NTA Level 5 • Semester 1 • PST05103

Activation of Lymphocytes

Pharmaceutical Microbiology • Source Session/Topic 47

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Session 47Activation of Lymphocytes

Total Session Time: 120 minutes

Pre-requisites

• Human anatomy and physiology

Students Learning Tasks

By the end of this session students are expected to be able to:

• Explain types and functions of lymphocytes

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing

STEP 2: Lymphocytes (15 minutes)

• Lymphocytes are a type of white blood cell (leukocyte) that are of

fundamental importance in the immune system

• Lymphocytes determine the specificity of the immune response to

infectious microorganisms and other foreign substances

• In adult humans lymphocytes make up roughly 20 to 40 percent of the

total number of white blood cells

• Lymphocytes are found in the circulation and in central lymphoid

organs and tissues where initial immune response occurs

• There are two primary types of lymphocytes;

o B lymphocytes and T lymphocytes, or B cells and T cells

• Both types of lymphocytes originate from stem cells in the bone marrow

and are initially similar in appearance

• The B cells produce antibodies that are used to attack invading

bacteria, viruses, and toxins

• The T cells destroy the body's own cells that have themselves been

taken over by viruses or become cancerous

STEP 3: Major Histocompatibility Complex (20 minutes)

• Major Histocompatibility Complex (MHC) is a group of genes that code

for proteins found on the surfaces of cells that help the immune

system recognize foreign substances. The genes are located on

chromosome 6 in human beings

• Each gene of MHC has an unusually large number of alleles therefore it

is very rare for two persons to have the same set of MHC molecules

(tissue types)

• MHC proteins are found in all higher vertebrates. In human beings the

complex is also called the human leukocyte antigen (HLA) system

• There two classes of MHC, class I and class II

• MHC Class I

o Molecules of MH Class I are found on membranes of almost all cells

o These help the immune system to recognize heathy cells from

infected cells and pathogens

• MHC Class II

o In MHC class II molecules are restricted to macrophages and

lymphocytes

o This class help cells of the immune system to interact and

communicate with one another

• The MHC also contains genes that code for other proteins e.g. the

complement proteins, cytokines and enzymes. These proteins are called

Major Histocompatibility Class III molecules

• MHC molecules are important components of the immune system because

they allow T lymphocytes to detect cells, such as macrophages, that

have ingested infectious microorganisms

• When a macrophage engulfs a microorganism, it partially digests it and

displays peptide fragments of the microbe on its surface, bound to MHC

molecules

• The T lymphocyte recognizes the foreign fragment attached to the MHC

molecule and binds to it, stimulating an immune response

• In uninfected healthy cells, the MHC molecule presents peptides from

its own cell (self-peptides), to which T cells do not normally react

STEP 4: Activation of Lymphocytes (40 minutes)

• Activation of lymphocytes occurs

o in the thymus to form T-cells or T-lymphocytes

o in bone marrow to form B-cells or B-lymphocytes

• Most lymphocytes are short-lived, with an average life span of a week

to a few months

• A few lymphocytes live for years, providing a pool of long-lived T and

B cells

o These cells account for immunologic “memory,” a more rapid,

vigorous response when the same antigen enters the body

• Activation process involves macrophages engulfing and digesting

pathogens

o Through receptor molecules on their surfaces, lymphocytes are able

to bind antigens and help remove them from the body

o Each lymphocyte bears receptors that bind to a specific antigen

o The ability to respond to virtually any antigen comes from the

enormous variety of lymphocyte populations that the body contains,

each of them with a receptor capable of recognizing a unique

antigen

o When macrophages engulf and digest pathogens, they take the

antigenic components of that pathogen and display them on their

membrane as antigens by attaching them onto major

histocompatibility complex class II (MHC class II)

▪ This is antigen presentation and the macrophages are antigen

presenting cells (APCs)

o T-lymphocytes such as inactivated helper T cells can now bind onto

these antigen complexes by using special receptors of their own (T

cell receptors that contain the glycoprotein CD4)

o Once bound, they begin releasing various chemicals such as

interleukin-1, which activates that helper T cell

o The helper T cell can then detach and bind to B-lymphocytes that

contain that same antigenic piece, which causes the two cells to

begin releasing cytokines and lymphokines

o This causes the cloning process in which the cells divide

mitotically to form many identical clones

o Some of these B-cell clones differentiate into plasma cells and

memory B cells while other T-cell clones differentiate into

cytotoxic T cells.

o The plasma cells produce antibody molecules which are released

into the blood and lymph

o Antibodies then bind to the target antigen and initiate its

neutralization or destruction

o Antibody production continues for several days or months, until

the antigen has been overcome

o The memory B cells, are stimulated to multiply but do not

differentiate into plasma cells; they provide the immune system

with long-lasting memory

o In the thymus, T cells multiply and differentiate into helper,

regulatory, or cytotoxic T cells or become memory T cells

o They are then seeded to peripheral tissues or circulate in the

blood or lymphatic system

o Once stimulated by the appropriate antigen, helper T cells secrete

chemical messengers called cytokines, which stimulate the

differentiation of B cells into plasma cells, thereby promoting

antibody production

o Regulatory T cells act to control immune reactions, hence their

name

o Cytotoxic T cells, which are activated by various cytokines, bind

to and kill infected cells and cancer cells

Formation of antibodies from activated B-lymphocytes

[pic]

Source:https://en.wikipedia.org/wiki/Danger_model#/media/File:Lymphocyte_act

ivationsimple.png

Activation of lymphocytes

[pic]

Source: http://www.immuneweb.com/images/Cell%20Lympho.htm

STEP 6: Key Points (5 minutes)

• Antigen presenting cells take antigenic components of a pathogen to

the major histocompatibility molecules to initiate activation of

lymphocytes

• T-lymphocytes are produced from lymphocytes activated in the thymus

• B-lymphocytes are produced from lymphocytes activated in the bone

marrow

• B-lymphocytes results in plasma cells which produce antibodies –the

humoral immunity

• The T-lymphocytes produce cytotoxic T-cells –the Cell mediated

immunity

• The complement system is a well regulated system that enhances

(complements) the ability of antibodies and phagocytic cells to clear

microbes and damaged cells from an organism, promotes inflammation,

and attacks the pathogen's cell membrane

STEP 7: Evaluation (5 minutes)

• What is the role of major histocompatibility complex in the activation

of lymphocytes?

• Which cells produce antibodies?

• What is a major histocompatibility complex?

References

Hugo and Russell (2011), Pharmaceutical Microbiology 8th Edition, Willey-

Blackwel publications

Karen C. Carroll et al (2013); Jawetz, Melnick and Adelberg’s Medical

Microbiology 26th Ed. McGraw Hill Co. Inc.

Greenwood et al (2012); Medical Microbiology, 18th edition Churchill

Livingstone

Andrey Kopot, Alex Volve http://www.aklectures.com/lecture/activation-of-

lymphocytes. Accessed 18th September, 2018

Srijana Khanal (2017) Complement Pathways: Types, Functions and Regulation,

https://microbeonline.com/complement-system-pathways-functions-

regulation/

Kindt TJ, Goldsby RA, Osborne BA, Kuby J (2007). Kuby Immunology. 6th W. H.

Freeman, New York. pp. 168-188.

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