Describe the lines of defence and explain the specific immune response and immunity

What this dot point is asking

SCSA wants you to organise immune defence into its three lines, distinguish non-specific from specific immunity, and explain how memory underpins long-lasting immunity and vaccination. A strong answer names the key cells and explains what each does.

The first line of defence: barriers

The first line is non-specific and acts to keep pathogens out. It includes physical and chemical barriers:

• Skin as a physical barrier.
• Mucous membranes that trap pathogens, and cilia that sweep mucus out of the airways.
• Chemical defences such as stomach acid, lysozyme in tears and saliva, and the acidity of the skin and stomach.
• Beneficial microbiota that outcompete pathogens.

If pathogens breach these barriers, the second line responds.

The second line of defence: innate response

The second line is also non-specific (it does not target a particular pathogen) and acts quickly. It includes:

• Phagocytosis: white blood cells such as macrophages and neutrophils engulf and digest pathogens.
• The inflammatory response: damaged cells release chemicals such as histamine, which dilate blood vessels and increase their permeability. This brings more blood, white cells and clotting factors to the site, causing the redness, heat, swelling and pain of inflammation.
• Fever: a raised body temperature that slows pathogen reproduction and speeds immune activity.

The third line of defence: specific immune response

The third line is specific and adaptive, and slower to develop but powerful and long-lasting. It is triggered by antigens, molecules (often proteins) on the surface of a pathogen that the immune system recognises as foreign.

The key cells are lymphocytes (a type of white blood cell), made in two types: B cells (mature in bone marrow) and T cells (mature in the thymus).

The cell-mediated response (T cells):

• Helper T cells are activated by antigen-presenting cells (such as macrophages displaying the antigen). Helper T cells release signalling chemicals (cytokines) that coordinate and stimulate the rest of the immune response.
• Cytotoxic (killer) T cells destroy body cells that have been infected by a pathogen (especially virus-infected cells) and abnormal cells.

The humoral response (B cells):

• A B cell whose receptor matches the antigen is activated, helped by signals from helper T cells.
• It divides and differentiates into plasma cells, which secrete large numbers of antibodies.
• Antibodies are Y-shaped proteins specific to one antigen. They neutralise pathogens, clump them together (agglutination), and mark them for destruction by phagocytes.

Memory and immunological memory

During the specific response, some activated B and T cells become memory cells that remain in the body. On first exposure (the primary response), the immune system is slow because it must find and multiply the right lymphocytes, and the person may become ill.

On a second exposure to the same antigen (the secondary response), memory cells respond much faster and more strongly, producing antibodies quickly so the person usually does not become ill. This is the basis of long-term immunity.

Types of immunity

Immunity can be classified by how it is acquired:

• Active immunity: the person's own immune system makes antibodies and memory cells. It is long-lasting. It is natural when gained by recovering from infection, or artificial when gained from vaccination.
• Passive immunity: antibodies are received from another source, giving immediate but short-lived protection with no memory cells. It is natural (antibodies passing from mother to baby across the placenta or in breast milk) or artificial (an injection of antibodies, such as antivenom).

Vaccination introduces a harmless form of an antigen (a weakened, killed or fragment of a pathogen). This triggers a primary response and produces memory cells without causing the disease, so a later real infection meets a fast secondary response.

Why this design works

Layering defences means most pathogens are stopped before the slow specific response is needed, while the specific response provides a tailored attack and lasting memory for those that get through. The specificity of antibodies and the speed of the secondary response together make recovery faster on repeat exposure, which is exactly what vaccination exploits.