Lymphocytes display the four attributes listed above. There are two
types of lymphocytes: B cells and T cells. B cells are involved in humoral
immunity. On first exposure to an antigen it recognizes on its surface, a
naïve B cell proliferates to produce a memory B cell and a plasma cell that
is the effector, synthesizing antibodies for release. T cells are unable to
recognizeantigen on their own. T cells require antigen-presenting cells
(macrophages, dendritic cells,andB cells) to process antigen and bind it
to membrane proteins known as major histocompatibility molecules
(MHC). Class I MHCis found on virtually all cells of the body, while
Class II MHC is specific for antigen-presenting cells (APCs).On first
encounter with a specific antigen-MHC II complex, the T cell proliferates
resulting in a memory T cell and an effector T cell. There are several sub-
types of T cells:
- THcells,which express the specific glycoprotein, CD4, are called CD4+
- TCcells,which express the specific glycoprotein, CD8, are called CD8+
- Tregcells, (suppressor T cells) are critical for maintenance of immuno-
logical tolerance.
The THcell secretes cytokines (small protein or peptide intercellular
communication molecules) that activate TCcells, B cells, and macrophages.
Release of cytokines by THcells induces TCcells to proliferate and differen-
tiate into effector cells. In that case the effectors are cytotoxic T lympho-
cytes (CTLs)that recognize Class I MHC-antigen complex on the surface
of altered self cells (e.g., cells of a foreign tissue graft or self cells infected
with virus). CTLs recognize altered self-cells and form a conjugate with
them; the target cell is killed by release of perforins(pore-forming pro-
teins) and serine proteases (granzymes)leading to apoptosisof the target.
THcells are regulated by APCs that internalize antigen and then
process it through the secretory pathway to a membrane-bound MHC II-
antigen complex. The THcell is activated by a co-stimulatory signal from
the APC. Cytokines from THcells are required for B cells to proliferate and
differentiate into a memory B cell and an antibody-producing plasma cell.
Two of the key cytokines involved in the interaction of those cells are IL-1
produced by activated macrophages and IL-2 that regulates T cell, but also
B cell proliferation. Another critical cytokine is interferon-gamma (IFN-γ)
that activates macrophages and is secreted by activated THcells. There are
two profiles of cytokines produced by THcells:
26 Anatomy, Histology, and Cell Biology