Recognition of extracellular pathogens
Innate recognition mechanisms
 Infection induces the production of inflammatory mediators by macrophages:
these include the cytokine interleukin-6 (IL-6). IL-6 induces the liver to
produce large quantities of acute-phase proteins that include the important
pattern recognition molecules C-reactive protein (CRP) and mannan-binding
lectin (MBL) that bind to pathogen associated molecular patterns (PAMPs) on
microbial surfaces.

Leucocytes express a range of pattern recognition receptors (PRRs). For
example, the PRRs expressed by macrophages include mannose receptor and
scavenger receptor.

Toll-like receptors (TLRs) are PRRs expressed by macrophages, dendritic cells
and various other cell types. They interact with a range of microbial PAMPs
leading to cellular stimulation and the generation of immune activity.
Components of bacteria, viruses, fungi and protozoa interact with TLRs and
include lipopolysaccharides, lipoproteins, carbohydrates, proteins and nucleic
acids.
Antigen recognition by antibodies and B lymphocytes
 Any molecule that is recognised specifically by a lymphocyte or an antibody is
referred to as an antigen. Proteins, carbohydrates, lipids and even nucleic
acids can serve as antigens for recognition by antibodies. Antibodies are
antigen recognition proteins secreted by B lymphocytes (B cells): ‘antibody’ is
a functional name, i.e. ‘against foreign bodies’, and the structural name for
antibody is immunoglobulin (Ig), i.e. ‘immune globular proteins’. They have a
Y-shaped structure with an antigen combining site at the tip of each arm:
these two sites are identical on a single antibody and therefore have the same
antigenic specificity. The precise region of an antigen that interacts with an
antigen combining site is termed the antigenic determinant or epitope.

The specificity of interaction between a combining site and an epitope is
determined by complementarity of shape and charge that maximises the
potential for attractive non-covalent interactions leading to high affinity
binding.

The antigen recognition receptors expressed by B cells are surface
immunoglobulins (sIg) that specifically bind antigens to the B cell surface. All
the sIg molecules expressed by a single B cell have identical antigen
combining sites, i.e. a single B cell is specific for a single antigen epitope.
When a B cell binds an antigen to its sIg and becomes activated, it
differentiates into a plasma cell that secretes antibodies with the same
combining sites (i.e. the same antigenic specificity) as the sIg: thus a B cell
produces antibodies that recognise the antigen it originally bound.

The body contains millions of families or clones of B cells, each expressing Ig
with a different antigen combining site. Thus, an antigen will interact with
those clones whose sIg bind it with highest affinity – termed clonal selection.
Activation of the B cells that bind antigen results in their proliferation, thus
increasing the number of B cells of the specific clones, and some of these are
maintained in the body as memory cells. If the same antigen enters the body
again, it will interact specifically with these memory cells, which are
reactivated more rapidly and in larger numbers than in the primary response,
hence giving the faster and bigger response that characterises adaptive
immunity. This is why exposure to certain microbes generates immunity to
re-infection by the same microbe. It is also the basis of vaccination
(immunisation) in which non-infective forms of microbial antigens are
deliberately inoculated into the body to induce a primary response generating
memory lymphocytes that confer immunity to the infective microbe
expressing the same antigens.
Immunoglobulin structure and classes (isotypes)
 The Y-shaped Ig molecule is composed of four polypeptide chains (held
together by disulphide bonds) – two identical heavy chains and two identical
light chains. The N-terminal regions of paired heavy and light chains form an
antigen combining site. Each chain is composed of a sequence of globular
regions called Ig domains: there are four or five domains in each heavy chain
and two in each light chain. Each domain is composed of about 110 amino
acids folded into two beta-pleated sheets with a characteristic tertiary
structure stabilised by a disulphide bond. The N-terminal domains of each
heavy and light chain, which form the antigen combining sites, vary in
structure between B cells or antibodies with different antigenic specificities,
and hence are called variable domains (VH and VL). The other constant
domains (CH and CL) have the same structure in different B cells and
antibodies. Between CH1 and CH2 is a flexible hinge region. The two arms of
the ‘Y’ above the hinge region are termed Fab regions (‘fragment antigen
binding’): each Fab is composed of VH/CH1 and VL/CL, and contains an antigen
combining site. The ‘stalk’ of the ‘Y’ below the hinge region is composed of
the other CH domains and is called the Fc region: this interacts with other
molecules of the immune system involved in generating defensive activities.
The only difference between secreted antibodies and sIg is that the latter
contains an additional amino acid sequence at the C-terminus of each heavy
chain that anchors the molecule in the B cell surface membrane.

There are two types of light chain (kappa and lambda). There are five types
of heavy chain, which differ in the amino acid sequences of their constant
domains, and are designated by the Greek letters , , , , : these identify
the five Ig classes IgM, IgG, IgA, IgE and IgD, respectively. There are four
subclasses of IgG (IgG1, IgG2, IgG3, IgG4) and two subclasses of IgA (IgA1,
IgA2). The differences in CH domain amino acid sequences are greater
between classes than between subclasses. The nine classes/subclasses are
called Ig isotypes.

All B cells, when they develop from stem cells in the bone marrow, initially
express IgM (and some also express IgD) as their sIg. When a B cell is
activated following binding of antigen, it may secrete IgM antibodies, or may
undergo class switching to produce IgG, IgA or IgE: this involves changing
the CH domains expressed in the newly synthesised immunoglobulins without
changing the VH domain or the light chains (VL + CL). Thus, the class of Ig is
changed without altering its antigenic specificity.

IgG, IgA and IgD each have three CH domains in their heavy chains, whereas
there are four CH domains in IgM and IgE heavy chains. IgG, IgE and IgD all
exist as monomers, i.e. single Y-shaped units. IgA antibodies are monomers
or dimers (two units) and IgM antibodies are pentamers (five units). In these
dimers and pentamers the monomers are linked together by a polypeptide
called the J-chain (joining chain). All the antigen combining sites on an IgA
dimer or an IgM pentamer are identical.

The antibody isotypes differ in their tissue distribution and so can help to
confer immunity in different parts of the body.

IgM is mainly restricted to the blood stream because it is too large to diffuse
readily across blood vessel walls (except in areas of inflammation).

IgG is the most plentiful class of Ig in the blood and is also found in tissue
fluids. A special property of IgG is that it is transferred across the placenta
from mother to fetus because of a special IgG binding receptor (FcRn)
expressed by the placenta. Thus much of the early immune protection of a
neonate, when its own immune system is still developing, is due to the
mother’s IgG acquired during gestation.

IgA is present in blood and internal tissue fluids as monomers. The dimeric
IgA is transported across mucosal epithelium into the mucosal secretions of
the gastro-intestinal, respiratory and genito-urinary tracts. This occurs
because of an IgA binding receptor (the poly-Ig receptor) expressed by
mucosal epithelial cells: part of this receptor remains attached to the secreted
IgA dimer and is called secretory piece. Thus, IgA is important in immunity at
mucosal surfaces. IgA is also secreted by the mammary gland into milk, and
so mother’s IgA is transferred to confer protection in the gut of suckling
infants.

IgE is present at very low levels in the blood because most binds to specific
receptors (FcR1) expressed by mast cells in tissues.

IgD appears to function only as an antigen receptor on B cell surfaces and
only very small amounts are secreted.
Recommended reading:
Todd I, Spickett G (2005) Lecture Notes: Immunology. 5th edition. Blackwell
Publishing. Chapters 2 and 5. OR
Todd I, Spickett G (April 2010) Lecture Notes: Immunology. 6th edition.
Wiley/Blackwell. Chapters 2 and 5.