What is blood?
Blood is a red colour pigment that circulate in body which comprises of plasma, various types of blood cells and platelets. The main function of blood is to deliver oxygen and nutrients to various cells and tissues of the body.
Various components of blood.
Blood is primarily broken down into the following components:
Types of blood cells found in human blood.
Blood cells are classified into the following types:
- Erythrocytes or red blood cells
- Leucocytes or white blood cells
Red Blood Corpuscles (RBC)
- RBC is also known as erythrocytes. They are disc-shaped cells concave in the middle and visible under a microscope. RBC carries oxygen from the lungs to all the cells of the body. They have no nucleus and contain a pigment called haemoglobin which is made up of an iron-containing pigment known as haema and a protein called globin. RBCs are produced in the spleen and the bone marrow and live for about four months because they lack a nucleus. So, when we donate blood to save the life of a person, then the loss of blood from our body is recovered within a day because red blood cells are made very fast in the bone marrow. The life of the RBC is about 100-120 days.
White Blood Corpuscles (WBC)
- WBC is also known as leukocytes. They fight with infection and protect us from diseases because they eat up the germs which cause diseases. That is why they are also known as ‘soldiers’ of the body’s defence system. They are round or irregular, semi-transparent cells containing a nucleus and visible under a microscope. They are a little larger than RBC. Some White blood cells make chemicals called ‘antibodies’ to fight against infection i.e why they provide immunity in our body. WBC in the blood is much smaller in number than red blood cells
Different types of white blood cells found in the blood.
White blood cells can be classified as follows:
- Broadly, WBC acts as a defence system in the body.
- There are several varieties of WBC performing specific functions such as, Neutrophils (65 to 70% of the total WBC) attack the invading bacteria and engulf them. Lymphocytes (25% of WBC) produces antibodies which protect the body against the antigen and thus provide immunity against infection. Basophils secrete anticoagulant called heparin which prevents clot within the blood cells. Eosinophils and monocytes also assist in the defence mechanism of the body by becoming active against specific antigens.
On the basis of structure and function, lymphocytes are divided into three main types:
- B Cells
- T Cells
- Natural Killer (NK) Cells
What are T Cells?
- T Cells also called T lymphocyte, type of leukocyte (white blood cell) that is an essential part of the immune system.
- T cells are one of two primary types of lymphocytes—B cells being the second type—that determine the specificity of the immune response to antigens (foreign substances) in the body.
- T cells originate in the bone marrow and mature in the thymus.
Why are they called memory cells?
- In the thymus, T cells multiply and differentiate into helper, regulatory, or cytotoxic T cells or become memory T cells.
- They are then sent to peripheral tissues or circulate in the blood or lymphatic system.
- Once stimulated by the appropriate antigen, helper T cells secrete chemical messengers called cytokines, which stimulate the differentiation of B cells into plasma cells (antibody-producing cells).
How do they control immunity?
- Regulatory T cells act to control immune reactions, hence their name.
- Cytotoxic T cells, which are activated by various cytokines, bind to and kill infected cells and cancer cells.
- Because the body contains millions of T and B cells, many of which carry unique receptors, it can respond to virtually any antigen.
Vaccination outcomes on T cells
- There is a possibility that pre-existing T cell memory might influence vaccination outcomes.
- Pre-existing immunity could help elicit better immune responses against novel coronavirus, and these responses can manifest faster.
- Meanwhile, pre-existing immunity could be mistaken as an enhanced efficacy of the vaccine in eliciting immune responses.
- This could be particularly confusing in Phase-1 trials where the vaccine is tested on a small group of healthy participants.
- The pre-existing immunity can reduce the immune responses that the vaccine causes through a mechanism called the “original antigenic sin”.
- It can also lead to antibody-mediated disease enhancement, where antibodies present at sub-neutralizing concentrations can actually augment virus infection and cause more severe disease.
This was seen in the case of chikungunya and dengue