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

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The key points covered of this video include:

1. Introduction to Primary and Secondary Responses
2. The Primary Immune Response
3. The Secondary Immune Response

Introduction to Primary and Secondary Responses

When a pathogen infects the body for the first time the initial immune response is slow. This slow response is called the primary immune response. The primary immune response is the initial response caused by a first infection. In the primary response antibodies specific to the pathogen’s antigens are produced. This takes a long time because there are many steps required to produce specific antibodies. When the infection is cleared the specific antibodies do not stay in the blood. When the body is infected for a second time by the same pathogen antibodies have to be made again. This immune response is much quicker the second time - this is called the secondary immune response. The secondary immune response is a more rapid and vigorous response caused by a second or subsequent infection by the same pathogens.

The Primary Immune Response

In the primary immune response the immune system has never come across the pathogen before. Lymphocytes have to first detect the pathogen’s antigens to produce a specific response. To produce specific antibodies B lymphocytes have to undergo clonal selection and clonal expansion. The correct B lymphocyte then has to differentiate into plasma cells and memory B cells. The plasme cells produce lots of the specific antibody which clears the infection. However this whole process takes a long time and symptoms usually appear.

The Secondary Immune Response

When the same pathogen invades the body for a second time there are already memory cells in the blood. Memory cells quickly recognise the specific antigens from the same pathogen. Memory B cells can then rapidly differentiate into plasma cells. Memory T cells are also activated and can differentiate into T killer cells and T helper cells. The secondary response is quicker because clonal selection and clonal expansion are skipped. Plasma cells produce antibodies much sooner and more rapidly in the secondary response than in the primary response. The concentration of antibodies thus reaches a higher concentration in a shorter period of time. This is usually quick enough to prevent any symptoms from appearing.

Summary

The primary response is much slower than the secondary immune response
In the primary response clonal selection, clonal expansion and differentiation have to occur to produce specific antibodies
In the secondary response these steps are skipped
In the secondary response memory cells immediately detect the same pathogen
This causes them to rapidly differentiate into plasma cells
This allows specific antibodies to be produced quickly and at a high concentration
Therefore the infection is cleared before any symptoms can appear

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