Immunity is the way a body protects itself against harmful foreign bodies which can invade it. These protective measures are divided into 2 categories: Non specific defence mechanism and Specific defence mechanism.

Non Specific Defence Mechanism:

This is the first line protection and prevents entry or minimize further passage of micro organisms into the body. It includes the following:

• Defence at body surface
• Phagocytosis
• Natural anti microbial substances
• Inflammatory response
• Immunological surveillance.

1. Defense at body surface:

   • Skin: If intact and healthy it is an efficient physical barrier to prevent entry of micro organisms.

   • Mucus: This is secreted by mucus membranes and creates a sticky surface which can trap foreign bodies.

   • Sebum and sweat: This is secreted on the skin surface and it contain an antibacterial substance.

   • Hairs in the nose: These act as a filter and the sweeping movement of the cilia moves inhaled foreign bodies towards the throat then it is swallowed or coughed up.

   • Urine: Since it is acidic it prevents multiplication of micro organisms. Its one way flow also helps to wash out any micro Organisms.

2. Phagocytosis: This is known as cell eating. Macrophages and neutrophils migrate to the site of inflammation and infection (chemostasis).Neutrophils release chemicals that attract the micro organisms, phagocytes engulf the micro organisms using their body mass or extending long pseudopodia towards them. These cells are non selective and engulf any foreign cells r particles. After engulfing they act as antigens presenting cells to stimulate lymphocytes.

3. Natural anti microbial substances:

   • Hydrochloric acid: Mostly in gastric juices kill ingested microorganisms.

   • Lysozyme: A protein with antibacterial properties found in tears, sweat, urine, cerebrospinal fluid.

   • Antibodies: Present in nasal secretions and saliva.

   • Saliva: Secreted in the mouth and washes away food debris that may attract microbes. It is slightly acidic.

   • Interferon: Produced by the T lymphocytes. It prevents replication of viruses within infected cells and spread to new cells.

   • Complement: A system of about 20 proteins found in blood and tissues. It is activated by immune complex (antigen and antibody bound together) and foreign sugars on bacterial cell walls. It acts in the following ways: Binds and damages bacteria walls thus destroying the microbe, Binds bacteria cell wall stimulating Phagocytosis, Attracts Phagocytosis cells into area of infection.

4. Inflammatory response: This is the physiological response to tissue damage. It mostly occurs when microbes have overcome other non specific defence mechanisms. It protects in the following ways: It isolates, activates, and removes both causative organism and damaged tissue to allow healing to take place.

5. Immunological surveillance: Lymphocytes known as natural killer cells constantly patrol the body searching for abnormal cells. These may be malignant, infected with virus, have abnormal markers on their cell membranes. Any abnormal cell recognized is killed.

Specific Defence Mechanism: There are cells responsible for immunity known as lymphocytes which are manufactured in the bone marrow. After release into the blood stream they are processed to form 2 types: a. T lymphocytes b. B lymphocytes.

• T lymphocytes: These are processed by the thymus gland. They are programmed to recognize only one type of antigen. They provide cell mediated immunity.

• B lymphocytes: These are processed by the bone marrow. They are responsible for producing antibodies (immunoglobulins) these are designed to bind and destroy antigens. Each B lymphocyte has a specific antigen it reacts to. They provide humoral (antibody) mediates immunity.

Cell Mediated Immunity: T lymphocytes are released into the blood after being activated by the thymus. When they meet an antigen for the first time they become sensitized to it. Four main types of lymphocytes are produced and each is directed towards the original antigen but deals with it in different ways.

• Memory cells: These are long lived cells which survive after the threat has been neutralized. They provide cell mediated immunity by responding rapidly to another attack of the same antigen.

• Cytotoxic cells: These directly inactivate any cells carrying the antigen. They attach themselves to the target cell and release powerful toxins. They destroy abnormal body cells e.g. infected or cancer cells.

• Helper cells: These correct function of both cell mediated and antibody mediated immunity. They are commonly destroyed in HIV infection and when their number is too low the immune system is suppressed. Their major functions are a. Produce special; chemical called cytokines e.g. interferons which support and promote Cytotoxic T lymphocytes and macrophages. b. Co operating with B lymphocytes to provide antibodies.

• Suppressor cells: These are known as brakes. They turn off activated T and B lymphocytes thus limiting the damaging effects of the immune response.

Antibody Mediated (Humoral) Immunity: B Lymphocytes are fixed on lymphoid tissue e.g. spleen and lymph nodes, they do not circulate freely. The B lymphocytes can recognize and bind antigens. Once the antigen is recognized it is bound with the help of helper T lymphocytes. The B lymphocytes enlarge and divide. It produces 2 types of cells: Plasma cells and Memory B cells.

Acquired Immunity: When antigens e.g. microbes are encountered for the first time a primary response occurs. Low levels of antibodies are formed after 2 weeks. The response may be enough to fight the antigen but after sometime the antibody levels reduce unless there is a second encounter within 2 – 4 weeks. The second encounter (secondary response) results into rapid response by memory B cells resulting in increase in antibody production. This increases until the maximum level is achieved. Immunity can be acquired naturally or artificially. Active immunity means the person responds to the antigen by producing his own antibodies. It commonly leads to long lasting immunity. Passive immunity means an individual is given antibodies produced by some one else. These antibodies gradually break down so passive immunity is usually short lived.

• Active Natural Acquired Immunity: This can be stimwulated by a person suffering from the disease or getting a sub clinical infection not causing signs and symptoms but strong enough to cause formation of antibodies e.g. suffering from measles results into life long immunity.

• Passive Natural Acquired Immunity: This is acquired by the baby while still in utero. Maternal antibodies cross the placenta barrier into the baby. These give immunity for a short time. (Note: OCR text incorrectly labels this as “Passive Artificial Acquired Immunity” on page 48, but the description matches passive natural).

• Passive Artificially Acquired Immunity: Ready made antibodies are given e.g. Anti tetanus serum, Antivenom, Antirabies. Since they are ready made they can be given to a person exposed to prevent development of disease (prophylaxis) or after disease has developed to prevent worsening (therapeutic).

Immunity Chart Summary
INNATE (Born with)
NON SPECIFIC: Defence at surface, Phagocytosis, Natural antimicrobial Substances, Inflammatory process, Immunological Surveillance
SPECIFIC
Cell mediated: memory T cells, Cytotoxic T cells, Helper T cells, Suppressor T cells
Antibody mediated (Humoral): Plasma cells, memory B cells

ACQUIRED
1. ARTIFICIAL: Active, Passive
2. NATURAL: Active, Passive