Four features characterize the immune system

The characteristic features of the immune system are specificity, the ability to respond to an enormous diversity of foreign molecules and organisms, the ability to distinguish self from nonself, and immunological memory.

I A fragment of a bacterium binds to the receptor CD14.

Toll receptor is activated.

A protein kinase cascade occurs.

I A fragment of a bacterium binds to the receptor CD14.

Toll receptor is activated.

Plasma Membrane Features

A protein kinase cascade occurs.

The transcription factor, NF-kB is phosphorylated.

mRNA

Translation

The transcription factor, NF-kB is phosphorylated.

mRNA

Translation

18.5 Cell Signaling and Defense Binding of a molecule from a pathogen to a receptor initiates a signal transduction pathway that results in the transcription of genes whose products are involved in defense against the pathogen.

specificity. As we saw above, the lymphocytes (B cells and T cells) are involved in specific defense mechanisms. T cell receptors and the antibodies produced by B cells recognize and bind to specific nonself substances. Organisms or molecules that are recognized by and interact with these cells to initiate an immune response are called antigens. The specific sites on antigens that the immune system recognizes are called antigenic determinants or epitopes (Figure 18.6). Chemically, an antigenic determinant is a specific portion of a large molecule, such as a certain sequence of amino acids that may be present in several proteins. A large antigen, such as a whole cell, may have many different antigenic determinants on its surface, each capable of being bound by a specific antibody or T cell. Even a single protein has multiple, different antigenic determinants. The host animal responds to the presence of an antigen by producing highly specific defenses—T cells or antibodies that are complementary to, or fit, the antigenic determinants of that antigen. Each T cell and each antibody is specific for a single antigenic determinant.

diversity. Challenges to the immune system are numerous: individual foreign molecules, viruses, bacteria, pro-tists, and multicellular parasites. Each of these types of

Antigenic determinants are a small part of antigens— for example, a part of a protein on a viral coat.

Antibodies react with antigenic determinants.

Antigenic determinants are a small part of antigens— for example, a part of a protein on a viral coat.

Antibodies react with antigenic determinants.

Antigen And Antibody Symbols
The antibody symbols used throughout the chapter are based on the structures shown in Figure 18.10.

18.6 Each Antibody Matches an Antigenic Determinant Each antigen has many different antigenic determinants that are recognized by specific antibodies. Each antibody recognizes and binds to its particular antigenic determinant to initiate defensive measures against the antigen.

potential pathogens includes many species, each species includes many subtly differing genetic strains, and each strain possesses multiple surface features. Estimates vary, but a reasonable guess is that humans can respond specifically to 10 million different antigenic determinants. Upon recognition of an antigenic determinant, the immune system responds by activating lymphocytes (B cells and T cells) of the appropriate specificity.

distinguishing self from nonself. The human body contains tens of thousands of different proteins, each with a specific three-dimensional structure capable of generating an immune response. Every cell in the body bears a tremendous number of antigenic determinants. A crucial requirement of an individual's immune system is that it recognize the body's own antigenic determinants and not attack them.

immunological memory. After responding to a particular type of pathogen once, the immune system "remembers" that pathogen and can usually respond more rapidly and powerfully to the same threat in the future. This immunological memory usually saves us from repeats of childhood diseases such as chicken pox. Vaccination and inoculation against disease work because the immune system "remembers" the antigenic determinants that are introduced into the body.

These four features of the immune response are seen in both components of the immune system, the humoral response and the cellular response.

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