining the respiratory, gastrointestinal, and urogenital tracts.
• Physiological Barriers: Prevent microbial growth in the body using secretions.
  • Examples: Acid ($HCl$) in the stomach, saliva in the mouth, and tears from the eyes (containing lysozymes).
• Cellular Barriers: Leukocytes (white blood cells) phagocytose and destroy invading microbes.
  • Examples: Polymorpho-nuclear leukocytes (PMNL-neutrophils), monocytes, natural killer (NK) cells, and tissue macrophages.
• Cytokine Barriers: Virus-infected cells secrete proteins called interferons, which protect non-infected surrounding cells from further viral infection.

2. Acquired Immunity (Specific Immunity)\nAcquired immunity is pathogen-specific and is characterized by memory. When the body encounters a pathogen for the first time, it produces a primary response of low intensity. Subsequent encounters with the same pathogen elicit a highly intensified secondary (anamnestic) response.

\nAcquired immunity is carried out by two special types of lymphocytes:

• B-lymphocytes (B-cells): Produce proteins called antibodies into the blood plasma to fight pathogens.
• T-lymphocytes (T-cells): Do not secrete antibodies themselves, but help B-cells to produce them.

Difference Between Humoral and Cell-Mediated Immunity

1. Humoral Immune Response (AMI):
   • Mediated by antibodies produced by B-lymphocytes circulating in body fluids (blood and lymph).
   • Effective against extracellular pathogens such as bacteria and viruses circulating in body fluids.
2. Cell-Mediated Immunity (CMI):
   • Mediated by T-lymphocytes directly destroying infected cells.
   • Responsible for recognizing 'self' and 'non-self' tissue and causes graft rejection during organ transplantation.