Introduction to the Immune System

Question 1

What is Immunology?

Answer 1

Immunology is the study of our body’s sytems for preventing and treating diseases.

Question 2

How is the immune system organised?

Answer 2

The Immune System is split into a frontline defence (Innate Immunity) and a second, more specific defence (Adaptive Immunity).
The adaptive immunity can be humoural (ie. B cells and antibodies) or it can be cellular (ie. T cells).

White Blood Cells (WBCs) are key players in the immune system.

Question 3

Briefly, describe the innate immune system.

Answer 3

The innate immune system is your ever-present defence against infection. It is made up of barriers that keep viruses, bacteria, parasites and other foreign particles out of your body or limit their ability to spread and move throughout the body.
The importance of innate immunity is demonstrated by the fact that you are normally healthy despite the barrage of potential infectious challenges that you face every minute.

Question 4

What does the innate immune system include?

Answer 4

• EPITHELIAL BARRIERS to the environment (eg. skin, gastrointestinal tract, respiratory tract) that prevent microbe entry
• SECRETIONS at mucosal surfaces - flushing action and antimicrobial properties
• CELLS that are resident in tissues (eg. mast cells) or circulating in the body (eg. neutrophils)
• Circulating PROTEINS in the blood (eg. complement proteins)
• CYTOKINES (eg. interferons) that are locally produced by infected cells

Question 5

What are the main functions of innate immunity?

Answer 5

• prevention, control and elimination of infection
• removal of damaged cells and initiation of tissue repair
• activate the adaptive immune response
• influence the type of adaptive response that will develop

Question 6

What are some important features of the innate immune system?

Answer 6

• it’s the first line of defence in preventing infection
• it responds to microbes and products of injured cells
• it has non-specific activity
• it has no “memory”, so if the challenge is repeated, the innate immune response will be the same

Question 7

Why is our innate immune system required?

Answer 7

We would not be able to survive on our adaptive immune system alone.
The adaptive immune system takes a few days to really kick in, while the innate immune system provides immediate and early protection.

Question 8

What is phagocytosis and why is it important?

Answer 8

Phagocytosis is a process by which cells internalise solid matter, including microbial pathogens.
This is a vital part of the innate immune response to pathogens, and plays an essential role in initiating the adaptive immune system.

Question 9

What are the ‘professional’ phagocytes?

Answer 9

Most cells are capable of phagocytosis, but it is the professional phagocytes of the immune system that truly excel at this process. These cells include NEUTROPHILS, MACROPHAGES and DENDRITIC CELLS.
In these cells, phagocytosis is a mechanism by which microorganisms can be contained, killed and processed for antigen presentation.

Question 10

How do immune cells know when to get to work?

Answer 10

The immune system detects ‘danger’ through a series of pathogen-associated molecular patterns (PAMPs) or damage-associated molecular pattern molecules (DAMPs) working in concert with both positive and negative signals derived from other tissues.

Question 11

Describe dendritic cells.

Answer 11

Dendritic cells are widely distributed in lymphoid tissues, mucosal epithelium and body organs. They are important in phagocytosis (as they are phagocytes).
This is because their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. They act as messengers between the innate and the adaptive immune systems.

Question 12

Briefly, describe the adaptive immune system.

Answer 12

The adaptive immune system is the second line of defence and responds to infection. It takes day-weeks to develop the immune response, but it is also:

• potent
• responsive to any potential foreign entry
• highly specific
• has ‘memory’

The importance of adaptive immunity is that it provides a powerful defence against infection if the innate immune system is breached.

Question 13

What are the main (cellular) components of the adaptive immune system?

Answer 13

• DENDRITIC CELLS: they capture, process and present antigens
• T-LYMPHOCYTES: they control the immune response by providing ‘help’ to B cells and macrophages (helper T cells), and directly kill off infected or tumour cells (cytotoxic T cells)
• CYTOKINES: they’re soluble proteins secreted mainly by T cells that control activities of other cells
• B LYMPHOCYTES: they produce and secrete antibodies, which are proteins that specifically bind target molecules (antigens) on microbes or cells

Question 14

What is the Lymphoid System?

Answer 14

Immunological cells are largely organised (not randomly) into tissues and organs for best efficiency. This is an advantage as it protects key areas of the body.
Collectively, these structures are known as the lymphoid system.

Question 15

Define primary and secondary lymphoid organs.

Answer 15

PRIMARY LYMPHOID ORGANS are sites of maturation of white blood cells. The WBCs differentiate from stem cells, multiply, are programmed and mature into functional cells.
SECONDARY LYMPHOID ORGANS provide the site of interaction between antigens and WBCs. They also allow the spread of the immune response. Secondary lymphoid organs are associated with systemic and mucosal immune compartments.

Question 16

Describe the Lymphatic System.

Answer 16

The lymphatic system is part of the circulatory system and an important part of the immune system. It comprises a network of lymphatic vessels that carry lymph fluid and cells from the tissues back into the blood stream.

Question 17

Describe lymph nodes.

Answer 17

At regular intervals along the lymph vessels are organised structures called lymph nodes. Major areas with a lot of lymph nodes are the neck, armpits and groin.
The lymph arrives at the node by an afferent lymphatic vessel, filters through multiple layers if antigen presenting cells, T cells and B cells, then finally exits via the efferent lymphatic vessel.

The dense concentration of immune cells provides an ideal environment for initiating immune responses and communication between immune cells. Lymph nodes are common battleground sites for the immune system and infections.

Question 18

The hallmark of adaptive immunity is the ability to specifically recognise foreign antigens. List, and describe, the two types of molecules involved in this process.

Answer 18

The two molecules involved in this process are immunoglobulins (Igs) and T cell antigen receptors (TCRs).

IMMUNOGLOBULINS:
Immunoglobulins (antibodies) are glycoproteins produced by plasma cells. They specifically recognise and bind strongly to particular antigens on pathogens, and prevent disease or aid in the destruction of the pathogen.
There are different classes and subclasses of immunoglobulins, which differ in their structure, biological features and distribution.
Immunoglobulins are produced by B cells, so they are a part of humoural immunity.

T CELL ANTIGEN RECEPTORS (TCRs):
T cell receptors are found on the surface of T cells. They are responsible for recognising processed fragments of antigens (peptides) which are ‘presented’ by host cells. The binding between the TCR and antigen peptides is relatively weak compared to antibodies.
When the TCR engages with the antigen peptide, the T lymphocyte is activated through a series of biochemical events (signal transduction), leading to cell proliferation and biological activity (eg. cytokine production).
TCRs are produced by T cells, so they are a part of cellular immunity.

Question 19

What are B cells?

Answer 19

B cells are lymphocytes which, when activated, become plasma cells and then produce antibodies. This is often known as humoural immunity.

Question 20

Describe the structure and function of antibodies.

Answer 20

Antibodies are glycoproteins that are made by B cells and plasma cells. They bind specifically and with high affinity to ‘non-self’ antigens. Antigens are usually proteins or carbohydrates attached to, or secreted by, microbes and infected cells.
Antibodies can neutralise targets, or recruit other components of the immune system to kill targets. Once bound to a cell or bacterium, antibodies can recruit other components of the immune system to kill targets by Fc-mediated effector functions. These include white blood cells (eg. macrophages, dendritic cells). It activates the complement cascade. The antibody Fc also binds to the neonatal Fc receptor.

Question 21

Describe the structure and function of T cells.

Answer 21

T cells control the immune response and combat microbes that are inside cells (intracellular). Different T cells have different functions and are distinguished by the array of proteins on the cell surface (Cluster of Differentiation (CD) markers). Two major groups are CD4 (helper T cells ) and CD8 (cytotoxic T cells).

Depending on how the antigen is presented, different T cells are engaged, leading to different T cell responses.

Question 22

How do antibodies aid immunological techniques in diagnostics and research?

Answer 22

There are two key features of antibodies: antigen specificity and high-affinity binding. These are extremely valuable in laboratory assays. They allow for rapid and highly sensitive identification of specific molecules.