Human Immune System
- It is the system that gives immunity to the body by recognizing, responding, and remembering foreign antigens.
- It plays a role in allergic reactions, auto-immune diseases, and organ transplantation.
- It includes lymphoid organs, tissues, cells, and antibodies.
Lymphoid Organs
These are the organs where origin, maturation, and proliferation of lymphocytes occur. They are of two types: Primary and Secondary.
Primary Lymphoid Organs
The organs where lymphocytes mature and differentiate into antigen-sensitive lymphocytes. They are of two types:
- Bone Marrow: The site of formation of all blood cells, including B and T-lymphocytes.
- Thymus: A bilobed organ near the heart and beneath the breastbone. It is large at birth but reduces in size by puberty. Immature T-lymphocytes from bone marrow migrate to the thymus and mature.
Secondary Lymphoid Organs
The organs to which mature lymphocytes migrate from primary lymphoid organs, interact with antigens, and proliferate to become effector cells.
- Examples: Spleen, lymph nodes, tonsils, Peyer’s patches, mucosa-associated lymphoid tissue (MALT), and appendix.
- Spleen: A bean-shaped organ containing lymphocytes and phagocytes. It removes worn-out RBCs and microorganisms from blood and serves as a reservoir of erythrocytes in the fetus.
- Lymph Nodes: Found in the lymphatic system, they trap microorganisms or antigens, activating lymphocytes for an immune response.
- MALT: Located within the lining of respiratory, digestive, and urogenital tracts. It constitutes 50% of lymphoid tissue.
Immunity
The ability of the immune system to fight pathogens. It is of two types: Innate and Acquired.
Innate (Inborn) Immunity
- It is non-specific immunity present at birth.
- It includes four types of barriers:
- Physical Barriers: Prevent entry of microbes. E.g., skin, mucus coating of respiratory, gastrointestinal, and urogenital tracts. Mucus traps microbes.
- Physiological Barriers: Prevent microbial growth. E.g., gastric HCl, saliva, tears.
- Cellular Barriers: Phagocytes like WBC (polymorphonuclear leukocytes, monocytes, natural killer lymphocytes), macrophages.
- Cytokine Barriers: Virus-infected cells secrete interferon, protecting non-infected cells from further viral infection.
Acquired (Adaptive) Immunity
- It is pathogen-specific immunity developed during a lifetime.
- It is characterized by memory: the first encounter with a pathogen produces a primary response (low intensity), while a second encounter causes a secondary (anamnestic) response (high intensity).
- Primary and secondary immune responses are carried out by B-lymphocytes (B-cells) and T-lymphocytes (T-cells).
- B-Lymphocytes: Produce antibodies, proteins that fight pathogens.
- T-Lymphocytes: Help B-cells produce antibodies.
Structure of an Antibody Molecule: An antibody has four polypeptide chains: two light chains and two heavy chains (H2L2). Types of antibodies: IgG, IgA, IgM, IgE, IgD.
Types of Acquired Immune Response
- Humoral Immune Response / Antibody-Mediated Immunity (AMI): Mediated by antibodies found in blood plasma, hence called humoral immunity.
- Cell-Mediated Response / Cell-Mediated Immunity (CMI): Mediated by T-lymphocytes (T-cells). It distinguishes ‘self’ from ‘non-self’ and causes graft rejection.
Tissue and blood group matching are essential before grafts or transplants. Patients require lifelong immunosuppressants.
Types of Acquired Immunity
Acquired immunity is of two types: Active and Passive.
1. Active Immunity:
- Antibodies are produced in the host body upon exposure to antigens (e.g., living/dead microbes or proteins). It is a slow process and occurs in two ways:
a. Natural Active Immunity: Developed during natural infection by microbes.
b. Artificial Active Immunity: Developed by injecting microbes deliberately during immunization.
2. Passive Immunity:
- Readymade antibodies are directly given to the body. It is of two types:
- Antibodies (IgG) from mother → placenta → fetus.
- Antibodies (IgA) in colostrum → infants.
- Anti-tetanus serum (ATS).
a. Natural Passive Immunity:
b. Artificial Passive Immunity:
Immunization
Based on the ‘memory’ of the immune system. It is of two types:
Active Immunization (Vaccination)
- A vaccine (antigenic proteins or inactivated pathogen) is introduced, resulting in antibody production.
- Antibodies neutralize antigens during actual infection.
- Vaccines generate memory B and T cells, which recognize pathogens quickly.
- Examples: Polio vaccine, Hepatitis B vaccine, DPT vaccine.
- Vaccines are produced using DNA recombinant technology (e.g., Hepatitis B vaccine from yeast).
Passive Immunization
- Direct injection of pre-formed antibodies or antitoxin for quick response.
- Examples: Immunization against tetanus, snake venom.