Significance and Result of Antigen Presented by MHC Class II Molecules on a B Cell

When a naive B cell binds antigen via its B-cell receptor, the cell brings the antigen into the cell, enclosed within a membrane-bound vesicle called an endosome. The fate of the proteins within the endosome is identical to that of proteins within the macrophage phagosome, resulting in peptides being loaded into the groove of an MHC class II molecule (see figure 16.9).

If a Th2 cell encounters a B cell bearing the peptide: MHC class II complex it recognizes, it responds by synthesizing cytokines and delivering them to that cell. These cytokines activate the B cell, enabling it to proliferate as well as undergo affinity maturation and class switching. The cytokines also drive the formation of memory cells. Note that the T-cell receptor could

Chapter 16 The Adaptive Immune Response

Chapter 16 The Adaptive Immune Response

Macrophage engulfs materials
Macrophage degrades proteins in phagosome into peptide fragments

T-cell receptor CD4

Secretion of cytokines

Targeted delivery of cytotoxins activate macrophage

Peptide fragments from engulfed material are presented by MHC class II molecules

Effector T-helper cell recognizes a peptide being presented by the macrophage and responds by activating the macrophage

T-cell receptor CD4

Targeted delivery of cytotoxins activate macrophage

Peptide fragments from engulfed material are presented by MHC class II molecules

Secretion of cytokines

Effector T-helper cell recognizes a peptide being presented by the macrophage and responds by activating the macrophage

Figure 16.19 Antigen Presentation by a Macrophage to an Effector T-Helper Cell be recognizing any one of the various peptides generated from the antigen during antigen processing and presentation. Thus, the epitope to which the effector T cell responds is most likely different from the one that the B-cell receptor recognized. In fact, a B cell that binds to a bacterium is probably recognizing an epitope on the surface of that cell, whereas the T-helper cell could very well be responding to a peptide from one of the bacterium's cytoplasmic proteins being presented by the B cell.

Understanding the mechanisms used in antigen processing and presentation is what led to an effective vaccine for children against what was the most common cause of meningitis in children, Haemophilus influenzae. Recall that young children are particularly susceptible to meningitis caused by this organism because it produces a polysaccharide capsule, an example of a T-independent antigen to which this age group responds poorly. Polysaccharide antigens can be converted to T-dependent antigens by covalently attaching, or conjugating, them to large protein molecules; this is done to make what is called a conjugate vaccine. The polysaccharide component of the vaccine binds to the B-cell receptor and the entire molecule is taken in. The protein component will then be processed and presented to a Th2 cell. Although the B cell recognizes the polysaccharide component of the vaccine, the Th2 cell recognizes peptides from the protein component. The Th2 cell then activates the B cell, leading to the production of antibodies that bind the capsule.

The requirement for antigen processing and presentation also explains how some people develop allergies to penicillin. This medication is a hapten, a molecule that binds a B-cell receptor yet does not elicit the production of antibodies unless it is attached to a protein carrier. In the body, penicillin can react with proteins, forming a penicillin-protein conjugate. The conjugate functions in a manner analogous to the Haemophilus influenzae conjugate vaccine, resulting in antibodies that bind penicillin. The reaction of IgE antibodies with penicillin can result in allergic reactions, precluding the further use of the antimicrobial medication in these reactive individuals. ■ allergy, p. 441 ■ penicillin, pp. 60,514

Activation of T Cells

Like B cells, naive T cells require supporting signals in order to become activated. Once activated, they proliferate and develop their effector functions. They also form memory cells. Dendritic cells, the scouts of innate immunity, play a pivotal role in this process. They bring antigens to naive T cells and help to educate those cells about the possible origins of various antigens. Activated macrophages can also activate naive T cells, but their role in this process is not nearly as significant as that of dendritic cells. ■ dendritic cells, p. 378

Upon activation, the T cell produces both the cytokine that stimulates T-cell growth (IL-2) and the receptor for that cytokine, allowing the cell to stimulate its own proliferation. Later, it begins producing additional cytokines and adhesion molecules that allow it to gain its effector functions. The effector T cells can leave the secondary lymphoid organs and circulate in the bloodstream; they can also enter tissues, particularly at sites of infection.

As activated T-helper cells proliferate, they differentiate into either Th1 or Th2 cells. The factors that dictate the outcome of this differentiation process are not fully understood, but they appear to involve cytokines as well as other stimuli.

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