Types of Adaptive Immunity, Antigens and Antibodies

Types of Adaptive Immunity:
1- Humoral immunity:
It is mediated by antibodies (produced by B cells). Humoral
immunity is the principal defense mechanism against extracellular
microbes and their toxins.
Antibodies:
Antibodies are a heterogeneous mixture of serum globulins. These
molecules have common structural features, which enable them to do two
things:
(1) recognize and bind specifically to a unique structural entity on an
antigen (namely, the epitope), and
(2) perform a common biologic function after combining with the
antigen.
Immunoglobulin molecules consist of two identical light (L) chains
and two identical heavy (H) chains, linked by disulfide bridges.
The resultant structure is shown in Figure 1.2. The portion of the
molecule that binds antigen consists of an area com- posed of the aminoterminal
regions of both H and L chains. There are five major classes of
H chains (termed ?, ?, ?, ?, and ?). On the basis of differences in their H
chains, immunoglobulin molecules are divided into five major classes—
IgG, IgM, IgA, IgE, and IgD—each of which has several unique biologic
properties. For example:
1- IgG is the only class of immunoglobulin that crosses the placenta,
conferring the mother’s immunity on the fetus.
2- IgA is the major antibody found in secretions such as tears and saliva.
2- Cell-mediated immunity (cellular immunity):
It is mediated by T lymphocytes (T cells). Several major subsets
of T cells:
A- helper T cells (TH cells), which express CD4.
B- cytotoxic T cells (TC cells), which express CD8.
C- T regulatory cell (Treg cells).
The functions related to the various subsets of T cells include the
following:
• B-cell help.
• Inflammatory effects.
• Cytotoxic effects.
• Regulatory effects.
• Cytokine effects.
3
In fact, at least two signals must be delivered to the antigen-specific
T cell for activation to occur:
Signal 1: involves the binding of the TCR to antigen, which must be
presented in the appropriate manner by APCs.
Signal 2: involves co-stimulators that include certain cytokines such as
interleukin-1 (IL-1), IL-4, and IL-6 as well as cell-surface molecules
expressed on APCs, such as CD40 and CD86.
Binding of T cell with APC called immunological synapse. Once T cells are optimally signaled for activation, a series of events
takes place and the activated cell synthesizes and releases cytokines. In
turn, these cytokines come in contact with appropriate cell surface
receptors on different cells and exert their effect on these cells.