Figure 24–47 Comparison of the co-stimulatory proteins required to activate a helper T cell and a b cell in response to the same foreign protein. (A) A naïve helper T cell is activated by a peptide fragment of a foreign protein bound to a class II Mhc protein on the surface of an activated dendritic cell. The co-stimulatory protein on the dendritic cell (a B7 protein— either cD80 or cD86) binds to the cD28 co-receptor on the T cell, providing a necessary co-stimulatory signal to the T cell; in addition, cytokines secreted by the dendritic cell (or other nearby cells) influence what subtype of effector helper cell the  T cell becomes (see Figure 24–44). (B) Once activated to become an effector cell, the helper T cell can help activate B cells that have the same peptide–Mhc protein complexes on their surface as the dendritic cell that activated the T cell. These B cells have Bcrs that bind an antigenic determinant on the surface of a folded foreign protein and endocytose the protein (red arrow); the protein is then cleaved into peptides, which are carried to the B cell surface by class II Mhc proteins, where some of them can be recognized by the Tcrs on the helper T cell (see Figure 24–39). Note that the Bcrs and Tcrs recognize different antigenic determinants of the protein. As indicated, the co-stimulatory protein used by the effector helper T cell is cD40 ligand, which binds to the cD40 co-receptor on the B cell; the T cell also secretes cytokines such as IL4 to help stimulate the B cell to undergo somatic hypermutation and class switching (not shown). The cD4 co-receptor on Th cells is omitted in both (A) and (B) for simplicity.
Figure 24–48 Some of the cell-surface proteins discussed in this chapter that belong to the Ig superfamily. The Ig and Ig-like domains are shaded in gray, except for the antigen-binding domains (not all of which are Ig domains—the class I and class II Mhc proteins are the exception), which are shaded in blue. The Ig superfamily also includes many cell-surface proteins involved in cell–cell interactions outside the immune system, such as the neural cell adhesion molecule (N-cAM) discussed in chapter 19 and the receptors for various protein growth factors discussed in chapter 15 (not shown). There are more than 750 members of the Ig superfamily in humans.
In both vertebrates and invertebrates, many proteins in the Ig superfamily are also found outside immune systems, where they often function in cell–cell recognition and adhesion processes, both during development and in adult tissues. It seems likely that the entire gene superfamily evolved from a primordial gene coding for a single Ig-like domain, similar to that encoding β2-microglobulin (see Figure 24–36). In present-day family members, a separate exon usually encodes the amino acids in each Ig-like domain, consistent with the likelihood that new family members arose during evolution by exon and gene duplications.
There are three main functionally distinct classes of T cells. Cytotoxic T cells (TC cells) directly kill infected cells by secreting perforins and granzymes that induce the infected cells to undergo apoptosis. Helper T cells (TH cells) help activate cytotoxic T cells to kill their target cells, B cells to make antibody responses, macrophages to destroy the microorganisms they harbor, and dendritic cells to activate T cells. Regulatory T cells (Treg cells) produce suppressive proteins (such as the cytokines IL10 and TGFβ) to inhibit other immune cells.