Immunity notes

MDCAT Biology

Specific defense is the acquired immune response that recognises particular foreign substances and responds through lymphocytes. It includes humoral immunity by B lymphocytes, cell-mediated immunity by T lymphocytes, antibodies, antigen presentation, vaccination and immunological memory.

Body Defence and Lines of Defence

The human body is protected by several defensive systems. The first line of defence prevents pathogens from entering the body. It includes physical barriers and chemical substances that act against microorganisms without recognising a particular antigen.

If pathogens pass the first line, internal non-specific defences act against them. Specific defence then recognises particular antigens and produces a targeted response. White blood cells are central to both non-specific and specific defence.

Skin forms a physical barrier against the entry of microorganisms.
Mucus secretions trap dust and pathogens in the respiratory and digestive passages.
Cilia move mucus and trapped particles out of the respiratory tract.
Lysozyme in tears and other secretions breaks down bacterial cell walls.
The first line of defence includes skin, mucus secretions and lysozyme in tears.
White blood cells protect the body by phagocytosis and specific immune responses.
Neutrophils and macrophages are phagocytic cells that engulf and digest pathogens.

White Blood Cells and Phagocytes

White blood cells, or leucocytes, are formed mainly in the bone marrow. They protect the body by recognising, engulfing or destroying harmful organisms and by coordinating immune responses. Different types of white blood cells have different functions.

Neutrophils are abundant phagocytic white blood cells. They are produced in the bone marrow, not by the thymus gland. Macrophages are larger phagocytes that remove pathogens, dead cells and cellular debris.

Neutrophils are white blood cells that engulf and digest pathogens.
Neutrophils are made in the bone marrow, not in the thymus gland.
Macrophages develop from monocytes and carry out phagocytosis in tissues.
Macrophages help clean up pus during the healing process.
Pus contains dead cells, dead microorganisms, tissue fluid and cellular debris.
Phagocytosis is the engulfing and digestion of solid particles by a cell.
Phagocytes also release substances that help attract other immune cells to an infected area.

Lymphocytes and Their Origin

Lymphocytes are white blood cells involved in specific immunity. The main types are B lymphocytes and T lymphocytes. They recognise foreign antigens through specific receptors on their cell surfaces.

Both B and T cells are produced from stem cells in the bone marrow. B lymphocytes mature in the bone marrow, while T lymphocytes leave the bone marrow and mature in the thymus gland.

B lymphocytes and T lymphocytes are the main types of lymphocytes.
Lymphocytes are produced from stem cells in the bone marrow.
B lymphocytes mature in the bone marrow.
T lymphocytes mature in the thymus gland.
The thymus is an organ of T-lymphocyte maturation, not the site where neutrophils are made.
Lymphocytes have receptors that bind to particular antigens.
B cells mainly produce antibodies, while T cells mainly perform cell-mediated functions.

Antigens and Antigen Recognition

An antigen is a molecule that the body recognises as foreign and responds to by making antibodies or by activating specific immune cells. Antigens may be present on bacteria, viruses, toxins, transplanted tissues or other foreign materials.

The immune system does not respond equally to every molecule. A lymphocyte is activated when its receptor matches a particular part of an antigen. This matching gives specificity to the immune response.

An antigen is any molecule the body recognises as foreign and responds to by making antibodies or activating immune cells.
Antigens may be proteins, polysaccharides or other molecules associated with pathogens or foreign cells.
A pathogen can carry many different antigens on its surface.
Antibody or lymphocyte receptors bind to specific regions of an antigen.
The antigen-binding region of an antibody is called the variable region.
Self molecules are normally tolerated and are not attacked by a healthy immune system.
Foreign antigens on a transplanted organ can be recognised as non-self.

Antibodies and Their Structure

An antibody is a specific protein produced by activated B lymphocytes and secreted by plasma cells. Antibodies are also called immunoglobulins. Each antibody binds to a particular antigen or a closely related group of antigens.

An antibody has a Y-shaped structure made of four polypeptide chains. It contains two identical heavy chains and two identical light chains. The ends of the chains form antigen-binding sites.

One antibody molecule is made of four polypeptide chains.
The four chains consist of two identical heavy chains and two identical light chains.
Disulphide bonds hold the polypeptide chains together.
Both heavy and light chains contain variable amino acid sequences.
The variable sequence in a light chain is shorter than the variable sequence in a heavy chain.
Variable regions form the antigen-binding sites and determine antibody specificity.
The constant regions are more similar among antibodies of the same class.
Antibodies may neutralise toxins and viruses, agglutinate particles, or help phagocytes destroy pathogens.

Humoral Immunity and Clonal Selection

Humoral immunity is the antibody-based part of specific immunity. When a B lymphocyte receptor matches an antigen, that B cell is selected and stimulated. This process is explained by the clonal selection theory.

The selected B cell divides by mitosis to form a clone of genetically similar cells. Some cells become plasma cells and secrete antibodies. Other cells become memory B cells, which remain in the body and produce a faster response when the same antigen enters again.

Humoral immunity involves B lymphocytes and antibodies in body fluids.
According to clonal selection theory, an antigen selects the lymphocyte clone with the matching receptor.
Activated B lymphocytes divide to form a clone of cells.
Plasma cells are activated B cells that secrete antibodies.
Memory B cells remain after the first immune response.
The secondary response is faster and stronger than the primary response because of memory cells.
Antibodies may neutralise toxins, block attachment of pathogens, cause agglutination and promote phagocytosis.
Secretion of cytokines is not a direct mode of action of antibodies.

Cell-Mediated Immunity and T Lymphocytes

Cell-mediated immunity is mainly carried out by T lymphocytes. It is especially important against virus-infected cells, abnormal cells and foreign cells in transplanted tissues. T cells do not usually destroy free pathogens by secreting antibodies.

Different T cells have different roles. Helper T cells coordinate immune responses, cytotoxic T cells destroy infected or abnormal cells, and memory T cells provide a quicker response during later exposure.

Cell-mediated immunity involves T lymphocytes.
Helper T cells release signals that activate B cells, cytotoxic T cells and other immune cells.
Cytotoxic T cells destroy body cells infected by viruses.
A virus-infected cell may be destroyed by cytotoxic T cells.
Memory T cells remain after an immune response and support a rapid secondary response.
T lymphocytes mature in the thymus after being produced in the bone marrow.
T lymphocytes recognise antigen fragments displayed on the surface of cells.
T cells are mainly responsible for rejection of a transplanted kidney.

Antigen-Presenting Cells and Transplant Rejection

Antigen-presenting cells process foreign material and display antigen fragments on their surface. This presentation allows T lymphocytes to recognise the antigen and begin a specific immune response.

A transplanted organ contains cells with antigens that may differ from those of the recipient. The recipient's T lymphocytes can recognise these graft antigens as foreign. Their response may damage and reject the transplanted tissue.

Antigen-presenting cells process and present antigens for recognition by other immune cells.
Macrophages, dendritic cells and B lymphocytes can act as antigen-presenting cells.
Antigen presentation helps activate specific T lymphocytes.
A graft is transplanted tissue or an organ placed into another body.
Rejection of a transplanted kidney is mainly caused by T lymphocytes recognising foreign antigens on the graft.
Transplant rejection is an example of cell-mediated immunity.
The immune system distinguishes between self antigens and foreign antigens.

Active and Passive Immunity

Immunity may be active or passive. Active immunity develops when the person's own immune system produces antibodies and memory cells after exposure to an antigen. Passive immunity occurs when ready-made antibodies are transferred to a person.

Active immunity usually develops more slowly but can last for a long time because memory cells are formed. Passive immunity gives immediate protection, but it is temporary because the recipient does not usually form memory cells against the transferred antibodies.

Acquired active immunity develops after infection or vaccination.
Natural active immunity develops after a person suffers an infection and mounts an immune response.
Artificial active immunity develops after vaccination.
Passive immunity results from receiving ready-made antibodies.
Natural passive immunity may pass from mother to child through the placenta or breast milk.
Artificial passive immunity is produced by giving antiserum or immunoglobulin.
Antiserum contains ready-made antibodies and gives temporary protection.
Active immunity forms memory cells, whereas passive immunity normally does not.

Vaccination, Antiserum and Serology

A vaccine contains an antigenic material that stimulates the immune system without causing the full disease. It may contain weakened or killed pathogens, parts of pathogens, or products of pathogens prepared to stimulate protection. Vaccination leads to artificial active immunity.

An antiserum contains antibodies already formed against a particular antigen. It acts quickly but provides temporary protection. Serology is the study of the interaction between antigens and antibodies in blood or serum.

Vaccination is an example of acquired or artificial active immunity.
Vaccines stimulate the production of specific antibodies and memory cells.
Vaccination can prevent diseases such as measles, mumps, polio and tetanus.
Polio vaccination may be given at different ages according to the immunisation programme or medical need.
The best way to prevent tetanus is vaccination.
Antiserum differs from a vaccine because antiserum contains ready-made antibodies.
A vaccine produces active, usually longer-lasting protection; antiserum produces passive, temporary protection.
Smallpox is an example of a disease against which infection can induce immunity.
Serology is the study of antigen and antibody interactions in blood serum.

Key terms

Specific immunity: A defence response directed against particular antigens and involving lymphocytes.
Antigen: A foreign molecule recognised by the body that can stimulate antibody production or a specific immune response.
Antibody: A specific immunoglobulin protein produced by plasma cells in response to an antigen.
Lymphocyte: A white blood cell involved in specific immunity, mainly a B cell or T cell.
B lymphocyte: A lymphocyte that matures in bone marrow and can form plasma cells, antibodies and memory cells.
T lymphocyte: A lymphocyte that matures in the thymus and performs cell-mediated immune functions.
Plasma cell: An activated B cell specialised for secreting large amounts of antibodies.
Cytotoxic T cell: A T cell that kills virus-infected, abnormal or foreign cells.
Helper T cell: A T cell that coordinates immune responses by activating and regulating other immune cells.
Memory cell: A long-lived B or T cell that produces a faster response on later exposure to the same antigen.
Clonal selection: The process in which an antigen selects and stimulates the lymphocyte clone with a matching receptor.
Humoral immunity: Specific immunity involving B lymphocytes and antibodies in body fluids.
Cell-mediated immunity: Specific immunity involving T lymphocytes acting against infected, abnormal or foreign cells.
Antigen-presenting cell: An immune cell that processes an antigen and displays its fragments for recognition by T lymphocytes.
Phagocytosis: The engulfing and digestion of pathogens or particles by a cell.
Vaccine: An antigenic preparation that stimulates artificial active immunity and immune memory.
Antiserum: Blood serum containing ready-made antibodies that gives temporary passive protection.
Serology: The study of antigen and antibody interactions in blood serum.

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