Immune system

What is the role of the immune system ?

To defend the body against infectious microorganisms/pathogenic agents

What is the lymphatic system ?

Its the counterpart of immune system and it consists of cells, organs and tissues that monitor body surfaces and internal fluid compartments

Where do all immune system cells derive from ?

From hematopoietic stem cells

What are lymphocytes?

They are the definitive cell type of the immune system and the effector cells in response to harmful substances

What is the lymphatic tissue important for ?

It serves as a primary site where lymphocytes proliferate, differentiate and mature

What are primary lymphatic organs ?

The thymus and bone marrow → here lymphocytes are born and educated and then migrate to secondary lymphatic organs

What happens in secondary organs ?

If the antigen is present, lymphocytes interact to become activated such as immunocompetent effector T and B

→ are able o recognise between self and non self and initiate appropriate response

What is an antigen ?

Any substance that can induce a specific immune response → the body is constantly exposed to pathogenic organism

• can be soluble substances or infectious organism, foreign tissue or transformed tissue

• its a molecule that can be recognised by cells of the immune system and does not imply pathogenic origin necessarily

What is an immunogen?

An specific type of antigen always able to elicit immune response

→ because not all antigens are able to induce specific immune response culminating with the production of antibodies

By what is adaptive immunity activated ?

By the presence of antigens

→ th longer the innate system, takes to clear pathogens more antigens accumulate and more robust adaptive response it

What are the cels of the immune system ?

Lymphocytes and various supporting cells

Where do cells of the immune system reside ?

In meshwork and in loose connective tissues

Where can we find the most lymphocytes?

In blood or lymph and represent the circulating pool of immunocompetent cells

→ participate in cycle during which they exit systemic circulation and enter lymphatic tissue

• remaining lymphocytes in blood do not circulate between lymphatic and system and they comprise of short lived, immature or activated cells destined for specific tissues → leave capillaries and go directly to tissues

What are lymphocytes responsible for ?

For immunologic surveillance

What are the 3 major lymphocytes ?

B, T and NK cells

What causes the activation of the humeral response ?

The recognition of antigens by BCRs

When is specific immune response activated ?

When immunocompetent cells encounter foreign antigens

Why are helper and cytotoxic T so important ?

They play a central role in initiating specific immune response by acting on immune system patrols

Where are self reactive T cells eliminated ?

They are selected and eliminated in thymus so presentation of self antigen allows for constant surveillance by Cytotoxic CD8+ T → only reacts to cells expressing aberrant non self antigens

What do CD4 cells require to become fully activated ?

3 stimulatory signals

What can the interaction of cytotoxic CD8 TCR with MHC I molecule presenting antigen peptide on target cell do ?

Results in secretion of perforin and granzyme B → induces apoptosis

With wha do B cells interact ?

With single antigen or type of antigenic side that it has been programmed to recognise

What is antibody-dependent cell-mediated cytotoxicity ?

Its the recognition and subsequent destruction of antibody-coated target cells by NK

What do activated T lymphocytes secrete ?

A variety of cytokines

What are cytokines ?

Soluble polypeptide substances that modulate immune responses, synthesised mainly by activated T lymphocytes

→ substances that are involved in immune defence mechanism and act on lymphocytes, growth factors instead act on other somatic cells

Why are cytokines important ?

They serve as chemical messengers between cells of immune system and act locally on same cell that secreted them or on neighbouring

→ may also communicate the state of immune system to cells of other systems

→ promote growth and differentiation of T, B, NK and hematopoietic cells

Signal transduction pathways downstream of cytokine receptors result in activation of specific transcription factors → affect different processes

How do cytokines function ?

Through specific receptors → cells regulated by cytokines possess cytokine receptors

• Interleukins are synthesised by CD4 lymphocytes, monocytes, macrophages and endothelial cells

Where do lymphatic vessels begin ?

As networks of blind capillaries in loose connective tissue

→ most numerous beneath epithelium of skin and mucous membranes

What do lymphatic vessels do ?

They remove substances and fluid from extracellular spaces of connective tissues producing lymph

What is a characteristic of walls of lymphatic capillaries?

Their wall are more permeable than walls of blood capillaries → large molecules entry more readily

What happens as lymph circulates through lymph nodes?

As it circulates and passes within lymph nodes antigens conveyed in lymph are trapped by follicular dendritic cells

→ antigen exposed on surface of follicular dendritic cells can processes by antigen-presenting cells

Where do lymphocytes circulate ?

Through both lymphatic and blood vessels

→ their circulation enables them to move from one part of lymphatic system to another at different stages in development and to reach sites where they are needed

Where can we find accumulations of lymphatic tissues not enclosed by capillaries ?

In alimentary canal, respiratory passage and genitourinary tract

→ Lymphocytes are found in lamina propria of these tracts

→ This kind of lymphatic tissue is diffuse lymphatic tissue or mucos associated lymphatic tissue

What happens with lymphocytes found in lamina propria of diffuse lymphatic tissues ?

When they are activated, they come in contact with antigen, they travel to regional lymph nodes where they undergo proliferation and differentiation

→ the progeny of these then return to lamina propria as effector T and B

Why is the diffuse lymphatic tissue important ?

1. Regular presence of large number of plasma cells → indication of local antibody secretion.

2. Presence of larger numbers of eosinophils → indication of chronic inflammation and hypersensitivity reactions

How do cells of reticular meshwork appear ?

Stellate or elongated with oval euchromatic nucleus and small amount of acidophilic cytoplasm

• cells can take up dyes and colloidal materials using immunocytochemistry and TEM we can observe several cells

Why are lymph nodes also important ?

They are important for phagocytosis and initiation of immune responses

How does the phagocytosis occur in lymph nodes ?

Some antigens become entrapped on surface of follicular dendritic cells, others are processed by macrophages, dendritic and B leading to activation of T and B

What do activated B cells differentiate into ?

1. Plasma cells - migrate to medullary cord where thy synthesise and release specific antibodies

2. Memory B cells - may leave lymph nodes and circulate to various regions → proliferate in response to contact with antigen

3. Plasmablasts - leave node and migrate to bone marrow → further differentiate into plasma cells → secrete antibodies

By what is the exit of T and B lymphocytes from lymph nodes regulated?

By S1P exit pathway → depends on the expression of sphingosine-1-phosphate receptor 1 on surface of lymphocytes and interactions with sphingosine-1-phosphate

What is the thymus ?

Its a bilobed lymphoepithelial organ in superior mediastinum, it develops bilaterally from third brachial pouch

What does the thymic medulla contain ?

Large number of epithelioreticular and loosely packed T

→ stains less intensely because contains large lymphocytes