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I have read that the body contains T-cells and B-cells specific to all antigens in small amounts and that these are activated when the antigen presents in the body. If this is true then why are we not able to fight all types of infection?
If this is not true, how does our body learn to fight new infection or new foreign substances?
Disclaimer: This is mostly like canonical or "textbook" immunology. As with many biological systems, there are caveats to everything and it can't all be covered here. If you have questions outside the scope of this answer, please make another question!
The immune system consists of two subsystems: adaptive and innate immunity. Adaptive immunity as you know consists of T cells and B cells, and fronts much of the resistance to established infections. In fact, without adaptive immunity, you end up with a disease called SCID (severe-combined immunodeficiency), and HIV depletes CD4+ T cells resulting in AIDS. Important no doubt, but adaptive immunity has a process to it which makes it useless against brand new threats to which immunological memory doesn't yet exist. This is where the innate immune system comes in.
So we can either use the mucosal layer in the GI tract or the skin as an example, either way. Of course, the primary defense against foreign threats are such physical barriers as mucus, skin, the ciliary system of the lungs, and so forth. But lets say you get a cut or abrasion and now the skin barrier is compromised and a slew of bacteria enter the tissue. Your tissues contain resident cells such as macrophages (macs) and dendritic cells (DC) which have pattern recognition receptors that either recognize PAMPs or DAMPs. So what's a good PAMP? Lipopolysaccharide (LPS or endotoxin) comes to mind, and is recognized by toll-like receptors on DCs and macs, specifically TLR-4. These cells begin to produce mediators of inflammation such as IL-8 and IL-1ß which attract cells from the blood. The primary innate immune cell in your blood is a neutrophil, a type of phagocyte, which kill bacteria in a variety of ways. You also have a system of plasma (blood) proteins produced in the liver called complement which attempt to coat and directly kill the bacteria, or direct innate cells to targets of interest.
So say the bacteria is in the tissue, the tissue is inflamed, and a dendritic cell engulfs and digests a bacterium. The dendritic cell is what's referred to as an antigen presenting cell or APC, and APCs act to bridge innate and adaptive immune systems. Through an exogenous pathway sort of like this, they break down the bacterial proteins and "present" them on these MHC-II complexes (HLA in human). Macrophages, B cells and T cells may also be APCs. So when the dendritic cell is digesting the bacterial proteins and preparing to present them on MHC complexes, they move out of the tissues into the lymph system by expressing lymph node trafficking receptors such as CCR7 which responds to chemokines secreted by activated endothelium in the lymphatic vessels. The endothelium is activated by the inflammatory signals we talked about earlier. The DC goes to the lymph node where naive T cells and B cells aggregate, circulate in the blood, and re-aggregate.
Naive T cells tend to go back to the lymph node because they too express trafficking receptors such as CCR7 and CD62L that say "follow signals back to the lymph node." So this is where the magic happens. I'm not going to cover adaptive immunity but in short, typically APCs present antigens derived from foreign threats to naive T and B cells in the lymph node, thus activating them. These cells differentiate into a variety of lineages including effector and memory cells. As a note, you first adaptive response against a new threat takes about 2 weeks from start to finish to get antigens to an expanded pool of antigen-specific clones. Memory cells improve the second response by largely forgoing the wall of text above, proceeding directly to clonal expansion and elimination.
Yes,it's true.In an overview, our body fights against all these foreign bodies and microbes in order to eliminate them and the threats caused by them.If u analyze deep, what you call antigen is just a protein on the surface of any microbe.Your body cells eg.RBCs, also have antigens on their surface.But your body's immune system doesn't attack them normally.Its because of the recognition of self antigens of our body by the immune system so as to prevent them from being attacked.If your body is attacked by a bacteria that has a new antigen on it that our system has never encountered, the so called antigen presenting cells for eg.,T helper cells, Dendritic cells ,etc. recognize each and every antigen on the microbe surface and present them to developing T and B cells.So the antibodies(the anti-proteins for the antigens) are produced to act against the antigens on the foreign cell surface and render them inactive.These antigen proteins have specific sites called epitopes an which the sites of antibodies ,called paratopes bind.In the set of disorders called autoimmune disorders the body's immune system attacks some self antigens because of some pathology.For some conditions such as AIDS by HIV the body's immune system is suppressed by initial attack on the T helper cells so that the recognition of antigens is suppressed and the immune system fails.And some microbes have the ability to modify the antigens on their surface.This is the reason for resistance of microbes to drugs.
Reference:Pathologic Basis of disease..- Robbins and Cotran
----Combining a second answer into one---
It's simple...it has already recognized what are all self and thus any new microbe attacks your body it doesn't need to recognize it earlier...since it is something new the defense system starts working. Naturally when it is not known as self, it's some new invader which is read by the patrolling WBCs and by the effects produced by it( such as inflammation) which draws the attention of other WBCs to the site through some chemical attractants called inflammatory mediators...these attacking happens only when the foreign substance crosses the first line of defense our body has(the so called innate immunity plays an important role in it).WHEN THAT MICROBE IS ALREADY KNOW BY RECOGNIZING THE ANTIGENS PRIOR TO EXPOSURE (i.e. during a second or successive attack) the reactions start immediately i.e. the antibodies already programmed are released immediately along with inflammatory mediators ( which accouts for allergic response).If it is new microbe a recognition process takes place and after that anibodies are produced.HAVING T AND B CELLS FOR ALL POSSIBLE ANTIGENS IS NOT POSSIBLE BECAUSE THE CORRESPONDING T AND B CELLS ARE PRODUCED ONLY AFTER FIRST EXPOSURE and stored as memory cells... during second attack the memory cells are cloned and response is produced.Antigens are just proteins and can be infinitely many(just a single alteration in a protein structure produces a new antigen)...IT DOESN'T NEED TO KNOW WHAT IT DOESN'T KNOW which is not possible if not exposed once... Because of the need for this first exposure attenuated antigens are given in vaccines so that during the first attack of the corresponding microbe T and B cells readily attack them.
