The Immune System: Primary & Secondary Immune Response | A-level Biology | OCR, AQA, Edexcel

Summary

The video explains how the immune system responds to a pathogen the first time it encounters it and how it responds more quickly if the same pathogen returns.

Primary immune response

A primary immune response is the response to a pathogen’s first infection. Because the immune system has not encountered the pathogen before, B and T lymphocytes must identify its antigens and mount a specific response. Finding lymphocytes with matching receptors takes time.

The primary response proceeds as follows:

1. Clonal selection: A B lymphocyte with a receptor complementary to the pathogen’s antigen binds to it and is selected.
2. Clonal expansion: The selected B cell divides by mitosis, producing many copies of itself.
3. Differentiation: The cloned cells become:
   • Plasma cells, which release large quantities of the specific antibody into the blood.
   • Memory B cells, which retain information about the pathogen.

The antibodies help the immune system deal with the pathogen. Once the infection is cleared, antibody levels fall. The video explains that the antibodies are eventually eliminated, while memory cells remain.

Because this process takes time, the pathogen may cause damage before it is cleared, so symptoms commonly appear.

Secondary immune response

A secondary immune response occurs when the same pathogen infects the body again.

Memory B cells quickly recognize the pathogen’s antigens and become activated, rapidly producing plasma cells. These plasma cells then release the matching antibodies.

Memory T cells also respond. As described in the video, they can develop into:

• T helper cells, which help plasma cells develop.
• T killer cells, which destroy infected body cells.

The secondary response is faster and more vigorous than the primary response because the immune system does not need to find and select the appropriate B cell from scratch or repeat the same initial clonal-selection and expansion stages.

The faster antibody response can clear the pathogen before it causes noticeable damage, so symptoms may be absent or reduced.

Comparing the responses

The video describes a graph with time in days on the horizontal axis and antibody concentration on the vertical axis:

• After the first infection, antibody concentration rises slowly, peaks after a delay, and then falls as the infection is cleared. The presenter notes that the peak may take around 10–30 days for some illnesses.
• After a second infection by the same pathogen, antibody concentration rises much sooner and reaches a higher level.
• The key distinction is that the secondary response produces more antibodies earlier, often quickly enough to prevent symptoms.

Speakers and sources featured

• Speaker: One male presenter or narrator; not named in the subtitles.
• Sources: No external sources are cited in the subtitles. The outro refers viewers to SnapRevise.