9.3 Third Line of Defence: Specific Defences
This section details the body's adaptive or specific immune system, which targets particular pathogens that have bypassed the first and second lines of defence. It is characterized by specificity and memory.
Antigen
A substance, typically a protein on the surface of a pathogen (like a virus or bacterium), that stimulates a specific immune response. The name comes from ANTI-body GENerating substance.
Specific Defence
Unlike the non-specific first and second lines, this defence system targets specific antigens, providing a powerful and tailored response to infections.
Lymphocytes
The primary white blood cells involved are B-lymphocytes (B cells) and T-lymphocytes (T cells).
Immune Response Types
- Humoral Immune Response: Mediated by B cells and the antibodies they produce.
- Cell-Mediated Immune Response: mediated directly by T cells.
Role of Monocytes in Initiating the Response
Monocytes are a type of white blood cell that act as a bridge between the non-specific and specific immune systems.
- They circulate in the blood and then move into tissues, where they mature into larger cells called macrophages or dendritic cells.
- Phagocytosis: These cells engulf and digest invading pathogens.
- Antigen Presentation: After digesting a pathogen, they display the pathogen's antigens on their own cell surface. This turns them into Antigen-Presenting Cells (APCs).
- Activation Signal: APCs secrete a protein called interleukin-1, which activates T cells and promotes a general inflammatory response (e.g., fever).
Cell-Mediated Immune Response (Role of T Cells)
This response involves the direct action of T cells against infected body cells and pathogens.
- Origin and Maturation: T cells originate from stem cells in the bone marrow and mature in the thymus gland, where they become "immunocompetent" (capable of an immune response).
Activation of T Cells
- Antigen Presentation: A macrophage (APC) presents a specific antigen on its surface, bound to its own protein called the Major Histocompatibility Complex (MHC).
- Recognition: A Helper T cell with a matching T cell receptor (TCR) binds to the antigen-MHC complex on the APC.
- Chemical Signaling: The APC releases interleukin-1, stimulating the Helper T cell.
- Proliferation: The activated Helper T cell releases interleukin-2, which triggers the rapid division (proliferation) of more Helper T cells, Cytotoxic T cells, and B cells.
Types of T cells
| T Cell Type | Surface Marker | Function |
|---|---|---|
| Helper T cell | CD4 | Secretes interleukin-2 to activate and promote the division of other T cells and B cells. They are the "generals" of the immune response. |
| Cytotoxic T cell | CD8 | Directly kills infected body cells or pathogens by releasing proteins like perforin (creates holes in the cell membrane, causing lysis) or cytotoxins (destroy DNA). |
| Suppressor T cell | CD8 | Inhibits the immune response once the infection is cleared, preventing excessive or autoimmune reactions. They "shut down" the response. |
| Memory T cell | - | Long-lived cells that remain after an infection. They provide a rapid and strong response if the same antigen is encountered again in the future. |
Humoral (Antibody-Mediated) Immune Response (Role of B Cells)
This response involves the production of antibodies by B cells to fight pathogens circulating in body fluids (humors), like blood and lymph.
- Origin and Maturation: B cells are produced and mature in the bone marrow.
Activation of B Cells
- Stimulation: Activated Helper T cells release interleukin-2.
- Antigen Binding: A B cell with a specific B cell receptor (BCR) binds to its corresponding antigen.
- Differentiation and Proliferation: The B cell is activated and divides into two main types of cells:
- Plasma Cells: Antibody factories. They produce and secrete millions of antibody molecules per hour but live only for a few days.
- Memory B Cells: Long-lived cells that remain in the body to provide a faster response to future infections by the same pathogen.
Structure and Function of Antibodies
Antibodies, also called immunoglobulins (Ig), are Y-shaped proteins.
- Structure:
- Composed of four polypeptide chains: two identical heavy chains and two identical light chains.
- Constant Region (C): The "stem" of the Y, which is the same for antibodies of a particular class.
- Variable Region (V): The "tips" of the Y, with a unique amino acid sequence that forms the antigen-binding site. Each antibody has two identical antigen-binding sites.
- Modes of Action:
- Neutralization: Antibodies bind to toxins or viruses, preventing them from attaching to host cells.
- Opsonization: Antibodies coat a pathogen, "marking" it for phagocytosis by macrophages.
- Complement System Activation: The antibody-antigen complex activates a cascade of proteins that can form a membrane attack complex, bursting the pathogen's cell membrane.
- Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC): Antibodies attach to infected cells, signaling Natural Killer (NK) cells to induce apoptosis (programmed cell death).
Primary vs. Secondary Immune Response
| Feature | Primary Immune Response | Secondary Immune Response |
|---|---|---|
| Trigger | First exposure to a specific antigen. | Subsequent exposure to the same antigen. |
| Speed of Response | Slower (takes several days to become fully functional). | Faster and more rapid. |
| Strength | Weaker response, lower antibody concentration. | Stronger response, higher antibody concentration. |
| Key Cells | Naive B and T cells are activated. | Existing Memory B and T cells are activated. |
| Outcome | Clears the infection and produces memory cells. | Rapidly clears the infection, often without symptoms. |
Monoclonal Antibodies (mAbs)
- Definition: Laboratory-engineered antibodies that are identical (clones) and designed to bind to a single, specific antigen.
- Impact and Applications:
- Research: Used to detect and quantify specific molecules, aiding in cancer research, protein analysis, and immunology.
- Diagnostics: Key components in tests like ELISA and Western blot, and used as imaging agents (e.g., in PET scans).
- Therapy: Used as targeted treatments for diseases like cancer, autoimmune disorders, and viral infections (e.g., COVID-19).
Inborn (Innate) vs. Acquired (Adaptive) Immunity
- Inborn/Innate Immunity: Non-specific defence mechanisms you are born with. Includes the first line (skin, mucous) and second line (inflammation, phagocytes) of defence. Found in all multicellular organisms.
- Acquired/Adaptive Immunity: Highly specific protection developed after exposure to a pathogen. Includes the third line of defence (cell-mediated and humoral immunity). It is unique to vertebrates.
Possible Questions/Answers
Q: What is the difference between an antigen and an antibody?
A: An antigen is a molecule (usually on a pathogen) that triggers an immune response. An antibody is a Y-shaped protein produced by plasma cells in response to an antigen, which binds to and helps neutralize the pathogen.
Q: What is the primary role of a Helper T cell?
A: A Helper T cell is the central coordinator of the adaptive immune response. It becomes activated by an antigen-presenting cell and then releases interleukin-2 to activate and promote the growth of both cytotoxic T cells (for cell-mediated immunity) and B cells (for humoral immunity).
Q: Differentiate between cell-mediated and humoral immunity.
A: Cell-mediated immunity involves T cells directly attacking and killing infected host cells. Humoral immunity involves B cells producing antibodies that circulate in the body's fluids (humors) to neutralize pathogens before they can infect cells.
Q: What makes the secondary immune response so much faster and stronger than the primary one?
A: The secondary response relies on memory B and T cells that were created during the primary response. These cells are already "primed" to recognize the antigen, allowing them to be activated much more quickly and mount a larger, more effective response upon re-exposure.
References
(Derived from FBISE textbook)