THE IMMUNE SYSTEM

The immune system is the body's defence to foreign invaders (known as Antigens) in our body.Every cell in our body has a specific protein on its membrane which works as an identification that  the cell is a part of our body and not a foreign invader . WBCs(also known as leukocytes)are specialised cells that check if there is any foreign substance in the body.There are 2 types of  WBCs

phagocytes-cells that engulf and kill the pathogens

leukocytes - cells that allow the body to remember and recognise previous invaders and help the body destroy them

Cells of the Immune system

Antigen-presenting cells  - cells that display the antigen fragments on their cell surface

Leukocytes - White blood cells.  These are the cells which provide immunity
Lymphocytes - Cells that allow the body to remember and recognise previous invaders and help the body destroy them 
phagocytes - A type of cell within the body capable of engulfing and absorbing bacteria and other small cells and particles
B cells - WBCs that spend their entire early life in the bone marrow.  Upon maturity, their job is to travel throughout the blood and lymph looking for antigens with which they can interlock
Cytotoxic T cells - T-cells that kills infected or cancerous cells in our body(also known as natural killer                            cells) 
Helper T cells - These cells travel through the blood and lymph, looking for antigens
Granulocytes - Leukocytes (white blood cells) containing granules in the cytoplasm. Also known as a granular leukocyte . They seem to act as a first line of defence, as they rush toward an infected area and engulf the offending microbes.Granulocytes kill microbes by digesting them with killer enzymes contained in small units called lysosomes.
Dendritic cells - Mostly found in the skin and mucosal epithelium, where they are referred to as Langerhan's cells. Unlike macrophages, dendritic cells can also recognise viral particles as non-self.

Macrophages - These are large, long-lived phagocytes which capture foreign cells, digest them, and present antigen from these cells and manifest them on their exterior.  In this manner, they present the antigens to the T cells.

Mast cells - Cells concentrated within the respiratory and gastrointestinal tracts, and within the deep layers of the skin.  These cells release histamine upon encountering certain antigens, thereby triggering an allergic reaction.

Memory cells - Specialised B cells which grant the body the ability to manufacture more of a particular antibody as needed, in case a particular antigen is ever encountered again.

Plasma cells - formed by B-cells when they recognise a particular type of antigen.They act as antibodies producing factories

Activation of Immune system

Activation of immune system begins when a pathogen enters the body.Macrophages(a kind of phagocytes)that encounter the pathogen engulfs and kills the pathogen and displays the antigen fragments on their cell surfaces.Macrophages that display the antigen fragments on their cell surfaces are known as antigen-presenting cells.An antigen presenting cell interacts with a helper T-cell that can recognise the same antigen.During the interaction,the macrophage releases a chemical signal called Interleukin 1,which stimulates the helper T-cell to release Interleukin 2 which causes proliferation of certain cytotoxic T-cells and B-cells.normal cells of the body that have become infected may also digest some of the pathogens and display  antigens fragments on their cell surface .The body makes millions of different types of cytotoxic T-cells.Each type is able to recognise a particular antigen.the cytotoxic T-cell that are capable of recognising the antigen displayed on the surfaces of the infected cells bind to the infected cells and produce chemicals that kill the infected cell.Death of infected cell results in the destruction of the pathogens.B-cells also come into millions of different types,each able to recognise a particular antigen.when B-cells become activated by helper T-cells they turn into plasma cells.These plasma cells become Antibody-producing factories,flooding the blood stream with antibodies that are able to recognise the antigens involved in that infection .Antigens bind to the surfaces of pathogens to stop the pathogen from infecting other cells in our body and marking them for destruction by macrophages.but some of the B-cells do not turn into plasma cells instead they turn into memory B-cells which stays in our blood stream for decades providing future protection 

              Three types of immunity in Humans

1. Innate immunity -

Everyone is born with innate (or natural) immunity, a type of general protection. Many of the germs that affect other species don't harm us. For example, the viruses that cause leukemia in cats or distemper in dogs don't affect humans. Innate immunity works both ways because some viruses that make humans ill — such as the virus that causes HIV/AIDS — don't make cats or dogs sick. Innate immunity also includes the external barriers of the body, like the skin and mucous membranes (like those that line the nose, throat, and gastrointestinal tract), which are the first line of defence in preventing diseases from entering the body. If this outer defencive wall is broken (as through a cut), the skin attempts to heal the break quickly and special immune cells on the skin attack invading germs. 

   2. Adaptive Immunity -

The second kind of protection is adaptive (or active) immunity, which develops throughout our lives. Adaptive immunity involves the lymphocytes and develops as people are exposed to diseases or immunised against diseases through vaccination.

   3. Passive Immunity-

Passive immunity is "borrowed" from another source and it lasts for a short time. For example, antibodies in a mother's breast milk provide a baby with temporary immunity to diseases the mother has been exposed to. This can help protect the baby against infection during the early years of childhood. 
Everyone's immune system is different. Some people never seem to get infections, whereas others seem to be sick all the time. As people get older, they usually become immune to more germs as the immune system comes into contact with more and more of them. That's why adults and teens tend to get fewer colds than kids — their bodies have learnt to recognise and immediately attack many of the viruses that causes cold. 

Vaccination

Vaccination is the process of introduction of antigenic material to stimulate an individuals immune system to develop adaptive immunity to a pathogen

Having little bit of the antigens(dead or weakened) inside the body makes  immune system build antibodies to fight off this kind of pathogen. Antibodies help trap and kill pathogens that could lead to disease.our body can make antibodies in two ways: by getting the disease or by getting the vaccine. Getting the vaccine is a much safer way to make antibodies without having the suffering of the disease itself .Antibodies stay in our body for a long time. They know how to fight off the germ. If the real germ that causes this disease (not the vaccine) enters the body in the future,the immune system system knows how to fight it off.Often, your immune system will remember how to fight a germ for the rest of your life.

AUTOIMMUNE DISORDERS

In autoimmune disorders, the immune system mistakenly attacks the body's healthy organs and tissues as though they were foreign invaders. Autoimmune diseases include:

• Lupus, a chronic disease marked by muscle and joint pain and inflammation (the abnormal immune response also may involve attacks on the kidneys and other organs)
• Juvenile rheumatoid arthritis, a disease in which the body's immune system acts as though certain body parts (such as the joints of the knee, hand, and foot) are foreign tissue and attacks them
• Scleroderma, a chronic autoimmune disease that can lead to inflammation and damage of the skin, joints, and internal organs
• Ankylosing spondylitis, a disease that involves inflammation of the spine and joints, causing stiffness and pain
• Juvenile dermatomyositis, a disorder marked by inflammation and damage of the skin and muscles

Allergic Disorders

Allergic disorders occur when the immune system overreacts to exposure to antigens in the environment. The substances that provoke such attacks are called allergens. The immune response can cause symptoms such as swelling, watery eyes, and sneezing, and even a life-threatening reaction called anaphylaxis. Medications called antihistamines can relieve most symptoms.

Allergic disorders include:

• Asthma, a respiratory disorder that can cause breathing problems, frequently involves an allergic response by the lungs. If the lungs are oversensitive to certain allergens (like pollen, molds, animal dander, or dust mites), it can trigger breathing tubes in the lungs to become narrowed and swollen, leading to reduced airflow and making it hard for a person to breathe.
• Eczema is an itchy rash also known as atopic dermatitis. Although atopic dermatitis is not necessarily caused by an allergic reaction, it more often occurs in kids and teens who have allergies, hay fever, or asthma or who have a family history of these conditions.
• Allergies of several types can occur in kids and teens. Environmental allergies (to dust mites, for example), seasonal allergies (such as hay fever), drug allergies (reactions to specific medications or drugs), food allergies (such as to nuts), and allergies to toxins (bee stings, for example) are the common conditions people usually refer to as allergies.

Blood Groups

just like all other cells our red blood cells have proteins on their surface in ABO blood grouping they are referred to as antigen .Our body makes several types of  antibodies but not not the antibodies that are found on the surface of red blood cells

in an antibody the binding site is known as variable

portion because it depends on the shape of the antigen it can bind to

You cannot have both A antigen and A antibodies in your blood or B antigen and B antibody because if you have then it will trigger an immune response.That's why only certain blood groups can receive from certain blood groups . There is a protein known as Rh protein if your body produces it  then your blood group is + but your body doesn't produce it then your blood group is - .If your blood is + you can accept both + and - but if it is - then you can only accept - because if Rh protein enters your body then it will trigger the immune system.

                                                 WATER AND ITS PROPERTIES

One of the things that makes our planet special is the presence of liquid water. Water is fundamental for all life; without it every living thing would die. It covers about 70% of Earth's surface and it makes up 65-75% of our bodies (82% of our blood is water). Even though water seems boring - no color, taste, or smell - it has amazing properties that make it necessary for supporting life.

The chemical composition of water is H₂O - two hydrogen atoms and one oxygen atom. Water has special properties because of the way these atoms bond together to form a water molecule, and the way the molecules interact with each other.

Polarity

When electrons are not shared equally in a covalent bond, the molecule is described as polar. Water molecules are ploar. This means that while water molecules are neutral as a whole, one end of the water molecule tends to have a positive charge while the other has a negative charge. The oxygen end has a slight negative charge while the hydrogen end has a slight positive charge. Each end of a water molecule is attracted to the opposite charged end of another water molecule. Water's polarity is responsible for the "stickiness" or cohesion between the molecules.

COHESION

Cohesion is the attraction between molecules of the same substance. Due to hydrogen bonding water is very cohesive. Water's cohesion causes molecules on the surface of the water to be drawn inward causing "surface tension". This property allows insects to walk on water. 

                                         

ADHESION

Adhesion is the attraction between molecules of different substances.Thats why water usually stick to subtances .

Capillary Action
Cohesion of water causes capillary attraction, which is the ability of water to move upward in small spaces. Cohesion makes it possible for water to move up the fibers of a plant. This is how plants get the water they need to survive. In addition, it moves water upwards in soil. Cohesion of water also causes surface tension, water's invisible skin which allows water striders to walk on water

Surface Tension
Water is considered the "universal solvent" because its bipolar molecule enables it to dissolve a wide variety of substances. Solubility is affected by polarity. Polar substances can dissolve other polar substances. Non-polar substances dissolve other non-polar substances. Polar substances and non-polar substances, however, do not mix.

Density
Another property of water is density during phase changes. The density of most substances increases when a liquid becomes a solid. Solid water is actually less dense than liquid water. It is for this reason that ice floats. Can you imagine a world where ice sank? Lakes would freeze from the bottom up, killing many fish. Frozen water in the polar regions would sink and change the ocean levels. The fact that ice floats is essential for the survival of many aquatic ecosystems and ultimately life on Earth.

Snow Flakes

Q: How are snowflakes formed? 

A:  A snowflake begins to form when an extremely cold water droplet freezes onto a pollen or dust particle in the sky. This creates an ice crystal. As the ice crystal falls to the ground, water vapor freezes onto the primary crystal, building new crystals – the six arms of the snowflake.

That’s the short answer. 

The more complex explanation is this: 

These ice crystals that make up snowflakes are symmetrical (or patterned) because they reflect the internal order of the crystal’s water molecules as they arrange themselves in predetermined spaces (known as “crystallization”) to form a six-sided snowflake. 

Ultimately, it is the temperature at which a crystal forms — and to a lesser extent the humidity of the air — that determines the basic shape of the ice crystal. Thus, we see long needle-like crystals at 23 degrees F and very flat plate-like crystals at 5 degrees F.  

The intricate shape of a single arm of the snowflake is determined by the atmospheric conditions experienced by entire ice crystal as it falls. A crystal might begin to grow arms in one manner, and then minutes or even seconds later, slight changes in the surrounding temperature or humidity causes the crystal to grow in another way. Although the six-sided shape is always maintained, the ice crystal (and its six arms) may branch off in new directions. Because each arm experiences the same atmospheric conditions, the arms look identical. 

Q: So, why are no two snowflakes exactly alike? 

A: Well, that’s because individual snowflakes all follow slightly different paths from the sky to the ground —and thus encounter slightly different atmospheric conditions along the way. Therefore, they all tend to look unique, resembling everything from prisms and needles to the familiar lacy pattern. 

some of their shapes depending on temperature

• 32-25° F - Thin hexagonal plates
• 25-21° F - Needles
• 21-14° F - Hollow columns
• 14-10° F - Sector plates (hexagons with indentations)
• 10-3° F - Dendrites (lacy hexagonal shapes)

If water and ice are clear, then why does snow look white?

In simple words the answer  snowflakes have so many light-reflecting surfaces they scatter the light into all of its colors, so snow appears white Or we can say beacuse the way the human eye perceives color. Even though the light source might not be truly 'white' light (e.g., sunlight, fluorescent, and incandescent all have a particular color), the human brain compensates for a light source. Thus, even though sunlight is yellow and scattered light from snow is yellow, the brain sees snow as white because the whole picture received by the brain has a yellow tint that is automatically subtracted.

Transparency

Water is relatively transparent to visible light, near ultraviolet light, and far-red light, but it absorbs most ultraviolet light, infrared light, and microwaves. Most photoreceptors and photosynthetic pigments utilize the portion of the light spectrum that is transmitted well through water. Microwave ovens take advantage of water's opacity to microwave radiation to heat the water inside of foods. The very weak onset of absorption in the red end of the visible spectrum lends water its intrinsic blue hue

It is the only natural substance found in all three physical states at the temperatures that naturally occur on Earth.

New research from the Aerospace Institute of the University of Stuttgart in Germany supports the theory that water has a memory—a claim that could change our whole way of looking at the world . Even though they do not have enough evidence it is a famous 
topic of discussion for many scientists.