Differences between Primary and Secondary Immune Response

In a primary immune response, naive B cells are stimulated by antigen, become activated, and differentiate into antibody-secreting cells that produce antibodies specific for the eliciting antigen. A secondary immune response is elicited when the same antigen stimulates memory B cells, leading to the production of greater quantities of specific antibody than are produced in the primary response. Some of the differences between Primary and Secondary Immune Response are as follows:

Figure: A schematic diagram showing primary and secondary response, Source: Textbook of Microbiology and Immunology, 2/e, Parija

S.N.	Characteristics	Primary Immune Response	Secondary Immune Response
1	Definition	Primary Immune Response is the reaction of the immune system when it contacts an antigen for the first time.	Secondary Immune Response is the reaction of the immune system when it contacts an antigen for the second and subsequent times.
2	Appearance	Appears mainly in the lymph nodes and spleen.	Appears mainly in the bone marrow and then, in the spleen and lymph nodes.
3	Occurrence	Occurs in response to the primary contact of the antigen.	Occurs in response to the second and subsequent exposure to the same antigen.
4	Antibody Peak	The antibody level reaches its peak in 7-10 days.	The antibody level reaches its peak in 3-5 days.
5	Affinity of Antibody	Low affinity to their antigens.	High affinity to their antigens.
6	Responding Cells	Naive B cells and T cells	Memory B cells
7	Antibodies	Both thymus-dependent and thymus-independent antibodies are involved in the primary immune response.	Only thymus-dependent antibodies are involved in the secondary immune response.
8	Lag Phase	Long (4-7 days)	Short (1-4 days)
9	Types of Antibodies	A large amount of IgM and a small amount of IgG are produced during the primary immune response.	A large amount of IgG, a small amount of IgM, IgA, and IgE are produced during the secondary immune response.
10	Amount of Antibody	Few antibodies are produced in primary immune response.	100-1000 times more antibodies are produced in secondary immune response.
11	Strength of the Response	Primary immune response is usually weaker than secondary immune response.	Secondary immune response is stronger.
12	Antibody level	Antibody level declines to the point where it may be undetectable.	Antibody level tends to remain high for longer.

Figure: Primary and secondary humoral immune responses, Source: Cellular and Molecular Immunology, Abbas, 9th Edition.

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