Introduction to immunology

1. Advances in Immunology

2. Immune system
 System which provide protection to our body from foreign antigen
(non-self).
 There are 2 important roles of immune system;
1) non-self antigen (foreign antigen)
2) Self altered antigen
 Without an immune system, our bodies would be open to attack from
bacteria, viruses, parasites, and more.
 It is our immune system that keeps us healthy as we are surrounded by
a sea of pathogens.
 The immune system is made up of special cells, organs, chemicals,
and tissues.
 If the immune system encounters a pathogen, for example, a
bacterium, virus, or parasite, the mechanism is so-called immune
response.

3. Parts of the immune system
 The main parts of the immune system that actively fight against
infections are white blood cells, antibodies, complement system,
lymphatic system, spleen, thymus, and bone marrow.
 The main parts of the immune system are:
1) White blood cells
2) Antibodies
3) Complement system
4) Lymphatic system
5) Spleen
6) Bone marrow
7) Thymus.

4. 1) White blood cells
 White blood cells are also called leukocytes.
 White blood cells are the key players in immune system.
 They are made in bone marrow and are part of the lymphatic system.
 White blood cells move through blood and tissue throughout the body,
looking for foreign invaders (microbes) such as bacteria, viruses,
parasites and fungi. When they find a target, they begin to multiply and
send signals out to other cell types to do the same.
 White blood cells include lymphocytes (such as B-cells, T-cells and
natural killer cells), and many other types of immune cells.
 Our white blood cells are stored in different places in the body, which
are referred to as lymphoid organs.

5.  These include the following:
 Thymus — a gland between the lungs and just below the neck.
 Spleen — an organ that filters the blood. It sits in the upper left of the
abdomen.
 Bone marrow — found in the center of the bones, it also produces red
blood cells.
 Lymph nodes —small glands positioned throughout the body, linked
by lymphatic vessels.

6.  There are two main types of leukocyte:
i) Phagocytes
 These cells surround and absorb pathogens and break them down, effectively eating them.
There are several types, including:
Neutrophils — 40-70%
 These are the most common type of phagocyte and tend to attack bacteria. The primary
role of neutrophils is the clearance of extracellular pathogens, through phagocytosis
 Neutrophils have long been regarded as the first line of defense against infection and
one of the main cell types involved in initiation of the inflammatory response.
 Neutrophils are hematopoietic-derived immune cells that are generated, and continue
to develop, in the bone marrow until recruited into circulation and then to sites of
infection or inflammation.
 Neutrophils can produce many cytokines and chemokines upon stimulation, and in
this way, they can interact with endothelial cells, dendritic cells, macrophages, natural
killer cells, T lymphocytes, and B lymphocytes. Through all these interactions,
neutrophils can either activate or downregulate both innate and adaptive immunity.

7. Monocytes —
 They are the largest type of leukocyte and have several roles.
 Monocytes represent 10% of leukocytes in human blood
 Play important roles in the inflammatory response, which is essential for the innate
response to pathogens. At the site of infection or injury in tissues, they can
differentiate into dendritic cells or macrophages that mediate both innate and
adaptive immune responses to disease.
 There are two types of monocytes in the human blood:
(a) the classical monocyte: which is characterized by high level expression of the
CD14 cell surface receptor (CD14++monocyte) and
(b) the non-classical: proinflammatory monocyte with low level expression of CD14
and with additional co-expression of the CD16 receptor (CD14+CD16+monocyte).
CD14+CD16+monocytes produce copious amounts of pro-inflammatory cytokines
like Tumor Necrosis Factor (TNF) and interleukin-12 (IL-12).

8. Macrophages —
 Macrophages are specialized cells involved in the detection, phagocytosis and
destruction of bacteria and other harmful organisms. Macrophages use the process of
phagocytosis to engulf particles and then digest them.
 In addition, they can also present antigens to T cells and initiate inflammation by
releasing molecules (known as cytokines) that activate other cells and also remove
dead and dying cells.
 The macrophages occur especially in the lungs, liver, spleen, and lymph nodes, where
their function is to free the airways, blood, and lymph of bacteria and other particles.
 Two Types of Macrophages: M1 and M2 Macrophages.
Mast cells —
 Mast cells are a type of white blood cell - play a major role in our immune system,
including helping to heal wounds and defend against pathogens.
 Mast cells are packed with histamine, a chemical released when you have an allergic
reaction.
 Mast cells are located at the boundaries between tissues and the external environment,
for example, at mucosal surfaces of the gut and lungs, in the skin and around blood
vessels.

9. ii) Lymphocytes
 Lymphocytes help the body to remember previous invaders and
recognize them if they come back to attack again.
 Lymphocytes begin their life in bone marrow. Some stay in the
marrow and develop into B lymphocytes (B cells), others head to the
thymus and become T lymphocytes (T cells). These two cell types
have different roles:
B lymphocytes —
 they produce antibodies and help alert the T lymphocytes.
T lymphocytes —
 they destroy compromised cells in the body and help alert other
leukocytes.

10. 2) Antibodies
 Also called immunoglobulin, a protective protein produced by the immune system
in response to the presence of a foreign substance (called an antigen) or the toxins
(poisons) they produce.
 To eliminate the invader, the immune system calls on a number of mechanisms,
including one of the most important—antibody production.
 Antibodies are produced by specialized white blood cells called B lymphocytes (or
B cells).
 When an antigen binds to the B-cell surface, it stimulates the B cell to divide and
mature into a group of identical cells called a clone. The mature B cells, called
plasma cells, secrete millions of antibodies into the bloodstream and lymphatic
system.
 Antibodies attack antigens by binding to them. The binding of an antibody to a
toxin, for example, can neutralize the poison simply by changing its chemical
composition; such antibodies are called antitoxins.
 In other cases, the antibody-coated antigen is subject to a chemical chain reaction
with complement, which is a series of proteins found in the blood. The complement
reaction either can trigger the lysis (bursting) of the invading microbe or can attract
microbe-killing scavenger cells that ingest, or phagocytose, the invader.

11. 3) Complement system
 The complement system is made up of proteins whose actions complement the
work done by antibodies.
 The term “Complement” refers to the ability of these proteins to complement i.e.,
augment the effects of other components of immune system e.g., Antibody.
 The complement system refers to a series of >20 proteins, circulating in the blood
and tissue fluids.
 Most of the proteins are normally inactive, but in response to the recognition of
molecular components of microorganisms they become sequentially activated.
 Three main effects
1. Lysis of cells such as bacteria, allografts and tumor cells
2. Generation of mediators that participate in inflammation and attract neutrophils
3. Opsonization i.e., Enhancement of phagocytosis

12. 4) Lymphatic system
 The lymphatic system is a network of delicate tubes throughout the
body. The main roles of the lymphatic system are to:
 manage the fluid levels in the body
 react to bacteria
 deal with cancer cells
 deal with cell products that otherwise would result in disease or
disorders
 absorb some of the fats in our diet from the intestine.
 The lymphatic system is made up of:
 lymph nodes (also called lymph glands) -- which trap microbes
 lymph vessels -- tubes that carry lymph, the colourless fluid that
bathes body's tissues and contains infection-fighting white blood
cells
 white blood cells (lymphocytes).

13. 5) Spleen
 The spleen is a blood-filtering organ that removes microbes and
destroys old or damaged red blood cells. It also makes disease-fighting
components of the immune system (including antibodies and
lymphocytes).
6) Bone marrow
 Bone marrow is the spongy tissue found inside bones. It produces
the red blood cells our bodies need to carry oxygen, the white
blood cells we use to fight infection, and the platelets we need to
help our blood clot.
7) Thymus
 The thymus filters and monitors blood content. It produces the
white blood cells called T-lymphocytes.

15. The body's other defences against microbes
 As well as the immune system, the body has several other ways to
defend itself against microbes, including:
 skin - a waterproof barrier that secretes oil with bacteria-killing
properties
 lungs - mucous in the lungs (phlegm) traps foreign particles, and small
hairs (cilia) wave the mucous upwards so it can be coughed out
 digestive tract - the mucous lining contains antibodies, and the acid in
the stomach can kill most microbes
 other defences - body fluids like skin oil, saliva and tears contain anti-
bacterial enzymes that help reduce the risk of infection. The constant
flushing of the urinary tract and the bowel also helps..