AS Level Biology (9700) Study Notes: Topic 11 – Immunity
Hello Biologists! Welcome to one of the most fascinating topics: Immunity. This chapter explains how your body fights off invaders—pathogens like bacteria and viruses—using an incredibly sophisticated defense system. Understanding immunity is crucial for explaining everything from why you get sick to how vaccines save millions of lives. Let's dive into your internal army!
11.1 The Immune System: Your Body's Defense Force
The immune system is usually divided into two main categories: the non-specific (innate) defense, which acts immediately against all pathogens, and the specific (adaptive) defense, which learns and remembers specific invaders.
11.1.1 Non-Specific Defense: Phagocytosis
Phagocytosis is the immediate, non-specific way your body deals with foreign particles and pathogens. Think of the phagocytes as the clean-up crew or the "Pac-Men" of the immune system.
Key Cells involved:
- Phagocytes: Cells that engulf and destroy pathogens. The two main types covered are Macrophages and Neutrophils.
Step-by-Step Mode of Action of Phagocytes:
Recognition: The phagocyte is attracted by chemicals released from the pathogen or damaged cells (chemotaxis).
Attachment: The phagocyte binds to the surface of the pathogen.
Engulfment: The phagocyte extends pseudopods (cytoplasmic extensions) around the pathogen, engulfing it via endocytosis to form a vesicle called a phagosome.
Digestion: Lysosomes (organelles containing hydrolytic enzymes) fuse with the phagosome, forming a phagolysosome.
Killing: The enzymes digest the pathogen, destroying it.
Presentation (Crucial for Macrophages): Macrophages often display fragments of the digested pathogen on their own surface, becoming Antigen-Presenting Cells (APCs). This links the non-specific and specific responses.
Quick Takeaway: Phagocytosis is fast and indiscriminate. It’s the first line of cellular attack.
11.1.2 Antigens: The Identification Tags
An antigen is any substance (usually a protein or glycoprotein) that, when introduced into the body, causes an immune response.
Self Antigens vs. Non-Self Antigens:
Self Antigens: Molecules found on your own body cells. Your immune system learns to recognize these as "friendly" and normally does not attack them. (Analogy: Your body's ID badge.)
Non-Self Antigens: Molecules found on pathogens (like bacteria, viruses) or foreign substances (like transplanted tissue). These trigger the immune response. (Analogy: An unknown, hostile ID badge.)
Did you know? Blood group markers and the proteins on cell membranes (like those discussed in Topic 4) are examples of self-antigens. When you get a transplant, the non-self antigens on the donor tissue trigger rejection.
11.1.3 The Specific Defense: Primary Immune Response
The specific immune response involves lymphocytes (B-cells and T-cells) and is initiated by the recognition of non-self antigens. This first time the body encounters a specific antigen is called the Primary Immune Response. It is relatively slow.
The Sequence of Events:
A. Macrophage Role (Antigen Presentation)
The macrophage engulfs the pathogen (via phagocytosis) and then presents the non-self antigen fragments on its surface.
B. T-Lymphocyte Activation
T-helper cells (TH cells): These cells possess receptors specific to the presented antigen. When they bind to the antigen presented by the macrophage, they become activated.
Activated T-helper cells then rapidly divide by mitosis and release chemical signals (lymphokines/cytokines) to stimulate other immune cells.
C. B-Lymphocyte Activation (Clonal Selection)
B-lymphocytes (B-cells): These cells have surface antibodies that are specific to one type of antigen. When a B-cell encounters its matching antigen AND receives chemical signals from activated T-helper cells, it becomes activated.
Activated B-cells divide rapidly (clonal selection) to form two groups of cells:
- Plasma Cells: Short-lived cells that function as "antibody factories," synthesizing and secreting large amounts of specific antibodies into the blood and tissue fluid.
- B-Memory Cells: Long-lived cells that remain in the bloodstream to provide long-term protection.
D. T-Killer Cell Role (Cellular Immunity)
T-killer cells (TK cells) (Cytotoxic T-cells): Activated by T-helper cells and antigens. They seek out and destroy infected body cells by releasing toxic substances (like perforin), causing the infected cells to lyse (burst).
Don't worry if this seems tricky at first! The key sequence is: Macrophage -> T-helper -> B-cell/T-killer activation. Everything multiplies (clones) once the threat is confirmed!
Quick Review: Primary Response vs. Secondary Response
After the first infection (primary response), most effector cells (like plasma cells) die off, but the memory cells remain.
When the same pathogen invades again, the memory cells (B and T) trigger a Secondary Immune Response:
Speed: Much faster than the primary response (often within hours).
Magnitude: Produces a much larger concentration of antibodies.
Duration: Lasts longer.
This rapid, powerful response usually clears the pathogen before you even feel symptoms, providing long-term immunity.
Analogy: The primary response is like studying a brand new topic for the first time—it takes time and effort. The secondary response is like revising that topic right before the exam—it’s quick and produces excellent results!
11.2 Antibodies and Vaccination
11.2.1 Molecular Structure and Function of Antibodies
Antibodies are globular proteins (immunoglobulins) produced by plasma cells. Their structure is highly specialized to their function.
Structure of an Antibody:
Y-shaped molecule, composed of four polypeptide chains (two identical heavy chains and two identical light chains).
Held together by disulfide bonds (a type of covalent bond, see Topic 2).
Constant Region (C): Determines the mechanism used to destroy the antigen (the 'stalk' of the Y).
Variable Region (V): Forms the antigen-binding site (the 'arms' of the Y). This region has a unique sequence of amino acids, making it highly specific to one type of antigen—like a lock and key.
Functions of Antibodies:
Agglutination: Clumping pathogens together, making it easier for phagocytes to engulf them.
Neutralisation: Binding to toxins or surface antigens on pathogens, preventing them from binding to or entering host cells.
Opsonisation: Coating the pathogen, tagging it for destruction by phagocytes.
11.2.2 Monoclonal Antibodies (MABs)
Monoclonal antibodies are antibodies that are identical because they are all produced by clones of a single plasma cell. They are highly valuable because of their absolute specificity.
The Hybridoma Method (Outline):
An animal (e.g., a mouse) is injected with the target antigen to stimulate a primary immune response, leading to the production of specific B-lymphocytes.
These specific B-lymphocytes are extracted from the animal’s spleen.
The B-cells are fused with myeloma (cancer) cells. Cancer cells divide indefinitely, but do not produce antibodies. The fusion creates a hybridoma cell.
The resulting hybridoma cell combines the properties of both: it produces the specific antibody AND divides indefinitely in culture.
These hybridoma cells are cultured to produce unlimited quantities of the specific monoclonal antibody.
Uses of Monoclonal Antibodies:
Diagnosis: Used in pregnancy tests (detecting the hormone hCG) and screening for disease antigens or markers (e.g., certain types of cancer).
Treatment: Used to target specific cell types, such as cancer cells. Antibodies can be attached to drugs or toxins and delivered precisely to the cancerous site, minimizing damage to healthy cells.
11.2.3 Types of Immunity
Immunity can be categorized based on how it is acquired (active vs. passive) and the source (natural vs. artificial).
A. Active Immunity
The body manufactures its own antibodies/memory cells in response to an antigen. It is slow acting but provides long-term immunity.
Natural Active Immunity: Immunity achieved after getting sick with an infectious disease (e.g., catching chickenpox).
Artificial Active Immunity: Immunity achieved through vaccination (deliberate exposure to altered antigen).
B. Passive Immunity
The body receives pre-formed antibodies from another source. It is fast acting but provides short-term immunity (as the antibodies are eventually broken down and no memory cells are made).
Natural Passive Immunity: Antibodies passed from mother to baby across the placenta or through breast milk.
Artificial Passive Immunity: Antibodies injected directly into the body (e.g., antitoxins for tetanus or rabies).
11.2.4 Vaccination and Disease Control
A vaccine contains antigens (from a dead, weakened, or fragmented pathogen) that are harmless but sufficient to stimulate a primary immune response.
How Vaccines Work (Providing Long-Term Immunity):
The vaccine (containing antigens) is introduced into the body.
The body mounts a primary immune response, producing plasma cells and specific antibodies, but crucially, memory cells (T and B cells).
If the real, virulent pathogen invades later, the memory cells trigger a rapid and powerful secondary immune response, destroying the pathogen before symptoms develop.
Vaccination Programmes and Disease Control:
Vaccination programs are essential in controlling infectious diseases by establishing herd immunity.
Herd Immunity: When a high enough proportion of the population is vaccinated, the disease cannot easily spread because there are too few susceptible hosts. This protects those who cannot be vaccinated (e.g., infants, people with compromised immune systems).
By breaking the chain of transmission, vaccination programs dramatically reduce the prevalence of transmissible diseases in the community, often leading to their control or even global elimination (like smallpox).
Key Takeaway: Antibodies link structure to function through their specific variable regions. Vaccination is artificial active immunity, relying on memory cells to prevent future illness and establish herd immunity.