Biology, Eighth Edition (Raven)

Chapter 51: The Immune System

The Immune Response

Why is it necessary to have so many different types of cells in the immune system? The immune system of vertebrates is characterized by acquired responses that are highly specific to particular antigens. This system has the advantage of having a ‘cellular memory’ for previous infections. Nonspecific, non-acquired defense is found in all animals and is effective in destroying invading microbes. However it is most effective outside of the host cells. The specific defenses of acquired immunity can, not only recognize specific pathogens, but also recognize cells that have been infected by those pathogens. The high degree of specificity of the vertebrate immune system requires many different kinds of cells.

A second advantage of the acquired responses of the immune system is a ‘cellular memory’ that provides a much stronger defense against re-infection. After exposure to a disease the cells producing antibodies for that disease persist in the body for long periods. If the disease re-enters the body they can quickly attack and destroy it without the time delay needed to build up large numbers of cells. The disease can be removed before it has time to take hold.

View the animation below, then complete the quiz to test your knowledge of the concept.


Foreign substances that elicit an immune response are termed

Macrophages secrete ________ which then activates ________ .
A)antibodies ; helper T cells
B)interleukin-1 ; cytotoxic T cells
C)interleukin-1 ; helper T cells
D)interleukin-2 ; cytotoxic T cells
E)interleukin-2 ; helper T cells

The human body makes
A)only one type of cytotoxic T cell.
B)tens of different types of cytotoxic T cells.
C)hundreds of different types of cytotoxic T cells.
D)thousands of different types of cytotoxic T cells.
E)millions of different types of cytotoxic T cells.

Helper T cells can stimulate the proliferation of cytotoxic T cells but not that of B cells.

The secondary immune response to a previously encountered pathogen is swifter and stronger than the primary immune response.
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