Edexcel A Level (IAL) Biology -6.9 Specific Immune Responses- Study Notes- New Syllabus

Antigens, Antibodies and the Immune Response

🌱 Introduction

The immune system protects the body by recognising and destroying pathogens. It relies on antigens, antibodies, and the activity of macrophages, antigen-presenting cells, and plasma cells. This topic explains how they all link together in one coordinated defence.

🔍 Antigens

What Antigens Are

• Antigens are unique molecules found on the surface of pathogens.
• Usually proteins or glycoproteins.
• The immune system recognises them as foreign.

Role of Antigens

• Act as signals telling the immune system that an invader is present.
• Trigger the production of specific antibodies.
• Help the body distinguish between self and non-self.

🧪 Antibodies

What Antibodies Are

• Y-shaped proteins made by plasma cells.
• Each type of antibody fits one specific antigen (like a key and lock).

How Antibodies Work

• Neutralisation: bind to pathogens or toxins to block their action.
• Agglutination: clump pathogens together to make them easier to engulf.
• Opsonisation: tag pathogens so phagocytes recognise them faster.
• Activation of complement proteins: helps destroy pathogens.

Role of Antibodies

• Make pathogens easier to kill.
• Prevent pathogens from entering or damaging cells.
• Allow phagocytes to clear infections quickly.

🧬 Key Cells in the Immune Response

Macrophages

• Large phagocytes found in tissues.
• First to respond to infection.
• Engulf and digest pathogens.
• After digesting pathogens, they display the pathogen’s antigens on their surface.

Antigen-Presenting Cells (APCs)

• Cells that hold the pathogen’s antigens on their membrane.
• Includes macrophages, dendritic cells, and B cells.
• Present antigens to T helper cells, activating the specific immune response.

Why APCs Are Important

• They link the non-specific response (phagocytosis) to the specific response (antibody production).
• Without antigen presentation, T helper cells cannot activate B cells.

Plasma Cells

• Activated B cells differentiate into plasma cells.
• Plasma cells produce large quantities of antibodies.
• They release antibodies into the blood and tissues.
• Short-lived but extremely active during infection.

🧷 Putting It All Together: Sequence of Immune Response

Step-by-step

• Pathogen enters the body and macrophages engulf it.
• Macrophage becomes an antigen-presenting cell by placing antigens on its surface.
• APC interacts with a T helper cell, activating it.
• Activated T helper cell stimulates B cells.
• B cells differentiate into plasma cells.
• Plasma cells release specific antibodies matching that antigen.
• Antibodies neutralise pathogens, help phagocytes, and stop infection.
• Leads to pathogen clearance and formation of memory cells.

📋 Summary Table