Immunology: One Mark Questions, Book Back Answers and Notes

About Immunology

Medium ~120 min study

Bacteria, viruses, fungi and parasites are all around us, yet most of the time we stay healthy. The reason is the immune system, which can tell the body's own cells (self) from anything foreign (non-self) and destroy the foreign material without harming healthy tissue.

The chapter builds this up in layers. First come the defences we are born with: skin, mucus, stomach acid, phagocytes and inflammation. Then comes acquired immunity, which is specific to each pathogen and remembers it. You will see where lymphocytes are made and trained (bone marrow and thymus), where they meet antigens (spleen, lymph nodes and mucosa-associated lymphoid tissue), and how B cells and T cells divide the work. The same ideas explain how vaccines work, why some people react to pollen, and how HIV weakens the whole system.

Book back questions often ask about the Y-shaped structure of an antibody, the difference between epitope and paratope, active versus passive immunity, primary versus secondary response, and how HIV multiplies. Draw the antibody once with its heavy chains, light chains, variable regions and binding sites labelled, and keep a small table of immune disorders for quick revision.

What you'll learn

• Distinguish innate immunity from acquired immunity, with examples of each.
• Classify lymphoid organs as primary or secondary and state the function of each.
• Draw and label the structure of an antibody and identify its antigen-binding sites.
• Explain how a vaccine produces active immunity and memory.
• Identify the causes of immunodeficiency and hypersensitivity reactions.
• Explain how the immune system detects and destroys cancer cells.

Before you start

• Structure of eukaryotic cells and their organelles.
• Components of blood, including red and white blood cells.
• Basics of protein synthesis and molecular biology.

Immunology explained

How the Body Defends Itself

Innate Immunity and Natural Barriers

Innate immunity is present from birth and is non-specific: it acts the same way against any invader, and it does not need earlier exposure. Anatomical barriers include the skin and mucous membranes. Physiological barriers include the acid in the stomach, lysozyme in tears and saliva, and body temperature. If a pathogen gets past these, phagocytes such as neutrophils and macrophages engulf it, and inflammation brings more defence cells to the site.

Acquired Immunity and Memory

Acquired immunity develops after birth, when the body meets a particular antigen. Its features are specificity, diversity, memory and the ability to tell self from non-self. It has two arms. Humoral immunity depends on antibodies made by B cells and carried in blood and lymph. Cell-mediated immunity depends on T cells, which kill infected or abnormal cells directly and help other immune cells.

Lymphoid Organs

Primary lymphoid organs are the bone marrow and the thymus. All lymphocytes arise in the bone marrow; B cells mature there, while T cells move to the thymus to mature. Here lymphocytes become immunocompetent and gain their antigen receptors. Secondary lymphoid organs are the spleen, lymph nodes and mucosa-associated lymphoid tissue (MALT), such as tonsils and Peyer's patches. They trap antigens from blood, lymph or mucosal surfaces and bring them into contact with mature lymphocytes.

Antigens and Antibodies

An antigen is a substance that can trigger an immune response. The small part of it that the antibody actually recognises is the epitope (antigenic determinant). Antibodies, or immunoglobulins, are made by plasma cells, which develop from B cells. Each antibody is Y-shaped, with two identical heavy chains and two identical light chains joined by disulphide bonds. The tips of the arms carry the paratope, the antigen-binding site. Antibodies clear antigens by agglutination (clumping), precipitation, opsonisation (coating the antigen so phagocytes can engulf it) and neutralisation of toxins.

Vaccination and Allergy

A vaccine contains a killed or weakened pathogen, or a harmless part of it such as a toxoid. It makes the body produce antibodies and memory cells without causing the disease, so a later infection meets a fast, strong secondary response. Allergy is an exaggerated response to a harmless substance such as pollen or dust. It involves IgE antibodies bound to mast cells; on re-exposure, these cells release histamine and other chemicals, causing sneezing, watery eyes, wheezing or, in severe cases, anaphylactic shock.

Immune Disorders and Tumour Immunology

In autoimmune diseases, the body loses self-tolerance and attacks its own tissues, as in rheumatoid arthritis. In immunodeficiency, the defence system is too weak. AIDS, caused by the human immunodeficiency virus (HIV), is the best-known example; the virus destroys helper T cells. Tumour immunology deals with how the immune system detects and kills cancer cells, and with immunotherapy that boosts this killing when normal controls on cell division fail.

Common mistakes to avoid

• Writing that T cells mature in the bone marrow. T cells arise in the bone marrow but mature in the thymus; B cells mature in the bone marrow.
• Confusing active and passive immunity. In active immunity, the body makes its own antibodies (after infection or a vaccine). In passive immunity, ready-made antibodies are given, as through a mother's milk or an antiserum.
• Assuming primary and secondary responses are the same. The secondary response is faster, stronger and lasts longer because memory cells are already present.
• Mixing up epitope and paratope. The epitope is on the antigen; the paratope is the binding site on the antibody.
• Treating immunodeficiency and autoimmunity as the same. Immunodeficiency is too little immune activity; autoimmunity is an attack on the body's own tissues.

Test yourself on these with the practice test, then check the worked reasoning in the solved MCQs.

Frequently asked questions

What is the difference between innate and acquired immunity?

Innate immunity is present from birth, is non-specific and acts immediately through barriers like skin, mucus and stomach acid, and through phagocytes. Acquired immunity develops after the body is exposed to a specific antigen. It is specific, takes some days to build up the first time, and leaves memory cells, so the next response to the same pathogen is much quicker.

How do primary and secondary lymphoid organs differ?

Primary lymphoid organs, the bone marrow and thymus, are where lymphocytes are produced and mature into immunocompetent cells. Secondary lymphoid organs, such as the spleen, lymph nodes and MALT, do not produce mature lymphocytes. They filter blood or lymph, trap antigens and provide the place where antigens meet mature B and T cells.

What is the basic structure of an antibody?

An antibody is a Y-shaped protein made of four polypeptide chains: two identical heavy chains and two identical light chains, joined by disulphide bonds. The tips of the two arms are variable regions that form the antigen-binding sites. The rest of the molecule, including the stem, is the constant region, which decides the class of the antibody, such as IgG or IgE.

How does a vaccine protect us from disease?

A vaccine exposes the body to the antigens of a pathogen in a safe form, as a killed or weakened organism or a toxoid. The immune system responds by making antibodies and memory B and T cells, but the person does not get the disease. If the real pathogen enters later, the memory cells start a fast, strong response and remove it before it can cause illness.

What happens during an allergic reaction?

An allergy is an over-reaction to a harmless substance called an allergen, such as pollen, dust or certain foods. On first exposure, the body makes IgE antibodies that attach to mast cells. On the next exposure, the allergen binds these antibodies and the mast cells release histamine and serotonin. This causes sneezing, a runny nose, watery eyes and difficulty in breathing.

What causes an autoimmune disease?

An autoimmune disease develops when the immune system can no longer tell self antigens from foreign ones. It produces autoantibodies and cytotoxic T cells that attack the body's own cells and tissues. Rheumatoid arthritis, where the joints are attacked, and multiple sclerosis, where the covering of nerve fibres is damaged, are common examples.

Why does HIV cause severe immunodeficiency?

HIV enters and multiplies inside helper T cells, and macrophages act as a reservoir that keeps producing the virus. Helper T cells coordinate the whole acquired immune response, so as their numbers fall, B cells make fewer antibodies and cell-mediated immunity weakens. The patient then becomes open to opportunistic infections and certain cancers that a healthy immune system would control.