Adaptive Immunity

• • • • • • • • Adaptive immunity develops as the need arises.

Two types: - Humoral immunity - Mediated by B cells (B lymphocytes) target - Produce antibodies that directly antigens of invaders - Cell-mediated immunity - Involve T cells (T lymphocytes) - Control antibody production and can directly kill infected host cells

• • The immune system does not recognize the whole microbe, but innumerable tiny pieces of it.

- Each small segment of an antigen that elicits an immune response is called an epitope or an antigenic determinant.

Figure 24.2

Antibody Structure and Diversity

• • An antibody (immunoglobulin) is a Y-shaped structure made up of four polypeptides: - Two large heavy chains and two smaller light chains, connected by disulfide bonds

Figure 24.7

• • • An antibody has constant and variable regions.

- Variable regions form the antigen-binding site.

Functional parts of the antibody can be separated following certain protease treatments.

Figure 24.9

• • All antibody isotypes have the same basic structure. - However, each has a unique “super” structure.

Figure 24.11

Time Course of an Immune Response

Primary vs. Secondary Antibody Responses

Figure 24.13

Levels of the Immune Response

Levels of the Immune Response

Adaptive Responses Are Specific to Individual Molecules  Antigen: the molecule recognized by the response  Each cell can only recognize one antigen

• • B cells differentiate by clonal selection.

- When a B cell contacts its cognate antigen, it is stimulated to proliferate and differentiate into plasma cells (secrete antibodies) and memory cells.

Figure 24.14

Basic Steps in Antibody Formation

Figure 24.17

The Major Histocompatibility Complex • • • Consists of membrane proteins with variable regions that can bind antigens: - Class I MHC: Found on all nucleated cells - Class II MHC: Found only on antigen-presenting cells

Figure 24.20

Figure 24.21

T Cell Receptors

• • • TCRs associatd with CD3 proteins on cell surface - Bind antigens only if attached to MHC - Complex transduces signal into cell, triggering T cell proliferation.

Figure 24.22

Cell-Mediated Immune Response

• Animation: The Activation of the Humoral and Cell-Mediated Pathways

Click box to launch animation

Figure 24.24

Figure 24.28

Figure 24.30