Immunity By : Idura Namira

What Needs To Be Learned :

Immune Response The key to a healthy immune system is its remarkable ability to distinguish between the body’s own cells, recognized as “self,” and foreign cells, or “nonself.” The body’s immune defenses normally coexist peacefully with cells that carry distinctive “self” marker molecules. But when immune defenders encounter foreign cells or organisms carrying markers that say “nonself,” they quickly launch an attack.Anything that can trigger this immune response is called an antigen.

Definitions To Know

Recognizing Them The difference between neutrophil and lymphocyte is that : Lymphocyte nucleus takes up most of the cell; there is very little cytoplasm. Neutrophils have lobed nucleus, with a larger amount cytoplasm. The neutrophil is larger than the lymphocyte.

Phagocytosis (Neutrophils) Neutrophils have a short life: after killing and digesting some pathogens, they die. Dead neutrophils often collect at a site of infection to form pus. Phagocytosis (Macrophage) Macrophage are long-lived antigen- presenting cells. The engulf bacteria by endocytosis. They do not destroy pathogens completely, but cut them up to display antigens that can be recognized by lymphocyte. *They are both made in the bone marrow.

Lymphocytes There are two types of lymphocytes, – B-Lymphocytes – T-lymphocytes They are both made in the bone marrow T-lymphocytes later goes and matures in the thymus whereas B-lymphocytes stays and matures in the bone marrow.

Mode of Action B-lymphocyte – B cell recognises a particular antigen and produces a specific antibody which will bind to it – All receptors on one B cell are the same i.e. can only recognise one antigen – When it binds an antigen the cell is stimulated to multiply forming lots of identical copies – Activation also requires presence of cytokines secreted by helper T cells (mandatory) – 2 types of B cell form by differentiation – memory cells and plasma cells – Plasma cells (effector cells) live for only a few days and produce huge numbers of antibodies – Memory cells live for a long time (months or years) – Population of memory cells is much larger than the original population of B cells which allows a faster and greater response to a second infection – Memory cells differentiate into antibody producing plasma cells on second contact

Mode of Action T-lymphocyte – T cells have receptors on their cell surface (similar to antibodies) which recognise fragments of antigen molecules. These antigens tend to be presented to them by other cells, usually macrophages. – Helper cells, work in association with macrophages – Macrophage presents antigen molecule to helper cell – this then produces large amount of cytokines: – Cytokines are small peptide molecules which: Stimulate T cells to multiply Promote inflammation Stimulate B cells to make antibodies – Killer cells produce smaller amounts of cytotoxic – Killer cells kill cells by punching holes in them causing lysis – They recognise foreign peptides which are not part of living antigens e.g. protein coat a virus leaves on the outside of cells hence they know the cell is infected

Role of memory cell in long term immunity Memory cells remains in the blood for many years. They are able to divide rapidly to produce plasma cells if the same antigen invades the body again.

Antibodies (Immunoglobulin) The sequences of amino acids in these regions make the specific three- dimensional shape which binds to just one type of antigen. The antigen-binding sites form the variable region, which is different on each type of antibody molecule produced. The ‘hinge’ region gives the flexibility for the antibody molecule to bind around the antigen.

Passive And Active Immunity

Vaccination Vaccination in involves introducing weakened pathogens into the body. The lymphocytes react to the antigens on the pathogens by producing antibodies and memory cells. There are several problem with vaccination, among them being : – Poor response – Antigenic variations – Antigenic concealment

Measles Measles is caused by a virus (Morbillivirus) which is spread by airborne droplets.

Vaccination : Why it has worked on smallpox but not measles. For measles, there are poor response to the vaccine shown by some children who need several booster to develop full immunity. Migrants and refugees can form reservoirs of infection, experiencing epidemics within their communities and spreading the disease to surrounding populations.