● Everyday we are exposed to large number of `color{Violet}"infectious agents"`. However, only a few of these exposures result in `color{Violet}"disease"`.

● This is due to the fact that the `color{Violet}"body"` is able to `color{Violet}"defend itself"` from most of these `color{Violet}"foreign agents"`.

● This overall `color{Violet}"ability"` of the host to fight the `color{Violet}"disease-causing organisms"`, conferred by the immune system is called `color{Brown}"immunity"`.

● Immunity is of two types:
(i) `color{Violet}"Innate immunity"`

(ii) `color{Violet}"Acquired immunity"`.

● Innate immunity is `color{Violet}"non-specific type"` of defence, that is present at the `color{Violet}"time of birth"`.

● This is accomplished by providing `color{Violet}"different types of barriers"` to the entry of the foreign agents into our body.

Innate immunity consist of `color{Violet}"four types"` of barriers. These are —

● `color{Brown} "Physical barriers"` :

`star` `color{Violet}"Skin"` on our body is the main barrier which `color{Violet}"prevents entry"` of the micro-organisms.

`star` `color{Violet}"Mucus coating"` of the epithelium lining the `color{Violet}"respiratory, gastrointestinal"` and `color{Violet}"urogenital tracts"` also help in trapping microbes entering our body.

● `color{Brown}"Physiological barriers"` : All prevent microbial growth.
`star` `color{Violet}"Acid"` in the stomach

`star` `color{Violet}"Saliva"` in the mouth

`star` `color{Violet}"Tears"` from eyes



● `color{Brown}"Cellular barriers"` : Certain types of `color{Violet}"leukocytes (WBC)"` of our body like `color{Violet}"polymorpho-nuclear leukocytes"` (`color{Violet}"PMNL-neutrophils"`) and `color{Violet}"monocytes"` and `color{Violet}"natural killer"` (type of lymphocytes) in the blood as well as `color{Violet}"macrophages"` in tissues can phagocytose and destroy microbes.



● `color{Brown}"Cytokine barriers"` : `color{Violet}"Virus-infected cells"` secrete proteins called `color{Violet}"interferons"` which protect non-infected cells from further viral infection.

● `color{Brown}"Acquired immunity"`, on the other hand, is `color{Violet}"pathogen specific"`.

● It is characterised by `color{Violet}"memory"`.

● This means that our body when it `color{Violet}"encounters a pathogen"` for the `color{Violet}"first time"` produces a response called `color{Brown}"primary response"` which is of low intensity.

● Subsequent encounter with the same pathogen elicits a `color{Violet}"highly intensified"` `color{Brown}"secondary"` or `color{Brown}"anamnestic response"`.



● This is ascribed to the fact that our body appears to have `color{Violet}"memory of the first encounter"`.

● The primary and secondary immune responses are carried out with the help of `color{Violet}"two special types"` of lymphocytes present in our blood, i.e., `color{Violet}"B-lymphocytes"` and `color{Violet}"T- lymphocytes"`.

● The `color{Brown}"B-lymphocytes"` produce an `color{Violet}"army of proteins"` in response to pathogens into our blood to fight with them.

● These proteins are called `color{Brown}"antibodies"`.

● The `color{Violet}"T-cells"` themselves do not secrete antibodies but `color{Violet}"help B cells"` produce them.



● Each antibody molecule has `color{Violet}"four peptide chains"`, two small called `color{Violet}"light chains"` and two longer called `color{Violet}"heavy chains"`.

● Hence, an `color{Violet}"antibody"` is represented as `H_2``L_2`.

● Different types of antibodies are produced in our body. `color{Violet}"IgA, IgM, IgE, IgG"` are some of them.

● Because these antibodies are found in the `color{Violet}"blood"`, the response is also called as `color{Brown}"humoral immune response"`.

● This is one of the two types of our acquired immune response – `color{Violet}"antibody mediated"`.

● The second type is called `color{Brown}"cell-mediated"` `color{brown}"immune response"` or cell-mediated immunity (`color{Brown}"CMI"`).

● The `color{Violet}"T-lymphocytes"` mediate `color{Violet}"CMI"`.

● Very often, when some `color{Violet}"human organs"` like heart, eye, liver, kidney fail to function satisfactorily, `color{Violet}"transplatation"` is the only remedy to enable the patient to live a normal life.

● Then a search begins – to find a `color{Violet}"suitable donor"`.

● `color{Violet}"Grafts"` from just any source – an animal, another primate, or any human beings cannot be made since the grafts would be `color{Violet}"rejected sooner or later"`.

● `color{Violet}"Tissue matching"`, `color{Violet}"blood group"` matching are essential before undertaking any `color{Violet}"graft/transplant"` and even after this the patient has to take `color{Violet}"immuno–suppresants"` all his/her life.

● The body is able to differentiate `color{Violet}"self and nonself"` and the cell-mediated immune response is responsible for the `color{Violet}"graft rejection"`.