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Basic Immunology (Revision)Dr Z Makatini

Introduction

• Immunity (derived from immunitas: Latin for exemption from civic duties and prosecution)

• Means protection from disease and especially infectious disease

• Cells and molecules involved in such protection constitute the immune system and

• The response to introduction of a foreign agent is known as the immune response.

Innate and Adaptive Immunity• The normal individual has two levels of

defense against foreign agents. • The first type is present in neonatal

animals and in invertebrates namely natural or innate immunity

• The second type of immunity is adaptive or acquired immunity and is confined to vertebrates.

Innate and Adaptive ImmunityInnate (natural) responses occur to the

same extent however many times the infectious agent is encountered, whereas

acquired (adaptive) responses improve on repeated exposure to a given infection.

Innate Immunity

• The innate responses use phagocytic cells (neutrophils, monocytes, and macrophages), cells that release inflammatory mediators (basophils, mast cells, and easinophils), and natural killer cells.

• The molecular components of innate responses include complement, acute-phase proteins, and cytokines such as the interferons.

Innate (or natural) immunity• Physical barriers are the first line of

defense against infection • Physiological factors • Protein secretions • Phagocytic cells

Physical barriers are the first line of defense against infection

The skin and mucous membranes provide a continuous surface which must be breached and back this up with mechanical protection through cilia and mucous.

Physiological factors pH, temperature and oxygen tension limit

microbial growth. The acid environment of the stomach

combined with microbial competition from the commensal flora inhibits gut infection.

Protein secretions into external body fluids

Eg lysozyme also help resist invasion. Soluble factors within the body such as

complement, interferons and collectins and other "broadly specific" molecules such as C-reactive protein are of considerable importance in protection against infection.

Phagocytic cells Critical in the defense against bacterial and

simple eukaryotic pathogens. Macrophages and Polymorphonuclear

leucocytes (PMN) can recognize bacterial and yeast cell walls through broadly specific receptors (usually for carbohydrate structures) and this recognition is greatly enhanced by activated complement (opsonin) as well as by specific antibody.

Acute Inflammation• The acute inflammatory response is a key part of the

innate immune system. • Many infections, especially where small wounds are the

route of entry, are eliminated by the combination of complement and recruitment of phagocytes, which flow from the acute inflammatory response.

• A defining aspect of the innate immune system is that it carries no memory of an encounter with a foreign organism.

Question?

• Which cells and molecules constitute the innate response?

What is an antigen?• An antigen is defined as "anything that can be

bound by an antibody". • This can be an enormous range of substances

from simple chemicals, sugars, small peptides to complex protein complexes such as viruses.

• The small antigens are not, however immunogenic in themselves and need to be coupled to a carrier to elicit an immune response.

• Such small antigens are referred to as haptens.

What is an antigen?• In fact antibodies interact specifically with

relatively small parts of molecules. These are known as antigenic determinants or epitopes.

• Sometimes the epitope is composed of a string of amino acids as might be found in a short peptide, such epitopes are said to be linear.

• Other epitopes are formed by more complex 3-dimensional structures present only as part of a native protein, such epitopes are called conformational

Adaptive immunity• The second level of defence increases in

strength and effectiveness with each encounter.

• The foreign agent is recognised in a specific manner and the immune system acquires memory towards it.

Primary Response• The first encounter with an antigen is

known as the primary response. • Re-encounter with the same antigen

causes a secondary response that is more rapid and powerful.

Adaptive Immunity• Acquired immunity is a useful evolutionary

adaptation because it improves the efficacy of the innate immune response by focusing the response to the site of invasion/infection as well as providing additional effector mechanisms that are unique to lymphocytes.

Lymphocytes• The difference between innate and acquired

immunity lies in the antigen specificity of lymphocytes. This property is conferred upon lymphocytes by the expression of cell surface receptors that recognize discrete parts of the antigen known as antigenic epitopes.

• The cell surface receptor of B lymphocytes, derived from Bone marrow in mammals or the Bursa of fabricius in chickens, is an immunoglobulin molecule which, when secreted by the B cell, is known as an antibody

• Antibodies have two ends. One end interacts with the antigen (the variable part) leaving the other (constant) end free to interact with the immunoglobulin receptors on these cells. During an immune response, a complex lattice of interlinked antigens and antibodies, known as an immune complex, will present an array of constant regions which can activate the various cells mentioned above through ligation of their immunoglobulin receptors.

Immune cells

LYMPHOCYTES

• These cells have receptors for antigen and confer specificity on an immune response.

• Lymphocytes express receptors with varying affinity for the antigen in question.

• The cell with the highest affinity for the most abundant antigen will have growth advantage and will preferentially generate progeny of itself. This process is called clonal expansion and is antigen driven

B lymphocytes

• Produce antibodies and some soluble mediators called cytokines. They arise in the bone marrow in adult mammals.

Antibodies• Antibodies work in three ways. • Neutralization. blocking the biological activity of

their target molecule e.g a toxin binding to it's receptor

• Opsonisation. interact with special receptors on various cells, including macrophages, neutrophils, basophils and mast cells allowing them to "recognise" and respond to the antigen

• Complement Activation. cause direct lysis by complement complement recruitment also enhances phagocytosis

T lymphocytes • Arise in bone marrow but mature in the thymus.

They do not produce antibody molecules but have surface receptors structurally related to Ig.

• T cells see antigen in a different way to B cells. They recognize peptide fragments of antigen complexed with cell surface MHC glycoproteins on neighbouring cells.

• The cell surface glycoproteins encoded by genes in the Major Histocompatibility Complex (MHC) bind fragments of antigen after it has been subjected to antigen processing

T cells• There are two sub-types of T cell defined

on the basis of function, accessory molecule expression and the type of MHC protein presenting antigen to them. This can be summarized as follows:

Natural killer (NK) cells • Are large granular lymphocytes that are cytotoxic in the

absence of prior stimulation. • NK cells represent a first line of defence to infections,

tumour growth and other pathogenic alterations of tissue homoeostasis.

• NK cells do not express antibodies or T cell receptors at their cell surface. They produce cytokines and express receptors for immunoglobulin.

• They also possess other receptor molecules which allow them to detect some infected host cells, including tumour cells, virus, or intracellular bacteria-infected cells.

MONONUCLEAR PHAGOCYTES• If you inject "vital" dyes into experimental

animals they will be taken up by various cell types including macrophages (mf), microglial cells in the CNS, endothelial cells of vascular sinusoids and reticular cells of lymphoid organs.

• These are the cells of the Reticulo-Endothelial System (RES). These cells all take up dye by pinocytosis.

• Only cells of the monocyte-macrophage lineage take up large particulate antigens, pieces of tissue, senescent cells, bacteria etc. by phagocytosis

MONONUCLEAR PHAGOCYTES

• Express receptors for antibody and complement which means that they bind immune complexes, especially if the antibody involved has complement components bound to it (if the antibody has fixed complement), and endocytose/phagocytose these rapidly.

• They act as scavengers for cell debris and senescent cells (Kupffer cells in the liver bind "old" erythrocytes).

DENDRITIC CELLS

• Cells of the dendritic cell (DC) lineage are bone marrow derived.

• In the skin they are known as Langerhans Cells (LC). These cells efficiently process antigen but cannot present it to T cells.

MONONUCLEAR PHAGOCYTES

• Have been shown to pick up antigen in skin and carry it via afferent lymphatic vessels to lymph nodes. Dendritic cells in lymph are known as "veiled" cells. In lymph nodes the cells, now known as tissue dendritic cells or interdigitating cells, may efficiently present antigen if they encounter the right T cell. In fact these are the best APC - far fewer DC are required to initiate an immune response than any other APC.

These cells have important properties

• they express a myeloid receptor (CD14) which serves as a recognition molecule for a wide variety of bacterial envelope molecules, such as LPS from Gram -ve organismsand components of Mycobacterial and Gram +ve cell walls. Ligation of this receptor leads to macrophage activation.

• they can act as antigen presenting cells (APC) for T cells.

• they are activated by T cell derived cytokines leading to increased phagocytosis and microbicidal activity (increased activity of degradative enzymes, nitrogen and oxygen free radical production and prostaglandins etc.).

• Follicular dendritic cells (FDC) are found in lymphoid follicles. They are called dendritic because of their morphology rather than any lineage relationship with DC. In fact, there is considerable uncertainty about their developmental origin [some evidence suggests they are long-lived bone marrow derived cells, other data that they are of epithelial origin].

• FDC have receptors for immunoglobulin and complement and are able to trap antigen at their cell surface, in the form of antigen/antibody/C3d complexes, for long periods of time. They cannot present antigen to T cells but are important in developing responses by B cells.

GRANULOCYTES • There are three types of granulocyte distinguished

according to their histological staining patterns. • Neutrophils, also known as polymophonuclear

leukocytes, express receptors for immunoglobulin and complement and are involved in the acute inflammatory response.

• Eosinophils carry receptors for IgE, are involved in the destruction of IgE coated parasites, such as helminths, and contribute to the response to allergens.

• Basophils are the circulating counterpart of tissue mast cells. They express high affinity receptors for IgE and are stimulated to secrete the chemicals responsible for immediate hypersensitivity following antigen induced aggregation of these receptors.

Lymphoid Tissue• Lymphoid tissue is conveniently divided into the central

or primary and peripheral or secondary organs. • Central organs include the bone marrow and thymus.

Lymphocytes, monocytes and granulocytes derive from precursor stem cells in the bone marrow.

• B lymphocytes migrate directly from marrow to the peripheral lymphoid tissue whereas T lymphocytes undergo further maturation in the thymus.

• The bone marrow and thymus are involved in generating precursor lymphocytes rather than immune responses

Lymphoid Tissue• Once released from the bone marrow and

thymus lymphocytes begin a life of patrol and respond. Some 2.5 x 1010 lymphocytes pass through a lymph node per day with 1-2% of the lymphoid pool traversing the whole lymphoid system every hour. This degree of patrol allows rapid response to infectious agents.

Lymphoid Tissue• The lymph nodes ( see diagram below) and spleen are

designed to optimize interaction between APC and T and B lymphocytes.

• The lymphatic system is a series of vessels which drain and filter the tissue fluids . Lymph fluid enters the node via afferent lymphatic, passes through the sinuses lined with macrophages and leaves via the efferent lymphatic (ultimately all drain into the portal vein).

Lymphoid Tissue• Lymphocytes enter the node primarily from the

blood via specialised endothelia also within the T areas and leave via the efferent lymphatics. Dendritic cells migrating from the tissues enter the node into the T cell areas. B cells entering nodes from the blood must cross the T rich area in transit to the B cell rich areas thus optimising the chance of T-B co-operation.

Lymphoid Tissue

• The B cell rich areas contain mature, resting B cells organised into structures around follicular dendritic cells (primary follicles).

• The spleen is fed by a single artery and does not receive afferent lymphatic drainage. Small splenic arterioles are surrounded by periarteriolar lymphoid sheaths mainly consisting of CD4 with some CD8 +ve T cells. The sheaths are associated with lymphoid follicles of similar organisation to the lymph node.

• The largest concentration of lymphoid tissue is, however, in the gut and other mucosal areas. Here aggregates of lymphoid tissue, similar to lymph nodes in organisation, constitute Peyer's Patches in the lamina propria of the small intestine, tonsils in the

• pharynx, and submucosal lymphoid follicles in the appendix and throughout the upper airways.

A VERY simplified overview of an immune response

• You receive a cut.

Bacteria enter the wound.

Many are destroyed rapidly by complement and the phagocytes recruited through acute inflammation (innate immunity).

Some of the dead bacteria or their breakdown products are taken up by the tissue resident dendritic cells.

The combined action of bacterial products and cytokines (from acute inflammation etc.) activate the tissue dendritic cells.

This causes them to migrate to the local lymph node via afferent lymphatics.

Dendritic cells enter the node in the T cell areas. They become resident there displaying their 'wares'

A VERY simplified overview of an immune response

• T cells enter the node from the blood, trafficking through the T cell area to the efferent lymph. Those which recognise the bacterial antigenic peptides displayed on the dendritic cells stop, activate, divide and differentiate; some later become memory T cells.

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A VERY simplified overview of an immune response

• B cells entering nodes from the blood must cross the T rich area in transit to the B cell rich areas. The antigen-specific ones must acquire antigen too, presumably via the lymph. Then they can have their MHC-peptide complexes recognised by activated T cells and receive help.

A VERY simplified overview of an immune response

• Some become IgM secreting plasma cells. Some migrate to the B cell rich areas and form germinal centres. Here B cells proliferate and give rise to progeny with high affinity for antigen through a process called affinity maturation. The products of germinal centres become IgG,A etc plasma cells and memory B cells