How Your Body Remembers a Germ It Has Met Before

We are surrounded by microbes, yet most of the time we stay healthy. The immune system blocks many invaders at once, learns to recognise particular ones and remembers them — a memory that vaccines use and HIV attacks.

This part covers innate and acquired immunity, antibodies and types of immunity, vaccination with allergy, autoimmunity and lymphoid organs, and HIV and AIDS.

Innate immunity is non-specific defence present at birth, built from physical, physiological, cellular and cytokine barriers, while acquired immunity is specific to each pathogen and has memory — so the first, primary response is weak and a later, secondary response to the same pathogen is fast and intense.

Innate immunity — four barriers:

- Physical — skin blocks entry; mucus in the respiratory, digestive and urogenital tracts traps microbes
- Physiological — stomach acid, saliva and tears prevent microbial growth
- Cellular — neutrophils, monocytes, natural killer cells and tissue macrophages engulf and destroy microbes
- Cytokine — virus-infected cells release interferons, which protect uninfected cells from the virus

Acquired immunity:

- Specific to each pathogen and characterised by memory
- Primary response — a low-intensity response on first encounter
- Secondary (anamnestic) response — a much stronger response on meeting the same pathogen again, thanks to memory cells

An everyday example. Tears washing dust and germs out of your eyes are part of your innate, physiological defence.

The substance. Most people get chickenpox only once because acquired immunity mounts a strong secondary response that prevents a second illness.

Humoral immunity uses antibodies from B-lymphocytes, cell-mediated immunity uses T-lymphocytes acting directly, an antibody is a Y-shaped molecule of two heavy and two light chains, and immunity is active if the body makes its own antibodies but passive if they are supplied ready-made.

Humoral immunity:

- B-lymphocytes produce antibodies into the blood

Cell-mediated immunity:

- T-lymphocytes do not secrete antibodies; they help B cells make them and attack infected cells directly
- Cell-mediated immunity causes graft rejection, which is why tissue matching and immunosuppressants are needed in transplants

Antibody structure:

- Four polypeptide chains — two light and two heavy — written **H${}_2$L${}_2$
- Arranged in a Y shape with antigen-binding sites at the tips

Active and passive immunity:

- Active — the body makes its own antibodies after infection or vaccination; slow to develop but long-lasting
- Passive — ready-made antibodies are given, as in colostrum carrying IgA to a newborn or antibodies crossing the placenta to the foetus; immediate but short-lived

An everyday example. A newborn fed colostrum, the first milk after birth, receives IgA antibodies — natural passive immunity.

The substance. Graft rejection shows a working immune system**, which correctly recognises transplanted tissue as foreign.

Vaccination introduces inactivated or weakened pathogens or their proteins so that the body makes antibodies and memory cells before real infection; antitoxins supply preformed antibodies for immediate protection; allergy is an exaggerated IgE-driven response to harmless substances; autoimmunity is an attack on the body's own cells; and lymphoid organs are where lymphocytes form, mature and meet antigens.

Vaccination and immunisation:

- A vaccine contains antigenic proteins or inactivated or weakened pathogens
- The body makes antibodies and memory B and T cells, which respond rapidly on later exposure

Preformed antibodies. For fast-acting threats such as tetanus or snakebite, ready-made antibodies or antitoxins give immediate passive immunisation.

Allergy:

- An exaggerated response to allergens such as dust mites, pollen and animal dander
- IgE antibodies trigger mast cells to release histamine and serotonin
- Causes sneezing, watery eyes, a running nose and difficult breathing; treated with antihistamines, adrenaline and steroids

Autoimmunity. The immune system mistakenly attacks self-cells, as in rheumatoid arthritis.

Lymphoid organs:

- Primary — bone marrow and thymus, where lymphocytes form and mature
- Secondary — spleen, lymph nodes, tonsils, Peyer's patches of the small intestine and appendix, where lymphocytes meet antigens and multiply

An everyday example. Anti-snake venom given in hospitals contains preformed antibodies that act at once.

The substance. Antitoxins protect immediately but leave no memory, unlike a vaccine.

AIDS is caused by the Human Immunodeficiency Virus, a retrovirus spread through unprotected sex, infected blood, shared needles and from an infected mother to the child; it multiplies inside macrophages and destroys helper T-lymphocytes, weakening immunity until opportunistic infections take hold; it is diagnosed by ELISA, managed with anti-retroviral drugs and prevented by safe practices.

Transmission:

- Sexual contact with an infected person
- Transfusion of contaminated blood or blood products
- Sharing infected needles, as among intravenous drug users
- From an infected mother to the child through the placenta
- It does not spread through touch or casual contact

Replication cycle:

- HIV enters macrophages, where reverse transcriptase copies its RNA genome into viral DNA
- The viral DNA joins the host cell's DNA and directs the making of new viruses, so macrophages act as an HIV factory
- New viruses attack **helper T-lymphocytes (T${}_H$), whose numbers fall steadily

Progression to AIDS. There is often a long gap between infection and symptoms. As helper T cells decline, the person becomes vulnerable to opportunistic infections** by bacteria, viruses, fungi and parasites such as Toxoplasma.

Diagnosis and treatment:

- ELISA, the enzyme-linked immunosorbent assay, is widely used for diagnosis
- Anti-retroviral drugs prolong life but cannot cure the infection

Prevention: screened blood, disposable needles and syringes, condoms, control of drug abuse and awareness campaigns.

An everyday example. Blood banks screen every donated unit for HIV to protect patients who receive transfusions.

The substance. HIV destroys the very cells that coordinate immunity, which is why normally harmless infections become life-threatening.