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

 Memory B cells

Plasma Cells: These secrete antibodies (immunoglobulins) into blood. The antibodies are carried in blood but the B lymphocytes remain fixed in the lymphoid tissue. Plasma cells produce one type of antibody which targets a specific antigen. Antibodies have the following functions: Bind antigens; label them as targets for other cells to attack. Bind to bacterial toxins neutralizing them. Activate the complement.

Memory B Cells: These remain in the body long after initial episode and respond rapidly in case of another attack by the same antigen by stimulating production of antibody secreting plasma cells. There is a balance in this system to prevent a body reacting to its own cells. If this balance is changed auto immune diseases occur.

Types of antibodies:

• IgA: Found in body secretions e.g. breast milk saliva. These antibodies prevent antigens crossing epithelial membranes and invading deeper tissues.

• IgD: Made by B cells and displayed on their surfaces. Antigens bind here to activate B cells.

• IgE: Found on cell membranes e.g. basophils and mast cells. If it binds with an antigen it activates the inflammatory response. In allergy it is excessive.

• IgG: This is the largest and most common antibody type. It attacks many different pathogens. It crosses the placenta and protects the fetus.

• IgM: Produced in large quantities in primary response and is a potent activator of complement.

Abnormal Immune Function
Hypersensitivity (Allergy): This is a powerful immune response to an antigen which is usually harmless to other people e.g. perfume. The body is harmed by the response but not the allergen. When a hypersensitive person is exposed to the allergen he is sensitized and the body forms a response which is beyond normal. Some symptoms are mild e.g. running nose and eyes or may be fatal e.g. anaphylactic shock. There are 4 types of hypersensitivity classified according to the part of immune system involved.

• Type 1: Anaphylactic: It occurs in a person who inherited very high levels of IgE. When exposed the antibodies activate basophils and mast cells which release histamines. This constricts the airway smooth muscles cause vasodilatatio0n and increase vascular permeability. Fluids move into the tissues, there is broncho constriction and shock due to vaso dilation. It can lead to death e.g. anaphylaxis due to drugs. Onset is immediate.

• Type II: Cytotoxic: The antibody reaction with an antigen on a cell surface marks it for destruction by Phagocytosis or lytic enzymes. This is useful in elimination of bacteria but antibodies may be directed to body tissues (auto immunity).This mechanism causes conditions e.g. haemoltytic disease of newborn and blood transfusion reactions. Onset is immediate.

• Type III: Immune Complex Mediated: Antibody- antigen complexes are cleared by Phagocytosis. If they are not cleared or are excessive they can be deposited in kidney, skin, joints and the eye thus causing g inflammation e.g.glomerulonephritis. Sensitivity to some drugs is type 3 where the immune complexes are deposited in tissues causing rashes, joint pains and haematuria. On set is 4 – 8 hours.

• Type IV: Delayed Type: This does not involve antibodies. It is over reaction of T lymphocytes to an antigen. If an antigen is recognized by memory T lymphocytes large numbers of Cytotoxic T lymphocytes are produced to eliminate the antigen. If the T lymphocyte system is not controlled normal tissues are damaged e.g. graft rejection (it becomes necrotic and sloughs off).Onset 24 – 48 hours.

Auto Immune Diseases:

Normally immune response is towards foreign antigens but sometimes the body fails to recognize its own tissues and attacks itself. This is an exaggerated type 3 hypersensitivity. The following conditions occur due to auto immunity.

• Rheumatoid Arthiritis: Body produces antibodies against synovial membranes. The antibody (Rheumatoid factor) can be detected in blood. It binds to synovial membrane and causes chronic inflammation of joints which are stiff, painful and swollen.

• Hashimoto’s Disease: The body makes antibodies against thyroglobulin leading to destruction of thyroid hormone and low secretion by the thyroid gland.

• Grave’s Disease: Body makes antibodies to thyroid cells. The antibodies stimulate the gland to produce high level of hormones and thus hyperthyroidism.

• Auto Immune Haemolytic Anaemia: The individual makes antibodies towards their own red blood cells and thus haemolysis.

• Myasthenia Gravis: Origin is unknown but it’s more common in women between 20 and 40 years. Antibodies are produced and block acetylcholine receptors in neuromuscular junction. Nerve impulses to muscles are blocked. This results into progressive and extensive muscle weakness although the muscles are normal. Eyelid muscles are affected first (Ptosis or drooping eyelids), diplopia (double vision).These are followed by muscles of the neck (Difficulty in chewing, swallowing, speech) Later limbs are involved. There are periods of improvement; relapses are precipitated by strenuous exercises, infections and pregnancy.