The non-specific and specific immune responses, the roles of phagocytes, B and T lymphocytes, the action of antibodies, immunological memory, and the principles of vaccination.

What this dot point is asking

Edexcel wants you to describe the non-specific and specific immune responses, explain the roles of phagocytes and B and T lymphocytes, describe the action of antibodies, explain immunological memory, and explain how vaccination provides immunity. The full sequence of the specific response and the basis of immunity and herd immunity are heavily examined.

Non-specific and specific responses

A phagocyte engulfs a pathogen by phagocytosis, then digests it with enzymes from lysosomes.

The specific immune response

An antibody is a Y-shaped protein (an immunoglobulin) with a specific shape. Its variable region has a binding site complementary to a specific antigen, forming an antigen-antibody complex. Antibodies work by neutralising toxins, marking pathogens for phagocytosis (opsonisation) and clumping pathogens together (agglutination) so they are easier to engulf. Because each antibody is a protein folded to a precise shape, its specificity comes from its tertiary structure, which links back to the proteins topic.

Immunological memory

After an infection, long-lived memory cells (memory B and T cells) remain in the body. In the primary response, the body is slow to make antibodies because the right lymphocytes must be selected and multiply, so symptoms appear. If the same pathogen returns, the memory cells recognise the antigen and divide rapidly, so the secondary response is faster and produces far more antibodies, usually destroying the pathogen before symptoms appear. This is the basis of long-term immunity.

Vaccination

A vaccine contains antigens (for example a weakened or dead pathogen, a fragment, or its antigens). It triggers a primary response and the formation of memory cells without causing the disease, so the person mounts a fast secondary response and is protected on future exposure. Widespread vaccination can produce herd immunity: when a high enough proportion of the population is immune, the pathogen cannot spread easily, indirectly protecting those who cannot be vaccinated.

Examples in context

Example 1. Measles and herd immunity. Measles is highly infectious, so about $95\%$ of a population must be immune to stop it spreading. When vaccination rates fall below this threshold, outbreaks return even among vaccinated communities because transmission chains can re-establish. This shows why the herd-immunity threshold depends on how infectious the pathogen is.

Example 2. Antigenic variation in influenza. The influenza virus changes its surface antigens frequently, so memory cells from a previous infection or vaccine no longer recognise the new strain and a fresh primary response is needed. This is why a new flu vaccine is produced each year, and it explains why immunity to one strain does not protect against another, a common exam application of the specificity of the response.

Try this

Q1. Describe the role of plasma cells in the immune response. [2 marks]

• Cue. They are formed from activated B lymphocytes and secrete antibodies specific to the antigen.

Q2. Explain why the secondary immune response is faster than the primary response. [2 marks]

• Cue. Memory cells made in the primary response remain, so they recognise the antigen and divide rapidly into plasma cells.