Introduction to the Immune System Flashcards
What does the innate immune system include?
- Epithelial barriers to the environment (e.g. skin, gastro-intestinal tract, respiratory tract) that prevent microbe entry.
- Secretions at mucosal surfaces – flushing action and antimicrobial properties.
- Cells that are resident in tissues (e.g. mast cells) or circulating in the body (e.g. neutrophils).
- Circulating proteins in the blood (e.g. complement proteins).
- Cytokines (e.g. interferons) that are locally produced by infected cells.
• Non-specific activity.
• No “memory”, the response is the same to repeated challenge.
How do immune cells know when to get to work?
- The immune system detects “danger” through a series of pathogen-associated molecular patterns (PAMPs) or damage-associated molecular pattern molecules (DAMPs).
- PAMPs are small molecular motifs conserved within a class of microbes. A vast array includes glycans, lipopolysaccharides, bacterial flagellin, lipoteichoic acid, peptidoglycan and nucleic acid variants normally associated with viruses, such as double-stranded RNA.
- DAMPs are molecules released by stressed cells undergoing necrosis. Some are proteins - heat-shock proteins and cytokines. Non-protein DAMPs include ATP, heparin sulphate, and DNA.
- PAMPS and DAMPS are recognized by Pattern Recognition Receptors (PRRs) on immune cells.
What is phagocytosis?
Phagocytosis is a process by which cells internalise solid matter, including microbial pathogens
Most cells are capable of phagocytosis, but it is the professional phagocytes of the immune system, includingneutrophils, macrophagesand dendritic cells, that truly excel in this process.
In these cells, phagocytosis is a mechanism by which microorganisms can be contained, killed and processed for antigen presentation.
This is a vital part of the innate immune response to pathogens, and plays an essential role in initiating theadaptive immune response
What is inflammation?
Inflammation is the process whereby immune cells (which are normally distributed throughout the body) can be recruited and concentrated to a site of infection or damage
Following PAMP or DAMP recognition, PRRs trigger proinflammatory and antimicrobial responses by inducing the release of a broad range of cytokines from white blood cells.
The main events in inflammation are:
• Increased blood supply to the affected area;
• Increased permeability of the vasculature ;
• Migration of WBCs out of the blood capillaries into the affected tissue.
What are the characteristics of the adaptive immune response?
The adaptive immune system responds to infection. It takes days/weeks to develop the immune response, but it is:
Potent
Responsive to any potential foreign entity
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
What are the main components of the adaptive immune system? (different cells involved)
The main components of the adaptive immune system are:
Dendritic cells – capture, process and present antigens.
T lymphocytes – control the immune response by providing “help” to B cells and macrophages (helper T cells); direct killing of infected or tumour cells (cytotoxic T cells).
Cytokines – soluble proteins secreted mainly by T cells that control activities of other cells.
B lymphocytes – produce and secrete antibodies, proteins that specifically bind target molecules (antigens) on microbes or cells.
What are the lymphoid system and the different lymphoid organs?
Immunological cells are largely organised into tissues and organs for best efficiency
Collectively, these structures are known as the lymphoid system
Primary lymphoid organs are the sites of maturation of white blood cells – they differentiate from stem cells, multiply, are programmed and mature into functional cells.
Secondary lymphoid organs provide the site for 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
What is the lymphatic system?
The lymphatic system is part of thecirculatory systemand an important part of theimmune system. It comprises a network oflymphatic vesselsthat carry lymph fluid and cells from the tissues back into the bloodstream.
What are the lymph nodes and what do they do?
At regular intervals along the lymph vessels are organised structures- lymph nodes
Lymph arrives at the node by an afferent lymphatic vessel, it filters through multiple layers of antigen-presenting cells, T cells and B cells, and 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.
How are foreign antigens recognised?
The hallmark of adaptive immunity is the ability to specifically recognise foreign antigens
The two types of molecules involved in this process are:
Immunoglobulins (Igs)
T-cell antigen receptors (TCRs)
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 – humoral immunity
T-cell receptors are found on the surface ofT cells. They are responsible for recognizing processed fragments ofantigen (peptides)which are “presented” by host cells. The binding between TCR and antigen peptides is relatively weak compared to antibodies.
When the TCR engages with antigenic peptide, the T lymphocyte is activated through a series of biochemical events (signal transduction), leading to cell proliferation and biological activity (e.g. cytokine production).
TCRs are produced by T cells – cellular immunity
How do B cells develop?
B cells are lymphocytes, which when activated, become plasma cells and then produce antibodies
This is often known as humoral immunity
What are antibodies?
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.
How does cellular immunity work?
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
T cells recognise antigens via cell-surface receptors (TCRs) proteins, that are related to antibodies.
But unlike antibodies, T cells do not recognise foreign molecules in their “native” state.
T cells only recognise antigens that have been processed within cells, and that are displayed on cell surfaces.
For example, antigens derived from pathogens that have infected cells, such as viruses or intracellular bacteria, or from pathogens that have been phagocytosed by antigen-presenting cells like macrophages and dendritic cells.
In both cases, foreign antigen fragments (peptides) are displayed at the cell surface by molecules of the major histocompatibility complex (MHC).
