Overview of Adaptive Immunity

Microbiology and immunology_2

The third line of defense

Overview of Adaptive Immunity

Adaptive immunity is the body’s ability to recognize and defend itself against specific pathogens and their products

Adaptive immunity

Specificity (acts against specific pathogens)

Induced (activated in response to specific pathogens)

Clonality (induced cells proliferate to form clones)

Self-tolerant (it does not act against self antigens)

Memory (faster and more effective response to a subsequent infection caused by the same pathogen)

Adaptive immunity involves the activity of lymphocytes

Two main types of lymphocytes

B cells

T cells

Adaptive Immunity

A resting lymphocyte

B cells form and mature in the red bone marrow

T cells form in the red

bone marrow and mature in the thymus

Both B and T cells are found in the blood and lymphoid organs

Adaptive immunity involves two types of responses

Humoral immune responses (humoral immunity)

Activated B cells produce antibodies that function against extracellular pathogens in the blood

Cell-mediated immune responses (cellular immunity)

Activated T cells function against infected host cells (intracellular pathogens)

Regulate adaptive immune responses

Adaptive Immunity

Components of adaptive immunity Lymphatic system

Allows for immune system surveillance

Lymphatic vessels conduct lymph from tissues and return it to the circulatory system Lymph: fluid leaked from blood vessels into surrounding

tissues

The lymphatic system includes lymphoid tissues and organs Red bone marrow and thymus Lymph nodes Tonsils, spleen Mucosa-associated lymphatic tissue-MALT

Lymphatic system

Lymph nodes are the site of activation of B cells and T cells Contain phagocytic cells (macrophages and dendritic cells)

Epitopes or antigenic determinants

Antigens are molecules the body recognizes as foreign

Their binding to defensive

cells can trigger adaptive immune responses

Antigens are recognized by

the shape of regions called epitopes

Microbial antigens include: Components of the cell walls, capsule, flagella or toxins, proteins and glycoproteins of viruses, fungi, or protozoa

Exogenous and endogenous antigens

Clonal deletion

The body eliminates lymphocytes that react against self-antigens

Lymphocytes that react to self-antigens undergo apoptosis

Leading to the development of self-tolerance

It is vital that immune responses are not directed against self-antigens

Humoral response involves activation of B cells

The precise binding of the receptor on the B cell to the epitope

Determines the specificity of a humoral immune response

All the BCRs on a single B cell are the same

The binding of the epitope to the BCRs stimulates the B cells to undergo cell division

Cell division gives rise to activated B cells called plasma cells

Humoral response involves activation of B cells

Plasma cells secrete antibodies or immunoglobulin (Ig) in the blood and lymph

Immunoglobulins have identical antigen-binding sites as the BCRs of the activated B cells

Y-shaped molecules with two antigen-binding sites

Five classes of immunoglobulins IgG IgA IgM IgE IgD

Immunoglobulins

T cells Cytotoxic T (Tc) cells

Kill viral (or other pathogens) infected host cells

Helper T (Th) cells

Help regulate the activities of B cells and cytotoxic T cells

Regulatory (suppressor) T cells Help prevent autoimmune disease

Cell-mediated immune responses involve T cells

Immunosystem cytokines act as intercellular signals among all cell types of the immune system

Viruses and other intracellular pathogens

Receptors of T cells are called TCRs

TCRs do not recognize epitopes directly

TCRs bind epitopes associated with proteins called MHC (Major Histocompatibility Complex)

T cells allow the body to fight against intracellular pathogens

MHC - Major Histocompatibility Complex

MHC proteins hold and position antigenic determinants for presentation to the T cells

First identified in graft patients - determine compatibility of tissues for tissue grafting

Found in the membrane of most cells of vertebrate animals

APCs B cells Macrophages Dendritic cells

APCs (Antigen presenting cells) and MHC

Dendritic cells typically found in skin and mucous membranes

APCs migrate to lymph nodes where they present the antigen to the T cells

Antigen is bound to MHC on the surface of the APCs

Initiate adaptive immune responses in lymphoid organs – lymph nodes - where the APCs interact with lymphocytes The initial event is the activation of cytotoxic T cells

Effector cytotoxic T cells will leave the lymph node ready

to attack virally infected cells

The body does not initiate adaptive immune responses at the site of an infection

Cell-mediated immune responses

Cell-mediated immune responses

Interaction between APC cells, T helper cells, inactive cytotoxic T cells, cytokines Leads to activation of cytotoxic T cells Formation of Memory T cells

Details of the figure will not be part of your next exam!!!

Cell-mediated immune responses

Effector cytotoxic T cells will attack virally infected cells

Recognize viral epitope on MHC I

of infected cells and

Induce apoptosis of infected cells

Memory T cells persist in lymphoid tissues Exposure to the same antigen

will trigger an effective response called a memory response

Humoral Immune Responses

Humoral antibodies are effective against pathogens that are circulating freely so that the antibody can contact them

Two types of humoral response

T-independent humoral immunity

T-dependent humoral immunity

Humoral immune responses are mounted against exogenous pathogens

T-independent humoral immune responses

Occurs without involvement of helper T cells

T-independent responses are reduced in children

Pathogens with T-

independent antigens can cause disease in children that are rare in adults

Haemophilus influenzae type b (capsule) causes meningitis in unvaccinated children

Most humoral responses are of the T-dependent type

Interaction between APC cells, helper T cells, B cells

Leads to activation of B

cells plasma cells will produce antibodies

Formation of Memory B cells

Lymph nodes/cytokines

mediate interactions Details of the figure will not be part of your next exam!!!

Functions of immunoglobulins

The binding of antibodies to antigens to form antibody-antigen complex tags the pathogen/toxins for destruction

Several mechanisms are involved

Agglutination by IgG and IgM

Opsonization

Neutralization

Functions of immunoglobulins

Activation of complement system (classical pathway)

Functions of immunoglobulins

Produced by B cell proliferation

BCRs complementary to antigenic determinant that triggered their production

Long-lived cells that persist in the lymphoid tissue

Initiate antibody secretion if antigen is encountered again

Memory B cells and the establishment of immunological memory

Immunological Memory

Primary response occurs after initial contact with an antigen Secondary (memory) response occurs after second exposure

Antibody titer is the relative amount of antibodies in serum

Administration of a tetanus toxoid in immunization

Subsequent exposure to active tetanus toxin

Immunological Memory

Colostrum IgG/IgA